Thomas Teillet

François Fournier

Jean Borgomano

Fei Hong

Origin of seismic reflections in a carbonate gas field, Lower Miocene, offshore Myanmar

Keywords: carbonate reservoir, diagenesis, acoustic properties, seismic reflectivity, seismic expression, time-lapse seismic, Yadana

The geological origin of seismic reflections, within a carbonate gas reservoir, has been determined on the basis of an integrative analysis of cores, well-logs, pre-production and 4D time-lapse-seismic data. The gas reservoir is subdivided into meter to decameter-thick diagenetic units which are characterized by distinct acoustic properties. Low impedance units are dominant and consist of highly microporous and vuggy foraminiferal/ coralline algal floatstones intervals whose porosity has been significantly enhanced in burial diagenetic environments. High to moderate acoustic impedance units (thickness: 1-20m) are related to various depositional and diagenetic fabrics: 1) early-lithified coral floatstones, 2) foraminiferal rudstone whose intergranular space is occluded by shallow burial sparry calcite and 3) carbonate breccia with neomorphosed corals occurring below an intra-formational subaerial exposure surface. Within the gas zone, seismic reflection horizons may form at the boundary between two diagenetic units with contrasted acoustic impedance or may result

Introduction

Seismic reflection has become a major and essential geophysical method for imaging subsurface architecture of carbonate systems and reservoirs (e.g., [START_REF] Eberli | Seismic imaging of carbonate reservoirs and systems[END_REF][START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF][START_REF] Rankey | Seismic architecture and seismic geomorphology of heterozoan carbonates: Eocene-Oligocene, Browse Basin, Northwest Shelf, Australia[END_REF] and to reconstruct their evolution pattern and related controlling factors (e.g., [START_REF] Bachtel | Seismic stratigraphic evolution of the Miocene-Pliocene Segitiga Platform, East Natuna Sea, Indonesia: The origin, growth and demise of an isolated carbonate platform[END_REF][START_REF] Fournier | Development patterns and controlling factors of Tertiary carbonate buildups: Insights from high-resolution 3D seismic and well data in the Malampaya gas field (offshore Palawan, Philippines)[END_REF]Lüdman et al., 2018). Most seismic reflections in sedimentary rocks are composites resulting from several contrasts in acoustic impedance [START_REF] Sheriff | Limitations on resolution of seismic reflections and geological detail derivable from them[END_REF]. In calcite-dominated carbonate reservoirs, such contrasts in acoustic impedance are mostly related to vertical changes in porosity and pore space architecture (e.g. [START_REF] Anselmetti | the Velocity-Deviation Log: A Tool to Predict Pore Type and Permeability Trends in Carbonate Drill Holes from Sonic and Porosity or Density Logs[END_REF][START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF]. Seismic interpretation of carbonates is therefore highly challenging since such changes in porosity and pore network structure are strongly related to diagenetic history [START_REF] Wagner | Seismic signatures of carbonate diagenesis[END_REF]. The seismic sequence stratigraphy method is based on the assumption that seismic reflections follow depositional surfaces and erosional unconformities [START_REF] Vail | Seismic stratigraphy and global changes of sea level: Part 5-Chronostratigraphic significance of seismic reflections[END_REF], and therefore, they have chronostratigraphic significance. The diagenetic evolution of reservoir rocks leads to a spatial porosity and pore type distribution that may significantly overprint the depositional architecture in both siliciclastic (e.g. [START_REF] Davies | Kilometer-scale chemical reaction boundary patterns anddeformation in sedimentary rocks[END_REF][START_REF] Zeng | Seismic prediction of sandstone diagenetic facies: Applied to Cretaceous Qinshankou Formation in Qijia Depression[END_REF] and carbonate settings (e.g. [START_REF] Zampetti | Architecture and growth history of a Miocene carbonate platform from 3D seismic reflection data[END_REF][START_REF] Warrlich | The impact of postdepositional processes on reservoir properties: Two case studies of Tertiary carbonate buildup gas fields in Southeast Asia (Malampaya and E11)[END_REF][START_REF] Léonide | Diagenetic patterns and pore space distribution along a platform to outer-shelf transect[END_REF]. As a consequence, the seismic imaging of depositional features and the sedimentological and stratigraphic interpretation of seismic data may be significantly disturbed especially when the diagenetic evolution is multiphase and involves meteoric and burial processes [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF]. However, the strong interaction between diagenetic architecture and seismic expression in carbonates represents a unique opportunity for detecting, from seismic data, diagenetic bodies of major impact for exploration and production. On the basis of the integrated analysis (carbonate sedimentology, diagenesis, petroacoustics and seismics) of a Lower Miocene carbonate buildup (Upper Burma Limestone, Yadana gas field, offshore Myanmar) the present paper aims at: (1) identifying the depositional and diagenetic processes that controlled the evolution and the spatial distribution of acoustic properties within the carbonate buildup, (2) assessing the relative impact of depositional facies, diagenetic processes, and seismic resolution on the seismic expression of these carbonates, (3) testing the ability of seismic data to reveal diagenetic heterogeneity of significant impact on recovery and flow during hydrocarbon production.

Geological setting

The structural framework as well as the marine geomorphology of the Andaman Sea area has been strongly controlled by the complex tectonic evolution of SE Asia during the Cenozoic [START_REF] Hall | Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations[END_REF][START_REF] Curray | Tectonics and history of the Andaman Sea region[END_REF]. During the Paleogene, the northeastward oblique subduction (Sunda subduction zone) of the Indian plate beneath the Eurasian plate resulted in the opening of the Andaman Sea as a back-arc basin (Fig. 1A) [START_REF] Curray | Tectonics and history of the Andaman Sea region[END_REF][START_REF] Chakraborty | Cenozoic geodynamic evolution of the Andaman-Sumatra subduction margin: Current understanding[END_REF]. In addition, such an oblique subduction led to: 1) major dextral strike-slip movements along the Sagaing-Andaman-Sumatran fault system, 2) the individualization of the Burma microplate, and 3) rifting and spreading of the Andaman Sea within the back-arc basin [START_REF] Chakraborty | Cenozoic geodynamic evolution of the Andaman-Sumatra subduction margin: Current understanding[END_REF][START_REF] Morley | Chapter 4 Cenozoic rifting, passive margin development and strike-slip faulting in the Andaman Sea: a discussion of established v. new tectonic models[END_REF]. During the Oligocene and early Miocene, the Yadana carbonate platform developed on the top of a volcanic basement, located in the northern Andaman Sea (Fig. 1A,1B) (Racey and Read, 2015;[START_REF] Paumard | Evolution of Late Oligocene -Early Miocene attached and isolated carbonate platforms in a volcanic ridge context (Maldives type), Yadana field, offshore Myanmar[END_REF], which separates the M5 basin to the west from the Moattama basin to the east (Fig. 1C,1D).

The lowermost deposits consist of Upper Eocene volcano-clastics sediments (Fig. 1E). The overlying Oligo-Miocene carbonates reach up to 700 m in thickness, and are subdivided into two distinct formations [START_REF] Paumard | Evolution of Late Oligocene -Early Miocene attached and isolated carbonate platforms in a volcanic ridge context (Maldives type), Yadana field, offshore Myanmar[END_REF]: 1) the Lower Burma Limestone formation (LBL), Late Oligocene in age, that is composed of two distinct carbonate platforms separated by a central trough which is filled by the Sein clastic formation and 2) the Upper Burman Limestone formation (UBL), Early Miocene in age after benthic foraminiferal biostratigraphy, that is made of one single carbonate platform, the top of which corresponding to the gas reservoir of the Yadana field. A long-term depositional hiatus (~10 Myr) is recorded at top of the UBL carbonates that are sealed by Late Miocene (N16, planktonic zone) pro-delta shales (Pyawbwe and Badamyar formations) from the Irrawaddy deltaic system. A major phase of eastward tilting of the Yadana platform, evidenced by seismic profiles (Fig. 1C), occurred during the Late Miocene (horizon M6: 8.2 Ma, after [START_REF] Paumard | Evolution of Late Oligocene -Early Miocene attached and isolated carbonate platforms in a volcanic ridge context (Maldives type), Yadana field, offshore Myanmar[END_REF]. The Yadana gas field is located at depths ranging from 1240 mCD (core depth in meter) to 1350 mCD, corresponding to an overburden pressure of 2510 to 2550 psi (17 Mpa) and a temperature of 60°C.

The Yadana buildup has been regarded as a reef-rimmed platform by [START_REF] Paumard | Evolution of Late Oligocene -Early Miocene attached and isolated carbonate platforms in a volcanic ridge context (Maldives type), Yadana field, offshore Myanmar[END_REF] on the basis of seismic facies interpretation, but the integration of paleoecological and sedimentological analysis of core data led to a revision of the former depositional models [START_REF] Teillet | Development patterns of an isolated oligo-mesophotic carbonate buildup, early Miocene, Yadana field[END_REF] and to the definition of three distinct carbonate factories producing sediment on the top of an open platform: (1) a scleractinian carbonate factory developing under mesophotic conditions during periods of high particulate organic matter supplies, (2) an echinodermal carbonate factory occupying dysphotic to aphotic area of the shelf coevally with the scleractinian carbonate factory, (3) a large benthic foraminiferal-coralline algal carbonate factories prevailing under oligo-mesophotic and oligo-mesotrophic conditions. Changes through time of such carbonate factories resulted in a layer-cake depositional architecture on top of the carbonate buildup (Fig. 2A).

A scenario of diagenetic and porosity evolution of the Upper Burman Limestones has been proposed by [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF] (Fig. 2B). The high (28%) average porosity of the reservoir is related to significant development of microporosity, moldic and vuggy porosity. Mudsupported, coral-rich sediments record significant early marine dissolution of aragonite and early lithification of matrix that has led to the development of dense layers. One intraformational subaerial exposure surface has been identified based on diagenetic features and stable isotopes. [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF] interpreted the top of the reservoir as a long-term depositional hiatus (middle to late Miocene) which was related to platform drowning and nondeposition in a deep marine setting. Moderate dolomitization has affected the top and flanks of the platform during the depositional hiatus. Microporosity development and sparry calcite cementation (C2 calcite cements) have been shown to have occurred in marine to marine shallow burial environments. A major decrease in porosity, (up to 15%), below the gas-water contact suggests a major phase of porosity evolution during the hydrocarbon emplacement: 1) increased moldic and vuggy porosity development within the gas zone, 2) microporosity preservation or enhancement in loosely-packed, subrounded micrites within the gas zone, and 3) microporosity reduction by calcite micro-overgrowth within the water zone leading to the formation of densely-packed, euhedral micrites.

Data and methods

The Yadana field has been penetrated by 20 wells and covered by a three-dimensional seismic survey. Four cored (WELL-1, WELL-2, WELL-3, and WELL-4) and four uncored (WELL-a, WELL-b, WELL-c, WELL-d) wells have been selected for the present study (see location in Fig. 1B). The database is summarized in Table 1 and detailed below.

Carbonate rock sample database and petrographic analyses

The identification of depositional and diagenetic features is based on the study of a total of 343 m of cores located within the UBL interval (WELL-1: 87.5 mCD (meter Core Depth), WELL-2: 84 mCD WELL-3: 59.5 mCD WELL-4: 112 mCD). A total of 694 thin sections have been prepared from cores (average spacing of 50 cm). The thin-sections were impregnated with a blue-color epoxy resin and half-stained with an alizarine-red + potassium ferrocyanide solution for identification of carbonate minerals. Thin-sections were observed under polarized-light microscopy for the characterization of depositional and diagenetic features as well as for the quantitative analysis of pore types. Macroporosity is defined as the pore space which is colored with blue-epoxy and whose size exceeds 2 pixels (i.e. diameter > 10 µm) on scanned thin-sections. The proportion of macroporosity on thin section is automatically extracted using the software ImageJ by gathering the blue pixels.

Microporosity is defined as pores non-visible on thin section pictures or whose size is lower than 2 pixels (i.e. pore diameter < 10 µm). The definition of microporosity used in the present work is therefore consistent with the definition given by Cantrell and Hagerty (1999). The proportion of microporosity was estimated as the difference between the laboratory porosity measured on plugs and the macroporosity computed from thin-section image analysis. Micrite microfabrics were characterized from scanning electron microscopy (SEM) images.

Petrophysical laboratory measurements

Laboratory helium-porosity (PHI_ plug ) measurements have been performed on approximately 1300 core-plugs at an interval of 50 cm in sub-atmospheric condition. Special core analysis (SCAL) consist of 14 MICP measurements giving information about the pore throat radius.

Mercury was injected into cleaned and dried samples and initially evacuated, at pressure stages up to 345 Mpa. Additionally, to include the mineralogical composition, density grain measurements, XRD analysis, element trace and fluorescence X analysis have been performed. The elemental trace determinations of major (Al, Ca, Mg, Na, K, Ti, Fe, Mn, P, S in percent) and minor elements (including Cu, Pb, Zn, As, U, Th, Sr, Ba in ppm) on whole rock and selected carbonate components (bioclasts, matrix and cements).

Well-log data and derived physical parameters

Conventional well logs data of the 8 studied wells have been used in the present work. They include neutron (NEUT), density (RHOB), gamma ray (GR), photo-electric effect (PE) and Pand S-sonic logs. Water saturation (S w ), total porosity (PHI_ welllog ) and mineralogical composition have been estimated at TOTAL from the quantitative interpretation of well logs. P-and S-wave velocities of dry rocks have been estimated from sonic logs and by using the [START_REF] Gassmann | Über die Elastizität Poröser Medien[END_REF] fluid substitution relationship. Effective bulk moduli of fluid mixtures are calculated by using the empirical Brie et al. (2015) mixing law. Core depths have been tied to log depths by matching vertical changes in plug porosity (PHI_ plug ) corrected from the overburden effect and well-log porosity (PHI_ welllog ) curves.

Rock physic modeling (DEM)

Differential Effective Models (DEM) [START_REF] Norris | A differential scheme for the effective moduli of composites[END_REF][START_REF] Zimmerman | Compressibility of sandstones[END_REF] have been used to model velocity-porosity transforms for various mineral compositions, porosity values, pore aspect ratios and fluid contents. Such an approach helps quantifying the relative impact of mineralogy, pore volume and pore architecture on elastic moduli and velocities (Xu and Payne, 2008;[START_REF] Fournier | The equivalent pore aspect ratio as a tool for pore type prediction in carbonate reservoirs[END_REF].

Seismic data

The pre-production 3D seismic survey (Base seismic) which has been used in the present work, was acquired by TOTAL in 1993 and covers the Yadana, the Badamyar and the Sein gas fields (~511 km 2 ). It is characterized by a trace spacing of 12.5 x 6.25 m and a signal sampling of 3 ms. A downward increase in acoustic impedance (positive reflection coefficient) is represented by a positive amplitude (SEG convention). The central signal frequency within the carbonate buildup averages 45 Hz which implies a vertical resolution averaging 20m. A monitor 3D seismic survey was acquired in 2012 after 14 years of production. In the present study, four key seismic reflection horizons have been defined within the gas-bearing interval (H9, H9A, H9B, H10 and H10B) from the pre-production 3D seismic survey. Additionally the expression of a seismic reflection horizon which is located at the pre-production gas-water contact (IGWC) has been analyzed in both pre-production and monitor 3D seismic data.

Integrative workflow for seismo-diagenetic characterization of the Yadana reservoir

The following workflow has been adopted to assess the geological significance of seismic reflections within the Yadana carbonate gas reservoir:

1. Sedimentological and diagenetic study based on petrographic description of cores and thin sections, cathodoluminescence, and interpretation of carbon and oxygen isotope ratios [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF][START_REF] Teillet | Development patterns of an isolated oligo-mesophotic carbonate buildup, early Miocene, Yadana field[END_REF]; 2. Interpretation of the relationships between depositional fabrics, diagenetic products, and physical properties (porosity, P-wave velocity, acoustic impedance); 3. Identification of diagenetic units on the basis of cores and well-log data; these are defined as carbonate rock intervals characterized by a pattern of diagenetic evolution, and a range of reservoir and acoustic properties; 4. Well-to-seismic ties 5. Determination of the relationship between seismic reflections and the major depositional surfaces and diagenetic unit boundaries.

Results

Depositional lithofacies

The UBL carbonates are characterized by a relatively monotonous, 100m-thick succession of bioclastic limestones. The biota is dominated by non-geniculate coralline algae and large benthic foraminifers. On the basis of textural and biological features, five lithofacies have been identified and summarized in Table 2: Coralline algal rudstone to floatstone (LF1: Fig. 3A), large benthic foraminiferal rudstone (LF2.1: Fig. 3B) to floatstone (LF2.2: Fig. 3C), scleractinian-echinodermal floatstone (LF3.1: Fig. 3D, 3E, 3F, 3G) and echinodermal wackestone (LF3.2).

Diagenetic features

Petrographic analysis of cores and thin-sections revealed a variety of diagenetic features related to carbonate cementation, neomorphism, dissolution, brecciation, dolomitization and compaction:

(1) Carbonate cements Two distinct cements have been evidenced from petrographic analysis of thin section. The isopacheous calcite cement (C1) consists of isopacheous rim of scalenohedral, sparry, non feroan calcite crystal averaging 10 µm in thickness which may occupy intergranulars or moldic pores after dissolved aragonite bioclasts (Fig. 3B). The cement (C2) consists of blocky, sparry, non-ferroan calcite (50 to 250 µm) occluding intergranular pores (Fig. 3B) in grainstone rudstones (in gas and water zones), moldic/vuggy pores and fractures (mainly in the water zone: Fig. 3E).

