Maxime Virolle
email: maxime.virolle@u-psud.fr

Benjamin Brigaud

Sylvain Luby

Eric Portier

Hugues Fenies

Raphaël Bourillot

Patricia Patrier

Daniel Beaufort

Influence of sedimentation and detrital clay grain coats on chloritized sandstone reservoir qualities: Insights from comparisons between ancient tidal heterolithic sandstones and a modern estuarine system

Keywords: Detrital clay, Chloritization, Reservoirs, Estuarine sandstones

Authigenic clay coatings (mostly Fe-rich chlorite coatings) affect sandstone reservoir qualities by inhibiting quartz overgrowth during burial diagenesis. It is still unclear what initial mineralogical assemblages and initial sedimentation conditions produce chloritized sandstone reservoirs. To address this question, sedimentological, petrographical and

Introduction

Factors controlling reservoir quality and heterogeneity -porosity (phi) and permeability (k)are prerequisites for improving modelling and for forecasting geothermal potential and oil or gas reserves. Quartz cementation is one of the main processes degrading reservoir quality in clastic rocks (Bjorlykke and Egeberg, 1993;Worden and Morad, 2000). However, authigenic clay coatings, mainly chlorite, may limit quartz overgrowth diagenesis and thereby maintain reservoir quality during burial [START_REF] Heald | Influence of coatings on quartz cementation[END_REF][START_REF] Pittman | Clay coats: Occurrence and relevance to preservation of porosity in sandstones. Origin, diagenesis, and petrophysics of clay minerals in sandstones[END_REF]Ehrenberg, 1993;[START_REF] Aase | The effect of grain-coating microquartz on preservation of reservoir porosity[END_REF][START_REF] Bloch | Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability[END_REF]. Both the proportion of the quartz grain surface covered by clay minerals (i.e. clay-coat coverage) and the coating mineralogy are crucial for the preservation of porosity [START_REF] Billault | A nanopetrographic and textural study of grain-coating chlorites in sandstone reservoirs[END_REF][START_REF] Lander | Toward more accurate quartz cement models: the importance of euhedral versus noneuhedral growth rates[END_REF], Griffiths, 2018). With extensive coat coverage by chlorites, less quartz cement nucleates [START_REF] Heald | Influence of coatings on quartz cementation[END_REF][START_REF] Walderhaug | Kinetic modeling of quartz cementation and porosity loss in deeply buried sandstone reservoirs[END_REF]. Clay coatings may have developed in a wide variety of depositional environments (Worden and Morad, 2003;Dowey et al., 2012, Wooldridge et al., 2017a, 2018, Griffiths et al., 2018). Shallow-marine water depositional environments, such as estuarine environments, form numerous heterolithic sandstone reservoirs [START_REF] Wood | Predicting tidal sand reservoir architecture using data from modern and ancient depositional systems. In: Integration of outcrop and modern analogs in reservoir modeling[END_REF][START_REF] Martinius | Reservoir challenges of heterolithic tidal sandstone reservoirs in the Halten Terrace, mid-Norway[END_REF][START_REF] Musial | Subsurface and outcrop characterization of large tidally influenced point bars of the Cretaceous McMurray Formation (Alberta, Canada)[END_REF][START_REF] Massart | Improved characterisation and modelling of heterolithic tidal sandstone reservoirs[END_REF]. The reservoir quality of these estuarine reservoirs is partly controlled by grain coating development during diagenesis (Worden and Morad, 2003;[START_REF] Morad | The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy[END_REF]Dowey et al., 2012;[START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF]. Wellcrystalized chlorite or illite coatings can originate either from direct precipitation from aqueous solutions (neoformation) without clay material precursors [START_REF] Haile | Experimental nucleation and growth of smectite and chlorite coatings on clean feldspar and quartz grain surfaces[END_REF][START_REF] Mamadou | