Thomas Teillet

François Fournier

Franck Gisquet

Lucien F Montaggioni

Jean Borgomano

Quentin Villeneuve

Fei Hong

Diagenetic history and porosity evolution of an Early Miocene carbonate buildup (Upper Burman Limestone), Yadana gas field, offshore Myanmar

Keywords: carbonates, diagenesis, porosity, microporosity, micrite, Miocene, gas reservoir

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The diagenetic evolution of Cenozoic carbonate buildups, from the humid equatorial tropics of SE Asia, has been widely documented, in particular because these form the reservoirs of many hydrocarbon fields in the region (e.g. [START_REF] Fournier | Paleoenvironments and high-frequency cyclicity from Cenozoic South-East Asian shallow-water carbonates: A case study from the Oligo-Miocene buildups of Malampaya (Offshore Palawan, Philippines)[END_REF][START_REF] Sattler | Late leaching under deep burial conditions: A case study from the Miocene Zhujiang Carbonate Reservoir, South China Sea[END_REF][START_REF] Wilson | Tectonic influences on SE Asian carbonate systems and their reservoir development[END_REF][START_REF] Zampetti | Controlling factors of a Miocene carbonate platform: Implications for platform architecture and offplatform reservoirs (Luconia province, Malaysia)[END_REF]. The diagenetic history of such shallow-water carbonate systems is commonly driven strongly by the occurrence of intraformational subaerial exposure events that have severely modified the mineralogical patterns as well as the volume and structure of the pore networks (e.g. [START_REF] Sun | Paleoclimatic Controls on Sedimentation, Diagenesis, and Reservoir Quality-Lessons from Miocene Carbonates[END_REF][START_REF] Saller | Depositional and diagenetic history of the Kerendan carbonate platform, Oligocene, Central Kalimantan, Indonesia[END_REF][START_REF] Sattler | Late leaching under deep burial conditions: A case study from the Miocene Zhujiang Carbonate Reservoir, South China Sea[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] Madden | Diagenesis of a SE Asian Cenozoic carbonate platform margin and its adjacent basinal deposits[END_REF] . In such a context, best reservoirs commonly developed beneath subaerial unconformities in highstand carbonate buildups that have been subject to intense meteoric water leaching and karstification [START_REF] Sun | Paleoclimatic Controls on Sedimentation, Diagenesis, and Reservoir Quality-Lessons from Miocene Carbonates[END_REF][START_REF] Moldovanyi | Regional exposure events and platform evolution of Zhujiang Formation carbonates, Pearl River mouth basin: evidence from primary and diagenetic seismic facies[END_REF][START_REF] Vahrenkamp | Growth architecture, faulting, and karstification of a middle Miocene carbonate platform, Luconia Province, offshore Sarawak, Malaysia[END_REF].

Additionally, the later diagenetic evolution and the reservoir architecture of such shallowwater carbonates have been shown to be largely controlled by porosity and permeability distribution inherited from early stages of subaerial exposures [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF]. In a few case studies such as the Miocene carbonates from the Liuhua oil field, offshore R.P. China [START_REF] Sattler | Late leaching under deep burial conditions: A case study from the Miocene Zhujiang Carbonate Reservoir, South China Sea[END_REF] and Miocene carbonate buildups from the Luconia Province [START_REF] Zampetti | Controlling factors of a Miocene carbonate platform: Implications for platform architecture and offplatform reservoirs (Luconia province, Malaysia)[END_REF], burial dissolution of carbonates has been proposed as a major process of porosity enhancement.

The Lower Miocene Yadana carbonate buildup (Upper Burman Limestone, offshore Myanmar) is known to be highly porous (av. porosity: 28%) and to be the main reservoir of a significant gas accumulation [START_REF] Racey | Petroleum geology of the Moattama Region, Myanmar[END_REF]. Based on the integration of detailed petrographic analyses of cores, thin-sections and SEM images, stable isotope (C and O) measurements, well-logs and seismic interpretations, the present paper aims at: (1) reconstructing the diagenetic history and the porosity evolution of the Lower Miocene Yadana carbonate buildup, (2) identifying the main diagenetic processes that has controlled the development of a highly porous reservoir in carbonates with little meteoric diagenetic overprint, (3) constraining the timing and processes of evolution and preservation of micrite microfabrics and associated microporosity and (4) assessing the impact of gas emplacement on the late diagenetic evolution of the reservoir and on the porosity distribution within the carbonate buildup.

Geological setting

The structural framework and the marine and landmass geomorphology of the Andaman Sea area has been strongly controlled by the complex tectonic evolution of SE Asia during the Cenozoic [START_REF] Lee | Cenozoic plate reconstruction of Southeast Asia[END_REF][START_REF] Hall | Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: Computer-based reconstructions, model and animations[END_REF]. During the Paleogene, the northeastward oblique subduction (Sunda subduction zone) of the Indian Plate beneath the Eurasian Plate (Fig. 1A) resulted in the opening of the Andaman Sea as a back-arc basin [START_REF] Chakraborty | Cenozoic geodynamic evolution of the Andaman-Sumatra subduction margin: Current understanding[END_REF][START_REF] Curray | Tectonics and history of the Andaman Sea region[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 formation 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 | Late Cretaceous-Early Palaeogene tectonic development of SE Asia[END_REF]. During the Oligocene and early Miocene, the Yadana carbonate platform developed on the top of a volcanic ridge east of the Sunda subduction zone and between the M5 Basin and the Moattama Basin [START_REF] Morley | Discussion of tectonic models for Cenozoic strike-slip fault-affected continental margins of mainland SE Asia[END_REF][START_REF] Racey | Petroleum geology of the Moattama Region, Myanmar[END_REF] (Fig. 1C).

The Moattama Basin contains Eocene to modern sediments. The lowermost deposits consist of Upper Eocene volcano-clastics sediments (Fig. 1E). The overlying Oligo-Miocene shallow-water carbonates may 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]. These are: (1) the Lower Burman Limestone Formation, late Oligocene in age, that is composed of two distinct carbonate platforms separated by a central incised valley filled by the Sein clastic formation and (2) the Upper Burman Limestone Formation, early Miocene in age, that is made up of one single carbonate platform, the top of which corresponds to the gas reservoir of the Yadana Field (Fig. 1D). A long-duration depositional hiatus (~10 Myr) is recorded at top of the Upper Burman Limestones which are sealed by Upper Miocene (N17, 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. 1D), 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]. From the Pliocene to Pleistocene, the Moattama Basin became a major depocentre, with up to 10 km-thick terrigenous deposits supplied from the Irrawaddy deltaic system.

Datasets and methods

The present work is based on the analysis of a subsurface database including well logs and cores. The Upper Burman Limestone has been penetrated by 20 wells. Four wells (WELL-1, WELL-2, WELL-3 and WELL-4) have been selected for the present study (location in Fig. 1B). A total of 343 m of cored sections are available through the reservoir (WELL-1: 87.5 mCD (meter Core Depth), WELL-2: 84 mCD WELL-3: 59.5 mCD WELL-4: 112 mCD).

These were described in terms of lithology, depositional features (texture, biological associations) and diagenetic features. A total of 694 thin sections have been prepared from cores (average spacing of 50 cm). The thin-sections were impregnated with a blue-dye epoxy resin and half-stained with alizarine-red and potassium ferrocyanide solution [START_REF] Dickson | Carbonate identification and genesis as revealed by staining[END_REF] for the 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 [START_REF] Cantrell | Microporosity in Arab Formation carbonates, Saudi Arabia[END_REF]. The proportion of microporosity was semi-quantitatively estimated as the difference between the laboratory porosity measured on plugs and the macroporosity computed from thin-section image analysis. Although based on two-dimensional images, image analysis has been shown to provide accurate quantifications of macroporosity in carbonate reservoirs [START_REF] Haines | The impact of carbonate texture on the quantification of total porosity by image analysis[END_REF]. Petrographic analysis of thinsections was supported by observations of selected samples of cements under cathodoluminescence using a 8200 MKII Technosyn (20 kV and 600 mA; Technosyn, Cambridge, UK) coupled to an Olympus microscope (Olympus Optical Corporation, Tokyo, Japan) and a digital MRc5 camera (Zeiss, Gottingen, Germany). Micrite microfabrics were characterized from environmental scanning electron microscopy (SEM) images performed at Aix-Marseille University by using a Philips XL30 ESEM ®. Laboratory porosity measurements have been performed on approximately 1000 core-plugs. Vertical changes of porosity in cored and non-cored intervals have been also investigated by means of well-log data: a porosity log has been computed from density and neutron logs and corrected from gas effect. Additionally, up to 1000 carbon and oxygen isotope ratios were measured at Erlangen University on bulk samples and selected carbonate components. Carbonate powders were prepared using a manual micro-drill and reacted with 100% phosphoric acid at 70°C using a Gasbench II connected to a ThermoFisher Delta V Plus mass spectrometer. All values are reported in per mil relative to V-PDB. Reproducibility and accuracy were monitored by replicate analysis of laboratory standards calibrated by assigning δ 13 C values of +1.95‰ to NBS19 and -47.3‰ to IAEA-CO9 and δ 18 O values of-2.20‰ to NBS19 and -23.2‰ to NBS18. Reproducibility for δ 13 C and δ 18 O was ±0.06‰. Elemental 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) were carried out at Bureau Veritas Commodities Canada Ltd., Vancouver, Canada on total of 17 rock samples, by using inductively coupled plasma spectrometry (ICP-ES/MS), with a PerkinElmer ICP-MS ELAN® 9000 (PerkinElemer, Inc., Waltham, Massachusetts, USA) which was calibrated using Bureau Veritas interlaboratory standards DS10 and Oreas 45Ea. Samples (~1 gram) were pulverized to a fine powder (~3.75 phi) and were digested by using the AQ250 Ultra Trace Geochemical aqua regia (HNO3-HCl) package for two hours at 95°C. Detection limits range from 1 to 5 ppm for minor elements and averages 0.01% for major elements.

Results

Depositional lithofacies of the Upper Burman Limestone

The cored interval from the Upper Burman Limestones is characterized by a relatively monotonous, 100 m-thick succession of bioclastic limestones (Fig. 2). Skeletal grains are dominated by non-geniculate coralline algae and large benthic foraminifers (Fig. 3A,B,C).

Corals may be common in a few intervals (Fig. 3D). Non-geniculate coralline red algae are dominated by encrusting to branching forms, commonly forming nodules (rhodoliths) (Fig. 3A). The dominant genera are Lithothamnion, Mesophyllum and Sporolithon. Larger benthic foraminiferal association is characterized by Lepidocyclinids, Spiroclypeus, Cycloclypeus commonly displaying flat-shaped morphologies (Fig. 3C). Scleractinian assemblage includes robust branching or massive forms. Subordinate components include echninoderms, bryozoans, green algae and rare planktic foraminifera.

On the basis of textural and biological features, lithofacies have been identified (Table 1): coralline algal and foraminiferal floatstone (LF1: Fig. 3A), large benthic foraminiferal grainstone/rudstone (LF2.1: Fig. 3B) and floatstone (LF2.2: Fig. 3C), scleractinianechinodermal floatstone (LF3.1: Fig. 3D) and echinodermal wackestone (LF3.2).

Petrography of diagenetic features

The main diagenetic processes observed in cores are dissolution, cementation and dolomitization. Compaction is relatively weak with rare presence of stylolites in cemented intervals and open fractures.

(1) Carbonate cements Two distinct carbonate cements have been evidenced from petrographic analysis of thin sections: Isopacheous calcite cement (C1) and blocky calcite cement (C2).

Isopacheous calcite cement (C1) consists of isopacheous rims of scalenohedral, sparry, nonferroan (stained pink with Alizarin Red S and potassium ferricyanide) calcite crystals, dull brown under cathodoluminescence, averaging 10 µm in thickness (Fig. 4A,B,C). C1 cements may rim large intergranular pores in rudstones or moldic pores after dissolved aragonite bioclasts.

Blocky calcite cements (C2) consist of sparry, nonferroan calcite (50 to 250 µm) occluding intragranular pores in grainstone and rudstones (in gas and water zones), moldic/vuggy pores and fractures (mainly within the water zone) (Fig. 4A,C). Calcite cements C2 have been observed overlying calcite cement rims C1 (Fig. 4 A,B,C). These cements are zoned and composed of (Fig. 4B): (1) a first fringe of non-luminescent to dull calcite lining pore wall,

(2) a fine and concentric dull to bright orange bands, and (3) a large internal zone occluding the pore and filled by a non-luminescent calcite.

Large syntaxial sparry calcite cements (up to few hundreds microns) may also develop around echinoderm pieces in grainstone facies (Fig. 4C).

(2) Neomorphic features Calcitized aragonitic bioclasts (dominantly coral fragments) display two types of neomorphic features:

1) Calcitized corals dominantly composed of equant calcite crystals (20 to 250 µm) (C2') (Fig. 4D).

2) Calcitized corals composed of large (250-1000 µm), orange-brown under natural light, sparry calcite crystals (CX) (Fig. 4E,F). CX calcites are non-luminescent and have been encountered only within brecciated scleractinian-rich limestones (LF3 lithofacies).

(3) Micrite microfabrics Four main micrite microfabrics have been identified under SEM in both micritized bioclasts and micrite matrix (Fig. 5).

In the gas zone, high porosity limestones (20-45%) exhibit dominantly loosely-packed, anhedral, subrounded calcimicrites (M1 microfabric: Fig. 5A,B). Subhedral micrite particles may be present in smaller proportions within the M1 micrite (Fig. 5A). Such micrite may contain interparticle micropores that are larger than the average size of the micrite particles (around 1µm), with pore diameters ranging typically from 2 to 10 µm (=micro-vugs: Fig. 5A).

Micrite M1 commonly display aggregates (5-10 µm) of coalescent micrite particles (Fig. 5B).

In a few intervals, M1 calcimicrites may coexist in samples with euhedral dolomicrite crystals (M4 microfabric: Fig. 5 B). In gas-bearing limestones with porosity < 20%, both micrite matrix and micritized allochems exhibit densely packed, coalescent, anhedral micrite crystals (M2 microfabric: Fig. 5C). The M2 micrite microfabric occurs typically in the matrix of coral-rich floatstones (lithofacies LF3)

In the water zone, carbonates display low to moderate microporosity. In both matrix and micritized bioclasts, micrite crystals are mainly euhedral (M3 microfabric: Fig. 5 D,E) and are coarser (typically 2-4µ) than gas-zone calcimicrites M1 and M2. Additionally, euhedral crystals may encase smaller, rounded micrite particles (Fig. 5E).

Finally, euhedral dolomicrite particles (from 0.5 to 4 µm) occur within micritized bioclasts (typically coralline algae) and matrix (Fig. 5 F) from the water and gas legs.

(4) Dissolution features

Selective dissolution of aragonitic skeletal elements (mainly sclereactinians) 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. 6 A,B). 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 [START_REF] Sanders | Syndepositional dissolution of calcium carbonate in neritic carbonate environments: Geological recognition, processes, potential significance[END_REF] Non-selective dissolution features affecting both aragonitic and calcitic bioclasts as well as micrite matrix may lead to the formation of moldic pores and vugs (Fig. 7 A), which are devoid of any sediment or cement infill. Such dissolution features contribute significantly to the porosity of the reservoir (typically 5-20% of the rock volume) within the gas zone but are rarer or absent within the water zone.

(5) Brecciation features Brecciated intervals are common within coral floatstones (LF3.1) and echinodermal wackestones (LF3.2) lithofacies. Two distinct carbonate breccias have been identified: 1) Type I breccia: thin beds (<20 cm) of low porosity (<20%) breccia, characterized by poorly sorted, irregular-shaped elements which are commonly interlocked and display very low displacement (Fig. 6C,7C). Micrite within the fragments is tight, (M2 microfabrics) and the space between fragments is filled with finely bioclastic wackestone material. The outlines of the clasts commonly display deep embayments (Fig. 6C).

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. 4F). 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.

(6) Dolomitization

In various intervals, intragranular and matrix micrite is partially replaced by euhedral dolomite micro-rhombs (<50 µm) (Fig. 7 B). As revealed by SEM observations (Fig. 5 F), dolomicrite rhombs (M4 microfabric) are associated with M1 calcimicrites. Such dolomitized intervals do not clearly follow the stratigraphic architecture of the platform: they are common and scattered in some wells (e.g. WELL-2) and almost lacking laterally, in time-equivalent intervals (e.g. in WELL-1).

(7) Stylolites

Compaction features are relatively scarce within the Upper Burman Limestone. In a few thinsections, stylolites are overprinted by dissolution vugs (Fig. 7 E).

Definition of diagenetic facies

Diagenetic facies have been defined from the combination of selected petrographic, and petrophysical parameters (Table 2): 1) diagenetic features, 2) micrite microfabrics, 3) range of porosity values, and 4) pore type association. The significant decrease in average porosity below the gas-water contact (Fig. 8) and the distinct diagenetic features between the gasbearing and the water-bearing reservoir led to differentiate the 2 zones in the facies classification. In the gas zone, in intervals where water saturation is lower than 50, porosity averages 28% (Fig. 8A,B), whereas at higher water-saturation and particularly below the gaswater contact, porosity decreases significantly (average porosity: 18%) over a 30 meter-thick interval (Fig. 8B) 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).

Carbon and oxygen stable isotope signatures of selected carbonate components

Measurements of carbon and oxygen isotope ratios have been performed from selected carbonate components including red algae, benthic foraminifers, calcitized corals, cements, dolomite crystals and micrite matrix (Fig. 9).

Calcitized corals C2' (δ 18 O= -4 to -0‰ V-PDB; δ 13 C= -0.5 to -0.3‰ V-PDB), red algae (δ 18 O= -0.6 to -2.2‰ V-PDB; δ 13 C= -0.4 to +0.7‰ V-PDB) and micritized large benthic foraminifera (δ 18 O= -0.7 to -2.9‰ V-PDB; δ 13 C= -0.5 to +0.1‰ V-PDB) display a relatively narrow range of carbon and oxygen signatures that are consistent with Miocene south-east Asian, shallow-water, marine calcite (Fig. 9A) [START_REF] Ali | Carbonate cement stratigraphy and timing of diagenesis in a Miocene mixed carbonate-clastic sequence, offshore Sabah, Malaysia-constraints from cathodoluminescence, geochemistry and isotope studies[END_REF][START_REF] Madden | Diagenesis of a SE Asian Cenozoic carbonate platform margin and its adjacent basinal deposits[END_REF].

Similar signatures are obtained for microporous micrites M1 (δ 18 O= -2.2 to -0.1‰ V-PDB; δ 13 C= -1.0 to +1.0‰ V-PDB) and densely packed micrites (δ 18 O= -2.3 to -0.7‰ V-PDB; δ 13 C= -0.6 to +0.2‰ V-PDB), whereas euhedral micrites M3 display slightly more negative δ 18 O values (δ 18 O= -2.9 to -2.7‰ V-PDB; δ 13 C= -0.6 to -0.4‰ V-PDB) (Fig. 9B).

Isopacheous cement (C1) displays a very narrow range of δ 13 C (from 0.0 to -0.7‰ V-PDB) and δ 18 O values (from -1.4 to -1.7‰ V-PDB) (Fig. 9C). In contrast, blocky calcite cement (C2) exhibits low variations of δ 13 C (from -0.7 to +0.1‰ V-PDB) for a wide range of δ 18 O values (from -5.2 to -1.9‰ V-PDB) (Fig. 9C). Calcitized corals CX are significantly depleted in both 18 O and 13 C compared to other carbonate components (δ 18 O: -5.2 to -6.8‰ V-PDB; δ 13 C: -2.7 to -6.2 ‰ V-PDB) (Fig. 8C). Finally, dolomite rhombs exhibit highly positive δ 18 O (from +2.6 to +3.1‰ V-PDB) and δ 13 C (from +0.8 to +1.0‰ V-PDB) values (Fig. 9C).

C and O isotopic signature of bulk samples

Measurements of carbon and oxygen isotope ratios have been conducted on bulk rock samples at a sampling increment averaging 0.50m on cores from wells WELL-1, WELL-2, WELL-3 and WELL-4 (Fig. 9D). Bulk rock δ 18 O and δ 13 C values are controlled by the δ 18 O and δ 13 C signatures of the individual carbonate phases constituting the carbonate rock. They are therefore indicative of the processes that have significantly altered the original isotopic composition of the carbonate sediments. The δ 18 O and δ 13 C domains identified for the different carbonate components (Fig. 9 A,B,C), together with petrographic analysis of cores and thin-sections, help interpreting the vertical variations of the carbon and oxygen isotope ratios of bulk carbonates in cored intervals (Fig. 8,10).

Within the gas zone, highly microporous, mud-supported limestones (coralline algal/foraminiferal G-DF1A and coral floatstones G-DF1B) and grainstones/rudstones (G-DF2), display δ 13 C values ranging from -1 to +0.5‰ and δ 18 O values ranging from -2.8 to +0‰. Both oxygen and carbon isotope signatures are consistent with those of micritized bioclasts, micrite matrix and sparry calcite cements C1 and C2. In contrast, with the water zone, floatstones and rudstones (W-DF1) are marked by more negative δ 18 O values (below -2‰ V-PDB) (Fig. 9E). Negative δ 18 O values coupled with highly negative δ 13 C (below -1 ‰ V-PDB) correspond to brecciated intervals with calcitized coral (CX) from both gas (G-DF3) and water zones (W-DF2) (Fig. 9D,E). Increasing bulk δ 18 O values toward significantly positive values (>+1‰PDB) coupled with moderately positive δ 13 C (0 to +1‰ V-PDB) characterize intervals with significant dolomite content in bulk rock (G-DF1C) (Fig. 9D).

Major and trace elements

The concentration values for a selection of major and trace elements have been measured on bioclasts (calcitized corals, large benthic foraminifers and red algae), micrite matrix and cements as well as on bulk rock samples (Table 3). Strontium concentrations in micritized bioclasts (large benthic foraminifers and red algae) range from 407 to 551 ppm. Such values are significantly depleted compared to values known in modern high-Mg calcite skeletal grains (typically higher than 1000 ppm) according to [START_REF] Veizer | Chemical diagenesis of carbonates: theory and application of trace element technique[END_REF]. Such low values are likely consistent with Sr loss during high-Mg to low-Mg calcite transformation [START_REF] Veizer | Chemical diagenesis of carbonates: theory and application of trace element technique[END_REF]. Strontium concentration in the blocky calcite cement C2 (613 ppm) is significantly lower than those expected in marine low-magnesium calcite (~1000 ppm). Corals neomorphised into calcite (CX) display the highest Sr concentrations with values of 644 and 2019 ppm. Manganese concentrations are generally low within micritized bioclasts (from 18 to 128 ppm), C2 (17 ppm) and CX (11 ppm) calcites. The iron concentrations in all of the measured samples are low (<0.04%). Magnesium concentrations are highly variable in the analyzed bioclasts (from 0.8 to 1.94%) and depleted in CX and C2 calcite (0.22 and 0.14% respectively). All of the analyzed samples have relatively high sodium concentrations ranging from 100 to 2200 ppm.

DISCUSSION

Origin of diagenetic features and diagenetic environments

The integration of selected petrographic, petrophysical and geochemical parameters led to the definition of various diagenetic facies (Table 2,Fig. 10,11) which are characterized by specific paragenesis.

Highly microporous and vuggy, mud-supported carbonates from the gaz zone (G-DF1)

Within the gas zone, mud-supported carbonates (foraminiferal, coralline algal and coral floatstones) are characterized by significant proportions of dissolution vugs which have been shown to post-date stylolites, thus suggesting a late phase of leaching in burial environments.

The significant drop in porosity (averaging 10%) below the gas-water contact, which is partly related to a decrease in moldic pore and vug abundance (Fig. 7A, 7B; Fig. 8; Fig. 11) strongly suggests that such a late stage of calcite dissolution occurred during and/or after gas emplacement.

In G-DF1 carbonates, dominant allochems (micritized coralline algae and large benthic foraminifers) have low-Mg mineralogy and exhibit highly microporous, subrounded micrite microfabric (M1) which is distinct from primary skeletal microfabrics. The carbon and oxygen isotopic signature of M1 micrites from micritized coralline algae and large benthic foraminifera are consistent with Miocene, south-east Asian, shallow-water, marine calcite values [START_REF] Ali | Carbonate cement stratigraphy and timing of diagenesis in a Miocene mixed carbonate-clastic sequence, offshore Sabah, Malaysia-constraints from cathodoluminescence, geochemistry and isotope studies[END_REF][START_REF] Madden | Diagenesis of a SE Asian Cenozoic carbonate platform margin and its adjacent basinal deposits[END_REF], thus suggesting that the calcite from M1 micrites formed from marine pore fluids at the sea-bottom or in marine, shallow burial conditions. The common occurrence of subrounded M1 micrite within the gas reservoir and its lack within the water zone may result from 1) preferential dissolution processes within the gas-bearing reservoir leading to rounded morphologies [START_REF] Lambert | Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic-Cretaceous): Keys for recognition and timing[END_REF][START_REF] Volery | Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir formation (Cenomanian to Early Turonian, Middle East)[END_REF][START_REF] Léonide | Diagenetic patterns and pore space distribution along a platform to outer-shelf transect (Urgonian limestone, Barremian-Aptian, SE France)[END_REF], or 2) preferential preservation of initially anhedral micrites within the hydrocarbon-zone [START_REF] Morad | Origin and evolution of microporosity in packstones and grainstones in a Lower Cretaceous carbonate reservoir, United Arab Emirates[END_REF]. In G-DF1 reservoirs, the micritic matrix mud displays the same micrite microfabric (M1) and the same range of carbon and oxygen isotope compositions as coralline algae and large benthic foraminifers (Fig. 9 A), thus suggesting similar diagenetic evolution. Although it has been shown that anhedral and subrounded or knobby micrites may form early in the marine or shallow-burial environment without the intervention of a dissolution process [START_REF] Morad | Origin and evolution of microporosity in packstones and grainstones in a Lower Cretaceous carbonate reservoir, United Arab Emirates[END_REF][START_REF] Lucia | Observations on the origin of micrite crystals[END_REF], the hypothesis of rounded micrite development during the late phase of leached that affected the gas zone cannot be ruled out.

Among the highly microporous and vuggy carbonates from the gas zone, coral-rich intervals (G-DF1B diagenetic sub-facies) exhibit distinct diagenetic patterns with regards to the early evolution of aragonitic components. In such intervals, coral are commonly dissolved and filled by a sediment which is closely similar to and in physical continuity with the matrix (=fossil ghost), thus forming a faint-ghost fabric (Fig. 6 A,B). Filling biomoulds with marine sediment below a subaerial exposure surface would require the existence of a continuous karstic porosity along which the sediment could infiltrate. The existence of such pathways is unlikely since in faint-ghost fabrics from Yadana, leached corals are generally sparse within the matrix, and typically non-touching. In contrast, the physical continuation of the biomould fill into the adjacent matrix or into the intragranular pore fill gives evidence of an early phase of aragonite dissolution in marine environments [START_REF] Sanders | Syndepositional dissolution of calcium carbonate in neritic carbonate environments: Geological recognition, processes, potential significance[END_REF].

Decameter-thick partially dolomitized intervals of mud-supported carbonates (foraminiferal floatstone LF2.2, echinodermal wackestone LF3.2) have been identified in Yadana wells (G-DF1C diagenetic facies). By considering the [START_REF] Zachos | Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present[END_REF] range for Miocene seawater δ 18 O (from 0 to -1‰ SMOW), and by using the equation linking the fractionation factors for dolomite with temperature [START_REF] Epstein | Revised Carbonate-Water Isotopic Temperature Scale[END_REF][START_REF] Irwin | Isotopic evidence for source of diagenetic carbonates formed during burial of organic-rich sediments[END_REF], the δ 18 O values of dolomite are consistent with a marine parent water at a temperature ranging from 12°C to 19°C. Such temperatures are significantly cooler than mean tropical sea surface temperatures (27-28.5°C in the modern Andaman Sea, after [START_REF] Raju | Some physical characteristics of Andaman Sea waters during winter[END_REF] which strongly suggests that dolomite formed from relatively deep marine water. The oxygen isotope signature of dolomite from Yadana is quite similar to that of Eocene carbonates from the Enewetak atoll (+2.1 to +2.7‰ PDB after [START_REF] Saller | Petrologic and geochemical constraints on the origin of subsurface dolomite, Enewetak Atoll: an example of dolomitization by normal seawater[END_REF] and from most of Cenozoic carbonate islands (+2 to +4‰ PDB after [START_REF] Budd | Cenozoic dolomites of carbonate islands: Their attributes and origin[END_REF][START_REF] Suzuki | Origin of atoll dolomites distinguished by geochemistry and crystal chemistry: Kita-daito-jima, northern Philippine Sea[END_REF] where dolomitization has been interpreted to result from the circulation of cool, deep sea waters.

Microporous cemented grainstones-rudstones from the gas zone (G-DF2)

In foraminiferal and coralline algal grainstones-rudstones, the isopacheous rims of equigranular calcite (C1) around the bioclasts exhibit δ 18 O and δ 13 C values (Fig. 9C) which strongly suggest that they form in shallow marine environments [START_REF] Ali | Carbonate cement stratigraphy and timing of diagenesis in a Miocene mixed carbonate-clastic sequence, offshore Sabah, Malaysia-constraints from cathodoluminescence, geochemistry and isotope studies[END_REF]. The calcite cement C2, that typically occludes entirely the intergranular porosity in grainstones and rudstones have δ 13 C values which are also consistent with a precipitation from marine water [START_REF] Ali | Carbonate cement stratigraphy and timing of diagenesis in a Miocene mixed carbonate-clastic sequence, offshore Sabah, Malaysia-constraints from cathodoluminescence, geochemistry and isotope studies[END_REF], but the significantly negative δ 18 O values (from -1.89 to -5.19‰PDB) would suggest a formation in shallow-burial conditions. By considering the [START_REF] Zachos | Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present[END_REF] range for Miocene sea-water δ18O (from 0 to -1‰SMOW), and by using the [START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF] equation, a temperature ranging from 20°C to 43°C has been estimated for C2 calcite formation, in the case of marine parent waters. In addition, the strontium concentration values in C2 (613 ppm) would be consistent with those for calcites precipitated from a shallow burial, deep marine water with a lowered Sr/Ca ratio with regards to surface seawater [START_REF] Swart | The geochemistry of carbonate diagenesis: The past, present and future[END_REF]. Additionally, the extremely low Fe (<0.01%) contents and the relatively elevated Na concentration (100 ppm) in C2 support the interpretation of a non-meteoric origin for parent fluids, even though the possibility of a contamination by Na from fluid inclusions cannot be discarded.

As for G-DF1 diagenetic facies, G-DF2 limestones are strongly affected by extensive nonselective dissolution features (vugs) cross-cutting stylolites. The lack of calcite cement within such vugs strongly suggest that dissolution postdates the formation of the early marine to marine shallow burial C2 cements.

Microporous and vuggy carbonate breccia from the gaz zone (G-DF3)

Highly porous carbonate breccia (G-DF3) form a 5 meter-thick interval with frequent brownish calcitized corals (CX). The negative signature in both δ 13 C (from -2.7 to -6‰ PDB) and δ 18 O (from -5.2 to -6.8‰ PDB) would be consistent with calcitization processes controlled by meteoric water. By considering meteoric water temperatures ranging from 20 to 30°C, a δ 18 O value of -2.5 to -6‰ V-SMOW has been calculated for the parent fluids by using the [START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF] equation. Such a value is consistent with the modern precipitation δ 18 O values recorded in the Andaman Islands (Laskar et al., 2011: +0.7 to -5.2‰

V-SMOW) and in low latitude in SE Asian area [START_REF] Bowen | Spatial distribution of δ 18 O in meteoric precipitation[END_REF] -4 to -6‰ V-SMOW). In addition, the meteoric origin of CX calcite is also supported by the elevated strontium concentration (2019 ppm) since aragonite calcitization in meteoric environments has been shown to retain significant proportions of the original strontium amount, with final concentrations in calcite commonly higher than 2000 ppm, particularly in the vadose zone [START_REF] Pingitore | Vadose and Phreatic Diagenesis: Processes, Products and their Recognition in Corals[END_REF][START_REF] Webb | Rare earth element geochemistry of scleractinian coral skeleton during meteoric diagenesis: A sequence through neomorphism of aragonite to calcite[END_REF]. Furthermore, the Sr/Ca and Mg/Ca weight ratio for CX calcite are within the range of published values for low-magnesium calcite replacing coral aragonite and also for interskeletal low-magnesium calcite in calcitized corals [START_REF] Rabier | Characterization of various stages of calcitization in Porites sp corals from uplifted reefs -Case studies from New Caledonia, Vanuatu, and Futuna (South-West Pacific)[END_REF]. Calcite CX exhibits crystal overgrowths developing into the inter-clast space. These are sealed by finely bioclastic micrite of probable marine origin as suggested by its stable isotope signature (Fig. 9C; δ 18 O: -1‰ PDB; δ 13 C: +0.2‰ PDB). This suggests that coral calcitization and CX calcite overgrowth occurred during or after brecciation but prior to marine micrite infill, and are therefore early diagenetic features. These indices strongly suggest that brecciation processes and calcitization of corals into CX occurred during an early, intraformational, stage of subaerial exposure.

Coral floatstones with low matrix microporosity from the gaz zone (G-DF4)

The low to moderate porosity values (10-25%) for G-DF4 coral floatstones is related to the low interparticle microporosity within matrix micrite (M2 microfabrics). As for G-DF1B diagenetic facies, early marine dissolution of aragonite is evidenced by the occurrence of faint-ghost textures. The carbon and isotope signatures of micrites and bulk carbonates from such dense G-DF4 intervals (Fig. 9B,D

Low microporosity carbonates (W-DF1 and W-DF2) from the water zone

As revealed by laboratory measurements and counting on thin-section, the sharp decrease in porosity below the gas water-contact is mainly related to low intragranular and matrix microporosity and in a decrease in moldic and vuggy pores (Fig. 8).

Carbonates from the water zone display the same early diagenetic features as those evidenced in the gas zone: 1) micritization and high-Mg to low-Mg calcite conversion of foraminifera and coralline algae, 2) early marine dissolution of aragonite in coral-rich intervals (faint-ghost texture), 3) brecciation and calcitization of corals into CX below subaerial exposure surface; 4) occlusion of intergranular pore by C1 and C2 calcite. SEM observations of micrites from the water zone revealed a densely-packed euhedral microfabric (M3: Fig. 5C). The occurrence of smaller (~1 µm) and rounded micrite particles which are encased within larger (~2-4µm) euhedral crystals may suggest that M3 euhedral micrite results from: 1) the transformation of rounded micrite into euhedral micrite and microspar (aggrading neomorphism), via precipitation of syntaxial calcite micro-overgrowths around micrite [START_REF] Morad | Origin and evolution of microporosity in packstones and grainstones in a Lower Cretaceous carbonate reservoir, United Arab Emirates[END_REF] or 2) recrystallization involving the dissolution of micrite and reprecipitation of euhedral calcite [START_REF] Richard | Development and compartmentalization of chalky carbonate reservoirs: the Urgonian Jura-Bas Dauphiné platform model (Génissiat, southeastern France)[END_REF][START_REF] Volery | Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir formation (Cenomanian to Early Turonian, Middle East)[END_REF][START_REF] Deville De Periere | Morphometry of micrite particles in Cretaceous microporous limestones of the Middle East: influence on reservoir properties[END_REF][START_REF] Léonide | Diagenetic patterns and pore space distribution along a platform to outer-shelf transect (Urgonian limestone, Barremian-Aptian, SE France)[END_REF]. In addition, lower δ 18 O values have been measured in micritized benthic foraminifers from the water zone (δ 18 O = -2.7 to -2.9‰) compared to the gas zone (δ 18 O =-2.0 to -2.2‰), which likely results from an increase in precipitation temperature in the water zone, assuming similar oxygen isotopic composition of the porewaters [START_REF] Dickson | Changes in carbon and oxygen isotopic composition during limestone diagenesis[END_REF]. The stable isotope composition of such micrites probably reflects that of a mixture between a micrite precursor (anhedral micrite?) and calcite micro-overgrowths, thus only minimum temperatures of euhedral micrite formation can be obtained using δ18O values on micritized grains. By considering the [START_REF] Zachos | Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present[END_REF] range for Miocene sea-water δ 18 O (from 0 to -1‰ SMOW), and by using the [START_REF] Anderson | Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems[END_REF] equation, a minimum temperature ranging from 23°C to 29°C has been estimated for calcite micro-overgrowth formation, in the case of marine parent waters. Higher temperatures (38-41°C) are obtained if precipitation is assumed to have occurred from evolved marine pore waters (δ18OV-SMOW = +2‰ after [START_REF] Egeberg | Origin and evolution of formation waters from oil fields on the Norwegian shelf[END_REF]. The latter values are closer to that of the present-day reservoir temperature (55°C).

Diagenetic history of the Upper Burman Limestone

Petrographic and stable isotopic studies allow the diagenetic history of the Upper Burman Limestone Formation to be reconstructed. The burial curve of the Yadana platform, the diagenetic model and the tectonic reconstruction of the region proposed in this paper are summarized in Fig. 12.

Step 1: Early Miocene: early marine and shallow-burial marine diagenesis A major feature of early marine diagenesis of the coral-rich limestones (LF3.1 lithofacies), occurring at the sea-bottom or near the sea-bottom is extensive aragonite dissolution and subsequent marine sediment infills that led to the development of faint-ghost fabrics. Such early dissolution features have been interpreted as resulting from undersaturation of shallowmarine waters with increased organic matter degradation at the sea-bottom (e.g. [START_REF] Reaves | Organic matter metabolizability and calcium carbonate dissolution in nearshore marine muds[END_REF][START_REF] Sanders | Syndepositional dissolution of calcium carbonate in neritic carbonate environments: Geological recognition, processes, potential significance[END_REF]. During periods of low sedimentation rates, low porosity M2 micrite microfabrics developed. Such an early lithification of mud matrix favored the early brecciation of LF3.1 and LF3.2 limestones. In coral-rich (LF3.1) and coralline algalforaminiferal limestones (LF1 and LF2 lithofacies), the conversion of high-Mg into low-Mg calcite and the micritisation of bioclasts (red algae and large benthic foraminifers) probably occurred during early marine to shallow-burial, marine diagenesis, as suggested by marine, low temperature stable isotope signatures (Fig. 9B) of M1 micrites. In grain-supported sediments (LF2.1 lithofacies), minor calcite cementation occurred (C1 sparry calcite cement) in shallow-marine environments at water temperatures ranging from 18 to 24°C, as calculated from the range of δ 18 O values of C1 calcite and Miocene seawater (0 to -1‰ SMOW, after [START_REF] Zachos | Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present[END_REF] .