(2) Neomorphic features Calcitized aragonitic bioclasts (dominantly coral fragments) display two types of neomorphic features: 1) calcitized corals composed of equant calcite crystals (20 to 250 μm) (C2') (Fig. 3D) and 2) calcitized corals composed of large (250-1000 μm), orange-brown under natural light, sparry calcite crystals (CX) (Fig. 3F). The CX brownish calcite has been encountered only within brecciated scleractinian-rich limestones (LF3 lithofacies).

(3) Dissolution features Selective dissolution of aragonitic skeletal elements (mainly sclereactinian) and subsequent infill of the moldic pores by sediment (micrite mud and fine-grained skeletal fragments), are common diagenetic features in coral-rich lithofacies (LF3) (Fig. 3G). The sediment replacing the dissolved septae is of similar nature and in physical continuity with the lime mudstone matrix in which the coral fragment are embedded, thus leading to a faint-ghost fabric (sensu Sanders, 2003).

Non-selective dissolution affecting both aragonitic and calcitic bioclasts as well as micrite matrix may lead to the formation of moldic pores and vugs overprinting stylolites (Fig. 3H) and devoid of any sediment or cement infill.

(4) Brecciation features Brecciated intervals are common within coral floatstones (LF3.1) and echinodermal wackestones (LF3.2) lithofacies. Two distinct carbonate breccia have been identified. 1) Thin beds (<20 cm) of low porosity (<20%) breccia (Type I breccia), characterized by poorly sorted, irregular-shaped elements which are commonly interlocked and displaying very low displacement (Fig. 3I: lower part).

2) Type II breccia: thick interval (up to 10 m) of highly porous, loose breccia with large (up to 10cm) angular and poorly sorted fragments which are composed of coral floatstones. In breccia type II, coral fragments are replaced by CX brownish calcite (Fig. 3F). The space between clasts may be partially filled with a finely bioclastic, matrix-supported sediment, but residual pore space may be preserved or filled by C2 sparry calcite cements.

(5) Dolomitization

In various intervals, intragranular and matrix micrite is partially replaced by euhedral dolomite micro-rhombs (<50 μm) (Fig. 3J). Dolomitization affects preferentially corallinealgal and foraminiferal lithofacies (LF1, LF2.1 and LF.2.2)

(6) Stylolites
Compaction features are relatively scarce within the Upper Burman Limestone. In a few thin sections, stylolites are overprinted by dissolution vugs (Fig. 3H).

(7) Micrite microfabrics

Four main micrite microfabrics have been identified under SEM in both micritized bioclasts and micrite matrix. In the gas zone, high porosity limestones (20-45%) exhibit dominantly loosely-packed, anhedral, subrounded calcimicrite crystals, typically 1-2 μm in diameter (M1 microfabric: Fig. 4A,4B). M1 micrites commonly display microvugs (2-10 μm in diameter) and aggregates (2-10 μm) of coalescent micrite particles (Fig. 4A,4B). In gas-bearing limestones with porosity < 20%, both micrite matrix and micritized allochems exhibit densely packed, coalescent, anhedral micrite crystals (M2 microfabric: Fig. 4C). The M2 micrite microfabric occurs typically in the matrix of coral-rich floatstones (lithofacies LF3). In the water zone, in both matrix and micritized bioclasts, micrite crystals are mainly euhedral (M3 microfabric: Fig. 4D,4E) and are coarser (typically 2-4μ) than gas-zone calcimicrites M1 and M2. Additionally, euhedral crystals may encase smaller, rounded micrite particles (Fig. 4E).

Finally, euhedral dolomicrite particles (from 0.5 to 4 µm) occur within micritized bioclasts (typically coralline algae) and matrix in both water and gas zone (Fig. 4F).

Pore type characterization

The analysis of 320 thin sections shows that macroporosity ranges from 0 to 15% and represents in average only 25% of the total porosity (PHI_ plug ) (Pore type distribution log, red curve on Fig. 5 and6) while 75% of the pore volume is microporosity. Macropore space is composed of a mixture of various pore types: (1) mesovugs (pore diameter between 10 to 50 µm) located within leached micrite matrix (M1); (2) moldic pores (pore diameter: from 50 µm to 5 cm) mainly after leached benthic foraminifers, coralline algae or corals; (3) macro-vugs (>50 µm) affecting both matrix and allochems; (4) intercrystalline pores in dolomitic limestone (pore size: 10-50 µm); and (5) intergranular pores (50-200 µm) which are very uncommon in the studied samples.

Microporosity is both located in micrite matrix and micritized allochems. MICP measurements performed on 14 samples revealed that microporosity in the UBL limestones is characterized by three classes in terms of pore throat radius distribution (Fig. 7). The first group, including 11 of the 14 samples, has a unimodal distribution with modal pore throat radii ranging from 0.6 to 1.2 µm. This group corresponds to samples with micrite microfabrics M1 (Fig. 4A,4B). The second group (only 2 samples) displays a unimodal distribution with a pore throat radius lower than 0.6 µm. These samples are coral-rich limestones with micrite microfabric M2 (Fig. 4C). Finally, one sample of dolomitic wackestone displays a bimodal distribution with modal pore throat radii of 0.8 µm and 3 µm, which are likely to be related respectively to micropores in M1 micrite and intercrystaline porosity between dolomite rhombs.

Diagenetic zonation of the Upper Burman Limestone

A diagenetic layering of the cored interval of the Upper Burman Limestone has been performed, based on the identification of diagenetic facies. Diagenetic facies have been defined from the combination of petrographic and petrophysical parameters (Table 3): 1) diagenetic features, 2) micrite microfabrics, 3) range of porosity values, 4) pore type association. The significant decrease in average porosity below the gas-water contact (Fig. 6) and the distinct diagenetic features observed in the gas-bearing and water-bearing reservoir led to differentiate the 2 zones in the facies classification. In intervals where water saturation is lower than 50%, porosity averages 28%, whereas at higher water-saturation and particularly below the gas-water contact, porosity decreases significantly particularly in the eastern part of the reservoir (average porosity: 18% in WELL-2) over a 20 meter-thick interval (Fig. 6).

Within the gas zone, four main groups of diagenetic facies have been identified: 1) highly microporous and vuggy, mud-supported carbonates (G-DF1), 2) microporous cemented grainstones-rudstones (G-DF2), 3) microporous and vuggy carbonate breccia (G-DF3), 4) floatstones with low matrix microporosity (G-DF4). This diagenetic facies G-DF1 has been subdivided into 3 sub-facies: calcitic red algal and foraminiferal floatstones (G-DF1a), calcitic coral floatstones (G-DF1b), and partially dolomitized floatstones (G-DF1c).

Within, the water zone, two diagenetic facies have been recognized in the cored interval: 1) floatstones (W-DF1a) and rudstones (W-DF1b) with low intragranular and matrix microporosity, and 2) carbonate breccia with low microporosity (W-DF2).

Physical factors controlling elastic properties at log scale

The impact of selected parameters (fluid content, mineralogic composition, porosity and pore shape) on the P-wave velocities measured from well logs in the Upper Burman Limestone, has been assessed. Figures 8A and 8B display graphs of P-wave velocity derived from sonic logs versus porosity. At a given porosity, P-wave velocity values (Vp) are significantly higher for water-saturated rocks (below the gas-water contact) compared to gas-saturated rocks. The shift in velocity averages 500 m/s for porosity values higher than 25%. Water-saturated P-wave velocity values have been calculated from velocities in the gas zone by using Biot-Gassman substitution theory (Gassman, 1951). For porosity values below 30%, the range of P-wave velocities predicted after Biot-Gassman water substitution from the gas-zone remarkably matches with that of the water zone (Fig. 8B). For porosity higher than 30%, the predicted water-saturated Vp values from the gas-bearing interval remain significantly lower than those from the water zone, at a given porosity (Fig. 8B).

-Mineralogic effect:

Petrographic observations of thin-sections, well-log signatures as well as laboratory mineralogical analyses of rock samples (trace element, XRD and fluorescence: Table 4) revealed a dominantly calcitic mineralogy with the occurrence of scattered, partially dolomitized intervals (WELL-2: Fig. 6). In such intervals dolomite content typically ranges from 5 to 15% with rare occurrences at concentrations up to 30%. In addition, XRD and fluorescence analyses (Table 4) indicate that Silica concentrations are very low (< 0.47%) suggesting a very low terrigenous content. Additionally, the very low values of gamma ray (GR), typically ranging from 10 to 20 API with rare peaks at 40 API, suggest very low clay content (Fig. 6). To quantify the impact of mineralogy on the acoustic properties of the Yadana reservoir, DEM models of spheroidal inclusions have been computed for various mineralogical compositions of the host. By keeping a constant pore aspect ratio (0.13: reference pore aspect ratio, after [START_REF] Xu | Modelling elastic properties in carbonate rocks[END_REF] and constant gas-saturation (Sw=0%), the substitution of 25% of dolomite in the host material (Fig. 8C) leads to a minor increase in Vp value (typically less than 3% at porosity>20%). This result is consistent with the Vp-porosity plot from Fig. 8D where the range of P-wave velocity values from dolomitized intervals largely overlaps that of purely calcitic limestones.

-Pore type effect: P-wave velocity versus porosity plots (Fig. 8D) exhibit a wide range of values, at a given values of porosity and for a given mineralogy and fluid. Such a scattering in velocity values is commonly interpreted as representing the pore type effect on acoustic properties (e.g. Eberli et al., 2003). To quantify such a pore type effect, DEM models of spheroidal inclusions within a calcitic host have been computed [START_REF] Fournier | The equivalent pore aspect ratio as a tool for pore type prediction in carbonate reservoirs[END_REF] and compared with well log-derived porosity and P-wave velocity (Fig. 8E and8F). Results for intervals with exclusive microporosity (and lacking macroporosity) revealed two domains of porosity and equivalent aspect ratio values which correspond to two distinct micrite microfabrics (Fig. 8E): -Domain 1 (porosity: 5-20%; equivalent aspect ratio<0.13): This domain represents less than 20% of samples and matches with the soft-pore domain [START_REF] Xu | Modelling elastic properties in carbonate rocks[END_REF]. The corresponding samples are limestones whose matrix and micritized grains are made of a densely packed micrite (M2 and M3 microfabrics).

-Domain 2 (porosity: 20-43%; equivalent aspect ratio>0.13): This domain represents more than 80% of the samples and corresponds to the highly micro-mesoporous limestones whose micritized grains and matrix are dominated by M1 microfabrics.

The range of velocity and porosity values for samples with macroporosity (Fig. 8F) largely overlaps that of purely microporous samples. Since the proportion of microporosity is always dominant (in average 75% of the whole pore volume), it is likely that the acoustic properties of macroporous limestones are largely controlled by those of the microporous host and therefore by micrite mirofabrics.

Geological control on acoustic properties and petroacoustic layering

As evidenced in section 4.5, vertical variations of acoustic impedance within the gas reservoir are mainly related to changes in porosity and pore network structure, and in lower proportions to changes in dolomite content. The depositional architecture of Yadana has been built based on vertical lithofacies changes in cores and well correlations (Fig. 2A). Well correlations are based on 4 key markers identified from cores: 1) the transition from N4 to N5-N6 biostratigraphical zones in the upper part of the cored interval; 2) a 10 meter-thick foraminiferal floatstone unit (LF2.2 lithofacies) identified on the four cored wells, with flat Spiroclypeus and Cycloclypeus, interpreted as representing a maximum deepening interval [START_REF] Teillet | Development patterns of an isolated oligo-mesophotic carbonate buildup, early Miocene, Yadana field[END_REF]; 3) a subaerial exposure surface [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF] and 4) the transition from N4a to N4 biostratigraphical zones identified in the lower part of the cored interval. The Figure 9B indicates that a significant overlap exists between the ranges of acoustic impedance for the distinct lithofacies. This implies that spatial variations of petroacoustic properties cannot be related to the depositional architecture only.

A petroacoustic layering of the Yadana reservoir has been also performed (Fig. 9A) based on the identification of diagenetic facies (Table 3) and associated acoustic properties (Fig. 9B). Contrasted ranges of porosity, P-wave velocity and acoustic impedance are evidenced for the distinct diagenetic facies. Three main groups of diagenetic facies (= Petroacoustic groups: Fig. 9B) were identified according to their petroacoustic signature (porosity, and acoustic impedance):

-

Petroacoustic group 1: Highly microporous and vuggy floatstones from the gas zone (G-DF1):

Highly microporous and vuggy floatstones (G-DF1) is the most common diagenetic facies from the gas-bearing interval (80%). They include purely calcitic limestones with M1 micrite microfabrics (G-DF1a and G-DF1b), and partly dolomitized limestones (G-DF1c). These diagenetic facies display a very similar and narrow range of porosity (mode: 30%) and low acoustic impedance (5600-6000 kg.m².s -1 ).

-Petroacoustic group 2: Microporous cemented grainstone/rudstone and breccia from the gas-zone (G-DF2 and G-DF3):

Diagenetic facies G-DF2 and G-DF3 are characterized by slightly lower porosity values compared to G-DF1 (mode: around 26%) and higher acoustic impedance (typically ranging from 6000 to 7000 kg.m².s -1 ). In such highly microporous diagenetic facies, lower porosity and higher acoustic impedance values are related to increased calcite cementation of intergranular pores (C2 blocky calcite) in grainstone-rudstone (G-DF2) and significant amount of calcitized corals (CX) in breccia (G-DF3).

-

Petroacoustic group 3: Low microporosity coral floatstones from the gas-zone (G-DF4), floatstone-rudstone and breccia from the water-zone (W-DF1, and W-DF2):

Diagenetic facies G-DF4 within the gas-bearing zone as well as W-DF1 and W-DF2 within the water zone are characterized by: 1) a wide range of porosity values, ranging from 5% to 35%, but usually lower than 20% and 2) a wide range of acoustic impedance values, typically higher than 7000 kg.m².s -1 and up to 10000 kg.m².s -1 . In the gas-bearing zone, the relatively low value of modal porosity (21%) of G-DF4 compared to other diagenetic facies appears largely related to the relatively low interparticle microporosity within densely packed matrix micrite (M2 microfabrics) in coral-rich intervals. In the water zone, low porosity and high acoustic impedance values in W-DF1 and W-DF2 diagenetic facies are related to significant development of micro-calcite overgrowths within intragranular and matrix micrite (M3 micrite microfabric).

Well-to-seismic tie

Synthetic seismograms have been computed from the well-logs in order to identify the geological origin of seismic reflectors. A good fit is obtained between reflections from the synthetic seismogram calculated for the four cored wells and the actual seismic traces extracted at well location (Fig. 10).

Within the gas-bearing interval, reflections are commonly low in amplitude, particularly in WELL-1 and WELL-4 (Fig. 10), where seismic and synthetic traces are relatively smooth.

The highest amplitude reflections located in WELL-2 and WELL-3 and below the gas-water contact (H9A, H9B, H10 and H10B) occur at the vicinity of high reflection coefficients (in absolute value) that are mostly related to boundaries between petroacoustic units. In the preproduction 3D seismic data a major seismic reflection horizon is located at the gas-water contact (IGWC= initial, pre-production gas-water contact) which is imaged on seismic profiles as a high amplitude, continuous, nearly horizontal but uneven reflection horizon (Fig. 11).

Analysis of 4D seismic data

Three-dimensional monitor seismic (Fig. 12B) has been compared with the pre-production 3D seismic survey (Base 1993, Fig. 12A). Three major results regarding changes in seismic expression during production have been revealed: 1) the flat-spot evidenced from the preproduction seismic around the IGWC is still present on the monitor seismic, at the same location, after 14 years of production (Fig. 12A,12B); 2) subsidence of the reservoir top during production ranges from 2 to 8 meters (Fig. 12A,12B); and 3) differential water rise occurred between eastern (~50 m) and western (~10 m) platform as revealed by the spatial distribution of 4D velocity anomaly (Fig. 12 E and12F). The top of the main anomaly is interpreted as the syn-production gas water contact (PGWC, Fig. 12F).

4.9 Geological origin of seismic reflections within the gas-bearing reservoir (preproduction 3D seismic)

In the gas-bearing reservoir, five main seismic reflection horizons (from top to base: H9, H9A, H9B, H10, H10A), interpretable throughout the Yadana buildup, have been identified and evaluated for their geological significance. The contribution of individual contrasts in acoustic impedance to the composite seismic amplitude has been quantified by comparing the synthetic seismograms computed from a selection of reflection coefficients with the full synthetic seismogram (Fig. 13). The match between the full synthetic seismic and the partial synthetic seismic is considered as good when the difference in amplitude is lower than 20% and when the shift in peak location is lower than /10, being the period of the wavelet (for a frequency mode around 45Hz,  averages 20 ms in the gas reservoir).

-H9 (WELL-2 and WELL-3): positive in amplitude and located just below the TOP UBL, the reflection is mostly controlled by two successive positive reflection coefficients, which represent a trend of increasing upward porosity at the transition between a coral floastone unit with low microporosity (GDF-4 diagenetic unit) and the overlying microporous foraminiferal rudstone (G-DF2) and foraminiferal/coralline algal floatstone (G-DF1) (Fig. 13A).