Step 1': Early Miocene: intra-formational subaerial exposure

The scattered brownish calcitized corals (CX) with distinct meteoric isotope signature, are the only potential evidence of carbonate precipitated in freshwater in the whole Yadana core dataset. Their occurrence within one correlatable brecciated interval may suggest the existence of one subaerial exposure event having interrupted the development history of the Yadana carbonate platform. According to [START_REF] Miller | The phanerozoic record of global sea-level change[END_REF], the amplitude of eustatic variations during the Early Miocene may have reached 30 m thus making possible the subaerial exposure during regional sea-level drops of shallow-water carbonates.

Step 2: Middle-Late Miocene: long-duration (~10 Myr) depositional hiatus A 10 Myr-long hiatus (Burdigalian to Tortonian) has been evidenced between the top of the Upper Burman Limestone (Early Miocene) and the overlying shales forming the Pyawbwe Formation (Late Miocene) (see burial curve: Fig. 12). Contemporaneously, a significant depocenter developed to the west, in the M5 Basin resulting in the deposition of 1500 m thick Middle terrigeneous sediments during the Late Miocene. The uppermost Upper Burman carbonates as well as the limestones reworked at the base of the overlying marls do not present any petrographic or geochemical evidence of meteoric weathering. All of the bulk and micro-sampling isotope measurements performed in the uppermost Upper Burman carbonates are consistent with early marine or marine shallow burial signatures. In addition, no trend of decreasing δ 13 C, that could indicate a subaerial exposure surface at top of Yadana limestone [START_REF] Matthews | Isotope signature associated with early meteoric diagenesis[END_REF], has been evidenced in the cored intervals from the four studied wells (Fig. 8, 10). The more comprehensive interpretation for the long-duration hiatus occurring at top of Upper Burman limestones is the drowning of the Yadana platform during the Burdigalian which prevented shallow marine sedimentation to occur. Long-term, marine depositional hiatuses are common features of submerged seamounts from the Pacific where Eocene to Miocene shallow-water carbonates are exposed in deep sea-water at the top of seamounts (Takayanagi et al., 2012) .

Even though microporosity development in Yadana may have started to develop during early marine diagenesis, high microporosity in calcitic sediments affected by cool seawater has been observed in Pacific atolls [START_REF] Saller | Meteoric diagenesis, marine diagenesis, and microporosity in Pleistocene and Oligocene limestones, Enewetak Atoll, Marshall Islands[END_REF]. The marine isotopic signature of dolomite, similarly to that from the Enewetak platform [START_REF] Saller | Petrologic and geochemical constraints on the origin of subsurface dolomite, Enewetak Atoll: an example of dolomitization by normal seawater[END_REF][START_REF] Saller | Meteoric diagenesis, marine diagenesis, and microporosity in Pleistocene and Oligocene limestones, Enewetak Atoll, Marshall Islands[END_REF] indicates dolomitization of the Yadana carbonates by cool (12-19°C), marine water and therefore likely occurred presumably prior to significant burial. The occurrence of a dolomitized horizon on top of the reservoir, just below the overlying Pyawbwe Shales may suggest that dolomitization occurred during the period of non-deposition, in relatively deepwater setting. Similarly, dissolution and dolomitization in deep seawater has been described in slope sediments from the Bahamas [START_REF] Mullins | Geology Authigenic dolomite in Bahamian peri-platform slope sediment[END_REF].

Step 3: Upper Miocene -Present: Cementation, euhedral calcimicrite development (M3) and late dissolution in burial environment

During the late Miocene, the opening and the spreading of the Andaman Sea resulted in increased subsidence in the Yadana area [START_REF] Chakraborty | Cenozoic geodynamic evolution of the Andaman-Sumatra subduction margin: Current understanding[END_REF][START_REF] Curray | Tectonics and history of the Andaman Sea region[END_REF]. After a 10 My-long hiatus, the Yadana platform has become buried below approximately 1000 mthick latest Miocene to Quaternary terrigenous marine sediments. Based on carbon and oxygen isotope signatures, the blocky calcite cements C2 that occlude most of the intergranular or intraskeletal porosity in the whole studied interval are regarded as having precipitated from marine pore water at temperatures ranging from 20 to 43°C. Such temperatures suggest that sparry calcite C2 cementation occurred therefore partially or entirely during the post-hiatus burial stage (Fig. 12).

The widespread development of open vugs is interpreted to have occurred during the burial

stage since these dissolution features overprint calcite C2 cements and stylolites. In addition, the increased abundance in moldic and vuggy porosity in the gas zone strongly suggests that most of the burial dissolution occurred during and after the gas emplacement. The dominance of highly porous M1 micrites within the gas zone and its scarcity within the water zone may be related to a differential preservation of early-formed anhedral, subrounded micrites in a reservoir with high hydrocarbon saturation [START_REF] Morad | Origin and evolution of microporosity in packstones and grainstones in a Lower Cretaceous carbonate reservoir, United Arab Emirates[END_REF]. However, the hypothesis of rounded micrite development by dissolution processes [START_REF] Lambert | Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic-Cretaceous): Keys for recognition and timing[END_REF]) cannot be ruled out. Various potential sources for aggressive fluids that have been used to explain burial porosity creation in carbonates can be identified: 1) carbon dioxide and organic acids from kerogen [START_REF] Mazzullo | An overview of dissolution porosity development in the deep-burial environment, with examples from carbonate reservoirs in the Permian Basin. Permian Basin Play[END_REF][START_REF] Morad | Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implication for mass transfer in sedimentary basins[END_REF], 2) high inorganic CO2 inputs from presumed metamorphic or igneous rocks [START_REF] Beavington-Penney | Reservoir quality variation on an Eocene carbonate ramp, El Garia Formation, offshore Tunisia: Structural control of burial corrosion and dolomitisation[END_REF], 3) hydrocarbon biodegradation [START_REF] Ehrenberg | Plagioclase dissolution related to biodegradation of oil in Brent Group sandstones (Middle Jurassic) of Gullfaks Field, Northern North Sea[END_REF], 4) hydrogen sulfide oxidation [START_REF] Hill | Sulfuric acid speleogenesis of Carlsbad Cavern and its relationship to hydrocarbons, Delaware Basin, New Mexico and Texas[END_REF], 5) mixing corrosion [START_REF] Esteban | Secondary porosity development during late burial in carbonate reservoirs as a result of mixing and/or cooling of brines[END_REF], 6) thermochemical sulfate reduction [START_REF] Machel | Bacterial and thermochemical sulfate reduction in diagenetic settingsold and new insights[END_REF], and 7) thermal convection [START_REF] Bjørlykke | Modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions[END_REF].

Since late dissolution features are mostly located within the gas-bearing reservoir, water acidification by CO2 and organic acids resulting from methane biodegradation [START_REF] Behar | Correlations between carboxylic acids and hydrocarbons in several crude oils. Alteration by biodegradation[END_REF] can be considered as a potential cause of undersaturation in the Yadana reservoir. CO2 is present within the gas zone (around 9% of the total gas volume) and may have contributed to significant carbonate undersaturation in residual water within the gas zone and at the gas migration front. As suggested by [START_REF] Lambert | Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic-Cretaceous): Keys for recognition and timing[END_REF], dissolution could have occurred during gas emplacement, with organic acids and CO2 moving downward with the advancing gas-water-contact. In this case, dissolved carbonates may have been transported downward into the water-zone, together with water moving downward during gas emplacement, and precipitated as calcite micro-overgrowths (M3 micrite) or C2 calcite within connected macropores. Dissolution may also have occurred after gas emplacement, by acidification of residual water saturation [START_REF] Lambert | Burial dissolution of micrite in Middle East carbonate reservoirs (Jurassic-Cretaceous): Keys for recognition and timing[END_REF], but in that case, a flow of residual water is needed to transport dissolved calcite [START_REF] Ehrenberg | Carbonate porosity creation by mesogenetic dissolution: reality or illusion?[END_REF] and maintain significant dissolution. The lack of preserved or relict sulfates, and sulfides in the Upper Burman limestone as well as the extreme scarcity of H2S (<4ppm) in the gas composition do not support the hypothesis of hydrogene sulfide oxidation or thermochemical sulfate reduction as major processes for secondary porosity development.