-H9A (WELL-2 and WELL-3): Theamplitude of reflection is dominantly controlled by reflection coefficients from base and top of a 10 meters-thick, moderate porosity (<20 %) coral-rich unit (G-DF4 diagenetic unit), interbedded within highly microporous limestones (porosity 30-35%; G-DF1 unit) (Fig. 13 B). As a consequence, H9A is not related to a single acoustic interface, but instead represents the composite contribution of a whole sub-seismic scale (i.e. below vertical seismic resolution) diagenetic unit.

-H10 (WELL-1 and WELL-4): In the western part of the buildup, H10 reflection amplitude is mainly controlled by negative reflection coefficients (Fig. 13 C), which represent a trend of decreasing upward porosity at the transition between a highly microporous foraminiferal/coralline algal floatstone (G-DF1) and the overlying, moderate porosity (<25 %) interval, dominated by cemented foraminiferal rudstones (G-DF2 diagenetic unit). H10 laterally pinches out to the east by interfering with the flat-spot reflection at the initial gas-water contact (IGWC) (Fig. 11A).

-H10B (WELL-1, WELL-2 and WELL-4): In the western part of the platform the H10B amplitude is mainly controlled by a negative reflection coefficient at the base of a moderately microporous carbonate breccia with calcitized corals (GDF3 diagenetic unit) (Fig. 13 D). Eastward (WELL-2, Fig. 10,11D) H10B significantly interferes with the IGWC flat-spot.

Discussion

Depositional vs diagenetic control on seismic reflection in carbonate gas reservoirs

Based on well-to-seismic tie and partial synthetic seismogram computation, the geological origin of the main seismic reflection horizons from the gas reservoir (from pre-production 3D seismic data) has been determined. Seismic reflections have been shown to result from (Fig. 14): 1) a vertical, transitional change in acoustic impedance, over 1-10 meter-thick intervals, at the boundary between two diagenetic units (H9, H10, H10B), or 2) the interference from two opposite-polarity reflections at top and base of a sub-seismic scale diagenetic unit (H9A).

Reflection horizon H9 has been shown to result from an upward decrease in acoustic impedance at the transition between a G-DF4 diagenetic unit and overlying G-DF2/G-DF1 intervals which is mostly related to a downward decrease in porosity (Fig. 10 and14). The significant shift in porosity (averaging 10%) between G-DF1/GDF2 and G-DF4 intervals results from both 1) early micrite cementation in marine environment (G-DF4) and 2) differential porosity enhancement or preservation as well as moldic and vuggy porosity development (G-DF1/GDF2) during late burial diagenesis [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF]. Coral floatstone intervals with low microporosity (GDF-4) have been interpreted to form during periods of high nutrient supplies associated with episodic high-energy events such storms or internal waves [START_REF] Teillet | Development patterns of an isolated oligo-mesophotic carbonate buildup, early Miocene, Yadana field[END_REF] and represent therefore time-correlatable units. The chronostratigraphic value of such a dense horizon is supported also by biostratigraphic constrains since the boundary between N4 and N5/N6 planktonic zones occurs in both WELL-2 and WELL-3 at base of the G-DF4 unit (Fig. 9,11E). As a consequence, in spite of the diagenetic control on porosity and acoustic properties, the main acoustic contrasts and associated seismic reflectors are conform to depositional time-line. This results from the importance of initial petrophysical properties (porosity and permeability), related to depositional textures and early diagenesis, on the subsequent evolution of pore network and acoustic properties [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF]. Similarly, the composite reflection H9A is also controlled by porosity contrasts at the boundary of a low-microporosity G-DF4 units which developed during the early marine diagenesis and have been enhanced during burial diagenesis by late dissolution processes [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF]. For both H9 and H9B horizons, the acoustic contrasts originating reflection occur at the boundary of coral-rich intervals and are therefore conform to lithofacies changes. However, such contrasts in acoustic impedance are created by differential early and later diagenetic evolution between coral-rich deposits and encasing foralgal facies. Early cementation of micrite mud matrix is restricted to some coralrich floatstone intervals (G-DF4) and the resulting reduction in porosity and permeability likely reduced or impeded the development of further secondary porosity (microporosity and moldic/vuggy porosity) which is widespread in foralgal facies (G-DF1a, G-DF2). In contrast, when the coral-rich intervals have not undergone early cementation (G-DF1b), their subsequent diagenetic evolution is similar to that of the encasing foralgal facies, and no significant contrast likely to generate a reflection forms at their boundaries. This clearly demonstrates that changes in lithofacies alone are not sufficient to generate reflections but they must be accompanied by differential diagenetic evolution.The H10 reflection horizon is controlled by an upward decrease in porosity at the transition between highly microporous floatstones (G-DF1) and overlying microporous, cemented foraminiferal rudstones (G-DF2).

In that case the shift in porosity is likely mostly related to differential volume of micropores between foraminiferal/coralline algal floatstones where both grains and matrix are microporous and cemented foraminiferal rudstones where intergranular space is occluded by sparry calcite C2 and microporosity is exclusively intragranular. As a consequence, the acoustic impedance contrast occurs at the boundary between two intervals which are characterized by distinct depositional textures even though pore network and acoustic properties have been significantly transformed during shallow and deeper burial [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF].

H10B reflection forms at an acoustic impedance contrast at base of a brecciated interval (G-DF3) which has been interpreted to develop during an intra-formational stage of subaerial exposure as suggested by its carbon and oxygen stable isotope signature [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF].

Since the thickness of such a meteorically-altered interval is low (<10m) compared to seismic resolution (~20m), H10B reflection horizon can be regarded as being conform to the intraformational exposure surface and therefore to be a valuable seismic stratigraphic marker.

As a consequence, the analysis of the main seismic horizons from the Yadana gas reservoir clearly show that the acoustic contrasts generating the reflections result from differential patterns of diagenetic evolution at the interface between intervals characterized by distinct lithofacies.

Origin of time-transgressive seismic reflections in carbonate gas reservoirs

The pre-production (1993) 3D seismic displays a relatively flat but uneven seismic horizon occurring at the gas-water contact (IGWC=Initial Gas-Water Contact), and cross-cutting the main seismic reflections identified in the gas reservoir (Fig. 11 and12). Surprisingly, the IGWC reflection remained unchanged in syn-production (2012) 3D seismic (Fig. 12) while the upward displacement of the gas-water contact may have reached 50ms (TWT). In addition no seismic reflection occurs at the syn-production (2012) gas-water contact position. Two possible origins can be therefore inferred from the persistence of a flat-spot at the preproduction gas-water contact: 1) the fluid-effect related to residual gas in the reservoir after production is sufficient to generate a significant and laterally continuous shift in acoustic impedance around the IGWC; 2) the flat-spot is controlled by a shift in acoustic impedance around the IGWC, which is related to differential porosity evolution between the gas and the water zone.

The comparison between the seismograms calculated from water-substituted and actual density and sonic logs (Fig. 13 B) shows that the position and the amplitude of the IGWC reflection is weakly affected by fluid saturation and therefore predicts the persistence of a time-transgressive reflection after gas production. As a consequence, the IGWC reflection is essentially related to the vertical and lateral variations of the intrinsic properties of the carbonate reservoir (porosity, mineralogy and velocity). A significant decrease in porosity has been evidenced in all of the studied wells, below the IGWC, within a 20 to 30 meter-thick interval (Fig. 13 A,B): the shift in average porosity between the uppermost 10 meters from the pre-production water zone and the lowermost 10 meters from the gas zone may reach 7 % in the western part of the reservoir and gradually decreases eastward, down to 0.2% (Fig. 14 C). The vertical variation in porosity is related with an increased abundance of moldic/vuggy pores and micropores in the gas zone and with a widespread development of euhedral calcite micro-overgrowths (micrite M3) in the water zone (Fig. 5). Such a significant and laterally continuous change in porosity and pore network structure below the IGWC has been interpreted to result from a major phase of diagenetic evolution during and/or after the hydrocarbon emplacement [START_REF] Teillet | Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field[END_REF] which is mainly related to 1) increased moldic and vuggy porosity development within the gas zone, 2) microporosity preservation or enhancement in loosely-packed, subrounded micrites within the gas zone, and 3) microporosity reduction by calcite micro-overgrowth within the water zone leading to the development of densely-packed, euhedral micrites (M3 micrite). As a consequence, the timetransgressive, IGWC reflection marks a major, reservoir-scale, diagenetic boundary.

Flat-spot reflections marking fluid contacts are extremely rare in carbonate reservoirs likely because of the relative stiffness of carbonate rocks which lowers fluid effect on velocity and because of the high vertical variability of porosity and acoustic impedance: fluid contacts generate reflection coefficients which are much smaller than those induced by porosity changes (e.g. [START_REF] Simm | Seismic Amplitudes: An Interpreter's Handbook[END_REF]. Diagenetic seismic reflectors cross-cutting stratal reflectors have been documented in various diagenetic settings. Most of the published examples concern bottom-simulating reflections which are related to diagenetic transformation of Opal-A into Opal-CT in biosiliceous deep-water deposits (e.g. [START_REF] Hammond | Anomalous seismic character -Bering Sea Shelf[END_REF][START_REF] Davies | A fossilized Opal A to Opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated palaeogeothermal gradient during the Neogene?[END_REF][START_REF] Nouzé | Geophysical characterization of bottom simulating reflectors in the Fairway Basin (off New Caledonia, Southwest Pacific), based on high resolution seismic profiles and heat flow data[END_REF]. In addition, kilometerscale geobodies, delineated by stratigraphy-cutting reflectors and characterized by faultrelated cementation within porous chalk, have been evidenced from high-resolution 3D seismic in the Upper Cretaceous from the North Sea [START_REF] Smit | Seismic geomorphology and origin of diagenetic geobodies in the Upper Cretaceous Chalk of the North Sea Basin (Danish Central Graben)[END_REF]. Stratigraphy-cutting reflections of mixed depositional and diagenetic origin have been also reported at the transition between shelf margin grainstones and slope wackestones from a Permian prograding carbonate platform from Texas [START_REF] Zeng | Seismic frequency control on carbonate seismic stratigraphy: A case study of the Kingdom Abo sequence, west Texas[END_REF]. The Yadana subsurface database documents an example of diagenetic, stratigraphy-cross-cutting reflection horizon which is controlled by differential porosity evolution within the gas and the water-zone, during and/or after gas emplacement.

Origin of seismic reflections in carbonate hydrocarbon reservoirs: comparison with other Cenozoic SE Asian case studies

The synthesis of published case studies of carbonate gas reservoirs from SE Asia (Table 5)

show that the main seismic reflection horizons occur at the boundary between decameterscale acoustic units where acoustic impedance is mainly controlled by porosity changes.

In the Miocene Elk-Antelope field (offshore Papua New-Guinea), some high amplitude reflections coincide with sedimentological facies boundaries, but other reflections may occur at porosity changes which are purely diagenetic and not facies-dependent [START_REF] Goldberg | Quantitative Seismic Interpretation for Characterizing Carbonate Diagenesisan Elk/Antelope Gas Field Study[END_REF]. In the same gas-field, the boundaries of dolomitized intervals are not characterized by major changes in acoustic impedance and are not related to major seismic reflection, thus suggesting that the mineralogical control is not of prime importance in such carbonate reservoirs.

In Miocene carbonate hydrocarbon reservoirs from Luconia Province, offshore Sarawak [START_REF] Zampetti | Architecture and growth history of a Miocene carbonate platform from 3D seismic reflection data[END_REF] and Liuhua, offshore R.P. China [START_REF] Zampetti | Well log and seismic character of Liuhua 11-1 field, South China Sea; relationship between diagenesis and seismic reflections[END_REF], the main seismic horizons mark the boundary of 10 to 100 meter-thick acoustic units whose properties are largely controlled by late dissolution and/or late cementation processes. In spite of the late diagenetic origin of the acoustic boundaries, seismic horizons have been shown to roughly follow the original depositional architecture.

In the Oligo-Miocene Malampaya carbonate gas field, offshore Palawan [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF], seismic reflection horizons are generated at the boundary between 10 to 80 meter-thick diagenetic units. Low impedance units may correspond to highly porous intervals (>20%) dominated by matrix microporosity and/or moldic/ vuggy porosity which developed during repeated stages of subaerial exposure as well as during burial. Highimpedance units are characterized by pedogenetic and dissolution features developed during repeated stages of subaerial exposure and by pervasive late burial sparry calcite cementation.

The reflectors located at top of high-impedance diagenetic units are major intra-formational unconformities associated with significant depositional hiatus and have therefore a chronostratigraphic value. In contrast reflectors located at their base have a questionable chronostratigraphic values since the late burial cementation processes have been shown to be independent of depositional facies.

As a consequence the present findings from the Yadana reservoir are consistent with the previous studies from various Oligo-Miocene carbonate reservoirs from SE Asia, in that diagenetically controlled boundaries can generate reflection events. However, the Yadana case study provided two additional results that complement the understanding of the origin of seismic reflections in carbonates: 1) thin (below vertical resolution) and dense diagenetic units may generate continuous seismic reflection horizons (H9A); 2) stratigraphy-crosscutting reflections of diagenetic origin may form at the gas-water contact as a result of differential porosity evolution in the gas and water zones during and/or after gas emplacement.

Genesis of seismic reflectivity: implications for seismic interpretation and fluid flow prediction

The few published examples of deeply buried, neritic carbonate reservoirs for which the geological significance of seismic reflectors has been assessed [START_REF] Zampetti | Architecture and growth history of a Miocene carbonate platform from 3D seismic reflection data[END_REF][START_REF] Zampetti | Well log and seismic character of Liuhua 11-1 field, South China Sea; relationship between diagenesis and seismic reflections[END_REF][START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF][START_REF] Goldberg | Quantitative Seismic Interpretation for Characterizing Carbonate Diagenesisan Elk/Antelope Gas Field Study[END_REF], indicate that their chronostratigraphic value is variable. The use of 3D seismic data for detailed reconstruction of the stratigraphic architecture of carbonate systems involves calibration between seismic traces and petrographic, sedimentological and diagenetic data from well-cores. The present study has shown that even though the main seismic reflectors are fundamentally related to diagenetic boundaries, they may nevertheless be conform to depositional surfaces, mainly in the following cases: 1) acoustic contrasts are related to changes in carbonate production and/or early diagenetic processes in response to oceanographic or climatic changes [START_REF] Eberli | The chronostratigraphic significance of seismic reflections along the Bahamas Transect[END_REF] this study) or 2) the acoustic properties of the diagenetic units are inherited from meteoric processes associated with an intra-formational subaerial exposure surface [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF]. Such diagenetic-acoustic units can be thin (sub-seismic scale), as evidenced by reflection H9A which results from reflection at boundaries of a 10 m-thick (~ 10), early-lithified coral floatstone interval. In contrast, the chronostratigraphic significance of reflections generated at a depositional texture interface or within the same lithological unit by vertical variation of cementation or late dissolution is more doubtful. Finally, some reflection horizons appear to be indisputably diachronous, like the IGWC reflector from Yadana which marks a late diagenesis boundary, associated with hydrocarbon emplacement.

Determining the geological significance of reflectors is therefore required prior to any seismostratigraphic analysis of carbonate reservoirs. Such a recommendation also concerns seismic stratigraphic approaches based on seismic facies analysis. Various studies have used a correspondence between seismic facies and depositional facies and environments (e.g. [START_REF] Paumard | Evolution of Late Oligocene -Early Miocene attached and isolated carbonate platforms in a volcanic ridge context (Maldives type), Yadana field, offshore Myanmar[END_REF]. Such an approach can only be valid if the geological significance of seismic facies has been previously calibrated to wellcores. For example, seismic facies which are characterized by continuous, parallel and subhorizontal reflections are extremely common in isolated carbonate buildups, and these have been reported from back-reef settings of rimmed platforms (Bachtle et al., 2004), sea-grass dominated euphotic open platform [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF] or oligo-mesophotic shelf with foralgal carbonate production [START_REF] Teillet | Development patterns of an isolated oligo-mesophotic carbonate buildup, early Miocene, Yadana field[END_REF]. The occurrence, continuity and amplitude of seismic reflections from isolated carbonate systems are largely determined by their zonation into seismic (> /4) to sub-seismic-scale (down to at least  /10) diagenetic units. Such a diagenetic zonation can affect carbonate sediments deposited in various environments, especially if late diagenetic processes predominate [START_REF] Zampetti | Architecture and growth history of a Miocene carbonate platform from 3D seismic reflection data[END_REF][START_REF] Zampetti | Well log and seismic character of Liuhua 11-1 field, South China Sea; relationship between diagenesis and seismic reflections[END_REF]. The use of seismic facies approaches without calibration to cores, in deeply-buried carbonate reservoirs which have been subject to polyphased diagenetic evolution, is therefore not recommended.