Finally, the major shift of porosity around the GWC (Fig. 8B) suggests a major phase of porosity evolution during and/or after the hydrocarbon emplacement which is mainly related to 1) differential moldic and vuggy porosity development within the gas zone, 2) microporosity preservation or enhancement in M1 micrites within the gas zone, and 3) microporosity reduction by calcite micro-overgrowth leading to densely-packed, euhedral M3 microfabric.

Contrasting patterns of porosity evolution in Oligo-Miocene SE Asian carbonate hydrocarbon reservoirs

Isolated carbonate buildups are common in Cenozoic of SE Asia and are known to be significant hydrocarbon reservoirs : e.g. Liuhua platform [START_REF] Sattler | Late leaching under deep burial conditions: A case study from the Miocene Zhujiang Carbonate Reservoir, South China Sea[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], Luconia platform [START_REF] Epting | Sedimentology of Miocene Carbonate Buildups Central Luconia Offshore Sarawak[END_REF][START_REF] Vahrenkamp | Miocene carbonates of the Luconia province, offshore Sarawak: implications for regional geology and reservoir properties from Strontium-isotope stratigraphy[END_REF][START_REF] Sattler | Late leaching under deep burial conditions: A case study from the Miocene Zhujiang Carbonate Reservoir, South China Sea[END_REF], Malampaya platform [START_REF] Fournier | Paleoenvironments and high-frequency cyclicity from Cenozoic South-East Asian shallow-water carbonates: A case study from the Oligo-Miocene buildups of Malampaya (Offshore Palawan, Philippines)[END_REF][START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF]. Such reservoirs are highly porous with porosity values commonly reaching 40%, and display a high proportion of secondary macropores (molds and vugs) and microporosity. [START_REF] Park | Porosity evolution in the Batu Raja carbonates of the Sunda Basin Windows of opportunity[END_REF] linked the increase in reservoir quality in the lower Miocene Batu Raja limestones from Indonesia to the repeated subaerial exposure events. [START_REF] Sun | Paleoclimatic Controls on Sedimentation, Diagenesis, and Reservoir Quality-Lessons from Miocene Carbonates[END_REF] emphasized the role of third-order sea-level drops and subsequent subaerial exposures on the regional development of Miocene carbonate reservoirs from South-East Asia. In addition, the development of elevated secondary porosity (moldic pores and microporosity) in the Luconia Province has been related to meteoric diagenesis during prolonged phases of subaerial exposure. However, for [START_REF] Zampetti | Architecture and growth history of a Miocene carbonate platform from 3D seismic reflection data; Luconia province, offshore Sarawak, Malaysia[END_REF], in the Luconia platforms, secondary porosity has been interpreted as having dominantly developed in burial environments. In the Liuhua isolated platform, there is little evidence of intraformational subaerial exposure events and secondary porosity has been shown to be largely related to burial leaching [START_REF] Sattler | Late leaching under deep burial conditions: A case study from the Miocene Zhujiang Carbonate Reservoir, South China Sea[END_REF]. In the Tonasa Platform, central Indonesia (Eocene to Miocene), diagenetic features related to subaerial exposure are uncommon, mainly localized around faulted highs, formed during the major-mid Oligocene eustatic fall although deposition occurred mainly close to sea surface on top of the platform [START_REF] Arosi | Diagenesis and fracturing of a large-scale, syntectonic carbonate platform[END_REF]. The scarcity of exposure related-features outside of faulted highs has been interpreted as resulting from the combined effect of relatively high subsidence rates and low rates of carbonate production and accumulation in foraminiferal-dominated carbonate systems [START_REF] Boudagher-Fadel | A revision of some larger foraminifera of the Miocene of Southeast Kalimantan[END_REF][START_REF] Wilson | Global and regional influences on equatorial shallow-marine carbonates during the Cenozoic[END_REF][START_REF] Arosi | Diagenesis and fracturing of a large-scale, syntectonic carbonate platform[END_REF].

Similarly, in Yadana, only one subaerial exposure event has been identified in the studied interval. The poor development of diagenetic features related to subaerial exposition is likely to be due to the low amplitude of eustatic variations during the Early Miocene and/or to high subsidence rates and in the case of effective emergence, the vadose zone is expected to be extremely reduced in thickness.

In contrast, the Oligo-Miocene Malampaya carbonate buildup (offshore Palawan, Philippines), was affected by cyclic, repeated subaerial exposure events recorded by thick (up to 5m) pedogenetized intervals [START_REF] Fournier | Paleoenvironments and high-frequency cyclicity from Cenozoic South-East Asian shallow-water carbonates: A case study from the Oligo-Miocene buildups of Malampaya (Offshore Palawan, Philippines)[END_REF]. However, in spite of such repeated subaerial exposure events, most of the secondary porosity (molds, vugs, microporosity) has been shown to have mainly developed during burial [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF][START_REF] Warrlich | The Impact of Post-depositional Processes on Reservoir Properties: Two Case Studies of Tertiary Carbonate Buildup Gas Fields in Southeast Asia (Malampaya and E11)[END_REF].

The different Cenozoic South-East Asian carbonates have responded differentially to the development of low porosity intervals. In the Yadana platform, low porosity units within the gas zone are rare and related to early marine lithification of micrite matrix below hardgrounds. In the Miocene Zhujiang carbonates in the Liuhua field (South-China Sea), tight intervals are related both to early meteoric phreatic and late burial calcite cementation [START_REF] Sattler | Late leaching under deep burial conditions: A case study from the Miocene Zhujiang Carbonate Reservoir, South China Sea[END_REF]. In the Miocene E11 field from Central Luconia, low porosity intervals results from increased clay content and early marine and/or meteoric cementation [START_REF] Warrlich | The Impact of Post-depositional Processes on Reservoir Properties: Two Case Studies of Tertiary Carbonate Buildup Gas Fields in Southeast Asia (Malampaya and E11)[END_REF]. In the Malampaya field, low porosity units result from the late burial cementation of previously highly permeable pedogenetized intervals [START_REF] Fournier | Geological significance of seismic reflections and imaging of the reservoir architecture in the Malampaya gas field (Philippines)[END_REF]. From the foregoing, it appears clearly that in the different Cenozoic South-East Asian carbonate reservoirs, early depositional and diagenetic processes have played a key role in the development of low-porosity units.

However, the diagenetic analysis of the Yadana reservoir revealed, for the first time in Cenozoic carbonate reservoirs from SE Asia, a significant porosity reduction below a hydrocarbon-water contact which resulted from preferential calcite micro-overgrowths in water zone micrites. Such a process of late porosity reduction in carbonate reservoirs probably exists in other microporous carbonates from South East Asia, but the scarcity of cores in water zones limits its detection.

CONCLUSIONS

Petrographic and stable isotopic analyses allow the diagenetic history of the Upper Burman Limestone Formation to be reconstructed.

1) The high (28%) average porosity of the Yadana gas reservoir is dominantly related to significant development of microporosity in micritized grains and matrix (loosely-packed, subrounded micrite M1) as well as moldic and vuggy porosity.

2) Mud supported, coral-rich sediments have undergone significant early marine dissolution of aragonite grains (faint-ghost texture) and lithification of micrite matrix (densely-packed and coalescent anhedral micrite M2), thus leading to the development of dense layers.

3) One potential subaerial exposure surface correlatable through wells has been evidenced at top of a brecciated interval which is characterized by the occurrence of scattered calcitized corals with negative δ 13 C signature (CX calcite). With the exception of this thin (<10m) interval, the Upper Burman carbonates show no petrographic or geochemical evidence of meteoric overprint. 6) The major decrease in porosity (averaging 10%) below the gas-water contact results from a major phase of porosity evolution during and/or after the gas emplacement. Such a porosity shift is mainly related to 1) increased moldic and vuggy porosity development within the gas zone, 2) microporosity preservation in loosely-packed, subrounded micrites (M1) 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).

The present findings provide therefore new insights into the porosity evolution in gas reservoirs from Cenozoic, South-East Asian carbonate buildups. and bulk-rock measurements (grey dots) in the background (B) δ 18 O-δ 13 C plot for the 3 calcimicrite microfabrics identified in the Upper Burman Limestone, and bulk-rock measurements (grey dots) in the background (C) δ 18 O-δ 13 C plot for calcite cements, dolomite, as well as for carbonate breccia clasts and matrix; bulk-rock measurements (grey dots) are plotted in the background (D, E) δ 18 O-δ 13 C plots for bulk-rock measurements: samples are labeled according to the diagenetic facies within the gas zone (D) and water zone (E). Fig. 10. WELL-1, -3 and -4, cored interval: lithofacies, depositional textures, carbon and oxygen isotope ratios, porosity (laboratory measurements on plugs), water saturation, dolomite and dissolution vug occurrence and diagenetic facies. 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. Brecciated at the top units, elements are gravel to pebble-sized (typically 0.5-5 cm), angular, para-autocthonous. The inter-intraclast space is filled with a micrite containing various proportions of small echinoderm fragments.

Figure 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.

  ) suggest that M2 micrites formed and became lithified in marine environments. Brecciated limestones (type I breccia) typically occur at top of G-DF4 intervals (Fig. 7 C, 10, 11). The clasts consist of paraautochtonous fragments of coral floatstone (LF3.1) displaying the densely-packed, anhedral micrite M2. They are embedded within an echinoderm-rich wackstone matrix with M1 micrite, thus suggesting early lithification and early brecciation of the coral floatstone. This also strongly demonstrates that the densely-packed, anhedral micrite M2 results from early lithification processes on the sea-bottom. Tops of these brecciated intervals are commonly bored by lithophagous molluscs and are sharply overlain by LF1 or LF2.1 facies (Fig. 7 C) thus suggesting hardground surfaces that formed during periods of non-deposition.

4)

  Stable isotope and petrographic data from the top of the UBL suggest that the associated long-term depositional hiatus (Middle to Late Miocene: ~10Myr) was related to platform drowning and non-deposition in a deep marine setting. Partial dolomitization, which affected preferentially the top and flanks of the platform, likely occurred during such a hiatus, in deep marine environment. 5) Microporosity development and formation of loosely-packed, subrounded micrites (M1 microfabric) likely mainly occurred in marine to marine shallow burial environments.

Fig. 1 .

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

Fig. 2 .

 2 Fig. 2. Well-correlations and stratigraphic architecture of the Yadana platform, based on large

Fig. 3 .

 3 Fig.3. Microphotographs of lithofacies from the Upper Burman Limestone. (A) WELL-1

Fig. 7 .

 7 Fig.7. (A) WELL-3 1319.92 mCD: microphotograph of moldic pores and vugs within a

Fig. 8 .

 8 Fig.8. WELL-2, (A) cored interval: lithofacies, depositional textures, carbon and oxygen

Fig. 9 .

 9 Fig.9. Carbon and oxygen isotopic signatures of the Upper Burman Limestone (A) δ 18 O-δ 13 C

Fig. 11 .

 11 Fig.11. Diagenetic facies zonation of the four studied wells, vertical variations of plug

Fig. 12 .

 12 Fig.12. Paragenesis and porosity evolution of the Upper Burman Limestone and subsidence

  Fig.7 A, E

Table captions Table 1 :

 captions1 depositional lithofacies and skeletal components of the Upper Burma Limestone

Table 2 :

 2 Definition and properties of the diagenetic facies from the Upper Burman Limestone

Table 2 :

 2 Concentrations of selected major and trace elements on bulk rock samples and various carbonate componentsHeterometric 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) Large benthic foraminifers are large (up to 2cm), thin-shelled, commonly well preserved, and typically horizontally-oriented. Dominated by Spiroclypeus tidoenganensis with common occurrences of Cycloclypeus… Laminar and loose Mesophyllum, together with branching Lithothamnion are common.

	Lithofacies	Skeletal components
	LF1. Coralline algal floatstone
	to rudstone with a coralline algal-
	foraminiferal wackestone to
	packestone matrix.
	LF2.1. Large benthic foraminiferal	The foraminiferal assemblage is dominated by Lepidoclyclina (Nephrolepidina)
	rudstone with common red algal	sumatrensis, L. (N.) oneatensis, and Spiroclypeus tidoenganensis with rarer
	fragments.	specimens of Amphistesgina, Heterostegina, Miogypsina, and Miogypsinoide.
		Coralline algae mainly include branching and warty Lithothamnion,
		Mesophyllum and branching Sporolithon.
	LF2.2. Large benthic floatstone with
	coralline algal wackestone /
	packestone matrix.
	LF3.1 Coral floatstone with an
	echinodermal wackestone matrix.
	Coral dominated intervals are
	frequently brecciated at the top.
	Thin intervals of echinodermal
	wackstone with sparse corals may be
	interbedded within coral-
	echinodermal floatstones.

Table 1 :

 1 depositional lithofacies and skeletal components of the Upper Burma Limestone

Table 2 :

 2 Definition and properties of the diagenetic facies from the Upper Burman Limestone

		Sample	Mn	Fe	Mg	Sr	Ba	Na
	Well	depth (m) Nature of the sample	ppm	%	%	ppm	ppm	%
		Detection limit	5	0.01	0.01	2	1	0.01
	WELL-1	1273.50 Large benthic foram.	128	0.03	1.94		90	0.22
	WELL-1	1257.50 Coralline algae	31	0.03	1.38		31	0.13
	WELL-1	1343.60 Coralline algae	24	0.02	0.48		1225	0.09
	WELL-1	1249.00 Coralline algae	41	0.01	0.71		35	0.12
	WELL-2	1277.50 Coralline algae	25	<0.01	0.9		76	0.15
	WELL-2	1314.30 Coralline algae	18	<0.01	0.8		583	0.09
	WELL-2	1319.16 Microporous matrix	35	0.03	1.19		16	0.09
	WELL-2	1357.66 C2 calcite	17	<0.01	0.22		6	0.01
	WELL-4	1326.50 CX calcite	11	<0.01	0.1		12	0.04
	WELL-2	1362.40 CX calcite	<5	<0.01	0.14	2019	17	0.04
	WELL-3	1298.36 bulk rock	69	0.01	0.16		27	0.03
	WELL-3	1308.89 bulk rock	103	0.02	0.17		954	0.01
	WELL-3	1309.00 bulk rock	34	0.01	0.21		2171	0.09
	WELL-3	1311.00 bulk rock	100	0.04	0.15		55	0.02
	WELL-2	1289.96 bulk rock	157	0.02	0.17		2850	0.04
	WELL-4	1270.82 bulk rock	55	<0.01	0.22		10	0.13
	WELL-4	1232.50 bulk rock (TOP UBL)	356	0.66	0.82		178	0.09

Table 3 :

 3 Concentrations of selected major and trace elements on bulk rock samples and various carbonate components

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. This manuscript has also benefited greatly from the detailed and constructive remarks from Conxita Taberner and from an anonymous reviewer.

under under polarized-light microscopy (A) and cathodoluminescence (B); Three zones are distinguished from cathodoluminescence: (1) C1 rim cement, lining the pore space, nonluminescent, (2) early phase of C2 cement formed by concentric, thin (20µm) bands with dull to bright luminescence and (3) later, non-luminescent, C2 cement (diagenetic facies: G-DF2).