The detection and mapping of significant horizontal heterogeneities which can impact fluid flow during hydrocarbon production is another major implication of the analysis of seismic reflection genesis. In Miocene carbonate gas fields from Central Luconia (Offshore Sarawak), extended and laterally continuous, meter to decameter-scale lithologic (argillaceous flooding event) and diagenetic (tightly-cemented and karstified intervals below exposure surfaces) units have been shown to represent major heterogeneities of strong impact on pressure behavior and gas-water contact rise during production [START_REF] Warrlich | What matters for flow and recovery in carbonate gas reservoirs: Insights from the mature Central Luconia Province, offshore Sarawak, Malaysia[END_REF]. In the Yadana field, 4D time-lapse seismic data evidence an uneven rise of the gas-water contact during production, ranging from 10 m in the western part of the accumulation to 50 m to the east (Fig. 12 B,12C, 12 E). In the western area, the gas-water contact was initially located below a moderate porosity, brecciated G-DF3 unit, whose base is marked by the H10B reflection horizon and which is related to meteoric diagenetic processes during an intra-formational stage of subaerial exposure. Such a diagenetic unit acted as a baffle during gas production and favored a preferential water rise in the western area of the reservoir, where the IGWC was located above the G-DF3 unit (and above H10B: Fig. 15B,15C). Around WELL-3, the synproduction gas-water contact (=PGWC) is locally parallel with and below the H9A reflection horizon which is related with a thin (<10m), early lithified coral-floatstone (G-DF4 diagenetic unit) which suggesting that such a low to moderate porosity unit behaved as a baffle during production. As a consequence, the analysis of the diagenetic significance of seismic reflections may help predicting and mapping major heterogeneities which baffle or accelerate flow during production. Additionnally, higher permeabilities parallel to depositional bedding rather than across bedding may have enhanced the differential water rise but this effect alone could not explain the step-like shape of the gas-water contact (Fig. 12E,15B). Two types of horizontal baffle for flow can been therefore identified and mapped throughout the Yadana buildup: (1) the brecciated and moderately porous unit related to intra-formational subaerial exposure (G-DF3 / reflection H10B) and (2) the early lithified and low porous coral-rich intervals (G-DF4 / reflection H9A).

Conclusion

-Within the gas reservoir, acoustic properties distribution is mainly determined by the reservoir zonation into meter to decameter-thick intervals which are individually characterized by a dominant diagenetic facies (G-DF1 to G-DF4). Such diagenetic facies are defined by a typical range of porosity, a dominant pore type association and a set of diagenetic features. Low impedance units are highly microporous and vuggy foraminiferal/ coralline algal floatstones intervals (G-DF1) whose total porosity has been significantly enhanced in burial environment, during or after gas emplacement. High to moderate impedance units may correspond to 1) thin (<10m), laterally continuous intervals of coral floatstones whose matrix has been partly cemented during early marine diagenesis (G-DF4), 2) foraminiferal rudstone geobodies of variable thickness (<1m to >10m) whose intergranular space is occluded by sparry calcite cements formed in shallow burial environments (G-DF2), or 3) relatively thick (up to 20m) and laterally continuous coral breccia interval whose aragonite has been calcitized during an intra-formational stage of subaerial exposure (G-DF3).

-Below the pre-production gas-water contact, both microporosity and moldic/vuggy macroporosity significantly drop (up to 15% decrease) as a result of a differential diagenetic evolution between gas and water zones during and/or after hydrocarbon emplacement: carbonate leaching occurred in the gas zone whereas calcite micro-overgrowths developed within the micrite in the water zone.

-Within the gas zone, the acoustic architecture is predominantly driven by the diagenetic architecture. The Yadana reservoir is subdivided into meter-decameter scale petroacoustic units whose acoustic impedance is mainly related to porosity and pore type and which may amalgamate distinct depositional lithofacies.

-Seismic reflections from the gas-leg may form at the boundary between two diagenetic units or may result from the interference between reflection at base and top of such intervals. The acoustic contrasts generating the reflections result from differential patterns of diagenetic evolution at the interface between intervals characterized by distinct lithofacies. Changes in lithofacies alone, without differential diagenetic evolution, do not create sufficient acoustic contrast to induce seismic reflection.

-A stratigraphy-cross-cutting reflection forms at a diagenetic boundary which coincides with the pre-production gas-water contact. This horizon remains at the same location after gasproduction and rising gas-water contact.

-The combined analysis of water rise from time-lapse seismic data and of the diagenetic significance of seismic reflections has been proved to help predicting and mapping the major heterogeneities which matter for flow during production. By using such an approach, two types of horizontal baffle for flow have been identified throughout the Yadana buildup: (1) the brecciated and moderately porous unit related to intra-formational subaerial exposure (G-DF3 diagenetic unit / reflection H10B) and (2) the early lithified and low porous coral-rich intervals (G-DF4 diagenetic unit / reflection H9A).

-The present approach which assimilate and analyze large and diverse data sets should be applicable to other mature carbonate gas field. The integration of petrographic, sedimentological and diagenetic inputs appears to be the key for the identification and 3D characterization of the relevant heterogeneities which impact on flow during production and for improving the use of quantitative seismic approaches in reservoir studies. Comparison with other carbonates case studies [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF][START_REF] Verwer | Acoustic properties of carbonates: Effects of rock texture and implications for fluid substitution[END_REF][START_REF] Janjuhah | Quantification of microporosity and its effect on permeability and acoustic velocity in Miocene carbonates, Central Luconia, offshore Sarawak[END_REF]

Figure and table captions

LF3.2. Echinodermal wackestone

Commonly brecciated, like LF3.1

Bioclastic wackestone is dominated by echinoderm pieces including ophiuroid ossicles and echinoids, small size fragments of coralline algae. Frequently interbedded between coral floatstone LF3.1.

Figure 1 :

 1 Figure 1: Geographical and geological setting of the Yadana gas field: (A) Geographical and

Figure 2 :

 2 Figure 2: (A): Well-correlation panel showing the depositional architecture of the Upper

Figure 4 :

 4 Figure 4: Micrite microfabrics identified under SEM: (A) Highly porous calcimicrite

Figure 5 :

 5 Figure 5: WELL-1 and -3, cored intervals: lithofacies, diagenetic zonations, porosity from

Figure 6 :

 6 Figure 6: WELL-2 cored interval: lithofacies, textures from thin section analysis, porosity

Figure 7 :

 7 Figure 7: Mercury injection capillary pressure (MICP) data on selected samples. (A) Pore

Figure 8 :

 8 Figure 8: Well-logs Vp-porosity crossplots of the Upper Burman Limestone Formation. (A)

  ; (B) Fluid effect on Vp-porosity relationship: comparison between log measurements (from all of the studied wells) in the water zone (light blue dots), in the gas zone (red triangles) and the corresponding water-substituted values (dark blue triangles) calculated by using Gassmann relationship; (C) Mineralogy effect on Vp: calculation of Vp-porosity relationships by using DEM models on spheroidal inclusions (aspect ratio=0.13) set in a carbonate mineral phase with various calcite and dolomite proportion, and values measured from logs in the gas zone from WELL-3; (D) Well log-derived Vp-porosity crossplot for limestones and dolomitized limestones from the gas zone (all wells); (E) Well log-derived Vp-porosity crossplot for intervals (WELL-2 and -3) displaying microporosity only; samples are labelled as a function of micrite microfabrics and DEM curves are displayed for various aspect ratios (calcitic host); (F) Well log-derived Vp-porosity crossplot for macroporous intervals (WELL-3); samples are labelled as a function of of dominant pore types observed on thin sections.

Figure 9 :

 9 Figure 9: (A): Petroacoustic architecture of the Yadana reservoir (Upper Burman Limestone)

Figure 10 :

 10 Figure 10: Well-to-seismic tie for WELL-1, -2, -3 and -4. Seismic trace is extracted from the

Figure 11 :

 11 Figure 11: Seismic profiles across the Yadana platform (pre-production 3D seismic). (A) W-

  (E) Comparison between biostratigraphic zonation of the Yadana buildup (fromTeillet et al., in press 1) and seismic reflections.

Figure 12 :

 12 Figure 12: Time-lapse seismic analysis. (A) and (B) W-E oriented profile from the pre-

Figure 13 :

 13 Figure 13: Comparison between full synthetic seismograms (computed from the full

Figure 14 :

 14 Figure 14: Fluid saturation vs diagenetic control on the IGWC reflection: (A) Well

Figure 15 :

 15 Figure 15: Seismic reflectivity, diagenetic/petroacoustic architecture of the Yadana reservoir

Table 1 :

 1 List of rock and well-log data. Elastic properties of fluids and mineralogical components.

Table 2 :

 2 Depositional facies from the Yadana carbonate gas field (Upper Burman Limestone)

Table 3 :

 3 Diagenetic facies from the Yadana carbonate gas field (Upper Burman Limestone)

Table 4 :

 4 Fluorescence and XRD analysis on bulk rock samples of various carbonate lithofacies. Origin of seismic reflections in selected Oligo-Miocene carbonate reservoirs from SE Asia.

	Tables 5:

Table 2

 2 Spiroclypeus tidoenganensis with common occurrences of Cycloclypeus… Laminar and loose Mesophyllum, together with branching Lithothamnion are common.With an echinodermal wackestone matrix. Coraldominated intervals are frequently brecciated. Breccia clasts display low displacement, are sub-angular to subrounded in shape and exhibit common deep embayments. The space between clasts is filled with a lime mud sediment containing various proportions of small echinoderm fragments.Scleractinian floatstone consists of transported fragile branches or massive fragments of Faviids and Pocilloporids. Ophiuroids, echinoids, small pieces of non-articulated coralline algae and occasional broken Spiroclypeus and lepidocyclinids.

			Lithofacies	Skeletal components
	Coralline algal	(LF1)	LF1. Coralline algal floatstone to rudstone with a coralline algal-foraminiferal wackestone to packestone matrix.	Heterometric spheroidal-ellipsoidal rhodoliths (1-10 cm in diameter) or pieces of branching coralline algae (Lithothamnion, Mesophyllum and Sporolithon. The foraminiferal assemblage is dominated by Spiroclypeus tidoenganensis and Nephrolepidina sumatrensis, with common occurrences of Miogypsinoides, Miogypsina and Heterostegina (Vlerkina)
			LF2.1. Large benthic foraminiferal rudstone	The foraminiferal assemblage is dominated by Lepidoclyclina
	Large foraminiferal benthic dominated Dominated by Coral with common red algal fragments. Intergranular space is occluded by sparry calcite cements. (Nephrolepidina) sumatrensis, L. (N.) oneatensis, and Spiroclypeus tidoenganensis with rarer specimens of Amphistesgina, Heterostegina, Miogypsina, and Miogypsinoide. Coralline algae mainly include branching and warty dominated Lithothamnion, losse Mesophyllum and branching Sporolithon. (LF2) LF2.2. Large benthic floatstone with coralline algal wackestone / packestone matrix. Large benthic foraminifers are large (up to 2cm), thin-shelled, commonly well preserved, and typically horizontally-oriented. (LF3) LF3.1. Coral floatstone

Table 3 :

 3

		Diagenetic facies	Diagenetic features	Micrite micro-fabrics	Typical porosity range	Pore type association	Occurrence
			A: calcitic red	Micritized bioclasts; blocky			High
			algal and foraminiferal	calcite cement (C2); leached bioclasts (red algae and	M1	25-45%	microporosity, high moldic and
			floatstones	foraminifers) and matrix			vuggy porosity
		Highly
	G-DF1	microporous and moldic/vuggy, mud-supported carbonates	B: calcitic coral floatstones	Neomorphosed corals into C2' calcite; micritized bioclasts; rare blocky calcite cement (C2); leached bioclasts (corals, red algae and foraminifers)	M1	25-45%	High microporosity, high moldic and vuggy porosity	Gas zone
			C: partially dolomitized floatstones	Dolomicrite to dolomicrosparite crystals replacing micrite matrix and micritized bioclasts; leached bioclasts (red algae and foraminifers) and matrix	M1 / M4	25-45%	High microporosity, high moldic and vuggy porosity
							High
	G-DF2	Microporous cemented grainstones-rudstones	Isopacheous calcite cement (C1) and blocky calcite cement (C2).	M1	18-30%	microporosity, intergranular moldic porosity; rarer	Gas zone
							porosity
							High
				Brecciation features (type II);			microporosity,
				neomorphosed corals into CX			inter-breccia
	G-DF3	Microporous and vuggy carbonate breccia	calcite; blocky calcite cement leached bioclasts (corals, red (C2); micritized bioclasts;	M1	25-40%	clast porosity; vuggy porosity; moldic and	Gas zone
				algae and foraminifers) and			rarer
				matrix			intergranular
							porosity
				Brecciation features (type I);
	G-DF4	Coral floatstones with low matrix microporosity	leached corals and micrite infills (corals, red algae and within molds (faint-ghost texture); leached bioclasts	M2	10-20%	Low porosity microporosity; moldic to vuggy	Gas zone
				foraminifers)
	W-DF1	Floatstones and rudstones with low intragranular and matrix microporosity	Micritized bioclasts; isopacheous calcite cement (C1) blocky calcite cement (C2); leached foraminifers) and matrix bioclasts (red algae and	M3	10-25%	Low porosity microporosity; moldic to vuggy	Water zone
	W-DF2	Carbonate breccia with low microporosity	Brecciation features (type II); neomorphosed corals into CX calcite; blocky calcite cement (C2); micritized bioclasts; matrix algae and foraminifers) and leached bioclasts (corals, red	M3	10-30%	Low microporosity; porosity moldic to vuggy inter-breccia clast porosity;	Water zone

Table 4 :

 4

	1.	FLUORESCENCE
	WELL	Depth	Lithofacies	SiO2 (%)	Al2O3 (%)	Fe2O3 (%)	CaO (%)	MgO (%)
		(m)
	WELL-2	1277.42	LF1	0.01	0.05	0.02	50.78	2.42
	WELL-2	1295.63	LF2.1	0.16	0.05	0.04	54.55	0.34
	WELL-2	1307.18	LF2.1	0.1	0.05	0.02	53.8	1.18
	WELL-2	1324.74	LF2.1	0.47	0.17	0.05	47.96	2.67
	WELL-3	1311.84	LF3	0.39	0.07	0.05	54.76	0.31
	WELL-3	1311.89	LF3	0.3	0.05	0.03	54.79	0.3
	WELL-3	1323.4	LF1	0.01	0.01	0.01	49.57	2.72
	WELL-3	1350.05	LF3	0.1	0.05	0.01	52.48	2.32
	WELL-3	1350.1	LF3	0.13	0.05	0.01	48.94	3.22
	2.	XRD POWDER
	WELL	Depth	Lithofacies	Calcite	Dolo.	Clay
		(m)
	WELL-1	1340.41	LF1 (red algae)	100	0	0
	WELL-2	1277.4	LF1	88	10	0
	WELL-2	1295.6	LF1	95	5	0
	WELL-1	1307.2	LF1	96	4	0
	WELL-1	1270.7	LF2.1 (foram)	100	0	0
	WELL-1	1300.2	LF2.1	69	31	0
	WELL-1	1319.5	LF2.1	100	0	0
	WELL-2	1328.6	LF2.1	97.6	2.4	0
	WELL-1	1324.7	LF2.1	96.5	4.5	0
	WELL-3	1278.6	LF2.2 (foram)	93.5	3	0
	WELL-3	1323.4	LF2.2	92.1	7.9	0
	WELL-1	1328.4	LF2.2	89.4	10.6	0
	WELL-1	1294.7	LF3 (coral)	100	0	0
	WELL-3	1286.4	LF3	100	0	0
	WELL-3	1311.9	LF3	100	0	0

Table 5 : Reference Origin of the main seismic reflectors Acoustic unit thickness Modal wavelet frequency Controls on acoustic properties Chronostratigraphic value

 5

	Goldberg et al., 2010	Reflection at acoustic impedance unit boundary	20-100 m ~20 Hz	Porosity changes controlled by diagenesis; porosity changes controlled by depositional facies;	Fair (Reflectors dominantly controlled by de positional facies) to doubtful (purely diagenetically-controlled)
	Zampetti et al., 2005	Reflection at acoustic impedance unit boundary	10-20 m ~120 Hz	Porosity changes controlled by late dissolution	Fair (roughly follow depositional architecture
	Zampetti et al., 2004	Reflection at acoustic impedance unit boundary	50-100 m ~30 Hz	Porosity changes controlled by late calcite cements	Fair (roughly conform to flooding surfaces)
				Porosity changes
	Fournier and Borgomano, 2007	Reflection at acoustic impedance unit boundary	15-80 m ~30 Hz	related to 1) pedogenesis and dissolution during repeated subaerial exposures, 2) late dissolution and calcite	Good (major exposure surfaces) to doubtful (base of cemented geobodies)
				cementation.
				Porosity changes
				related to 1) early
	this paper	1) Composite reflection from base and top of acoustic impedance units; 2) Reflection at acoustic impedance unit boundary	10-20 m ~45 Hz	lithification in coral floatstone; 2) differential dissolution and calcite cementation controlled by depositional dissolution and micrite texture; 3) differential	Good (early lithified coral floatstone units); doubtful (texture-controlled porosity change); time-transgressive (diagenetic boundary controlled by fluid-contact)
				cementation between
				gas and water zone.

Gamma ray(GR) X X X X Density (RHOB) X X X X Neutron (NEUT) X X X X Sonic_P (Vp) X X X X Sonis_S (Vs) X X X X Porosity (PHI_well-log) X X X X Water sat. (SW) X X X X KEPAR X X X X Seismic trace X X X X Synthetic trace X X

Acknowledgments

The present study is part of a PhD work (PhD student: Thomas Teillet) funded by TOTAL R&D CARBONATES, Pau, France. TOTAL and partners (CHEVRON, MOGE) are also greatly acknowledged for the database, the technical support and the clearance for publishing the study. Alain Tonetto (Aix-Marseille University) is greatly thanked for his support in the acquisition of SEM images. Anonymous reviewers are also acknowledged for their constructive remarks.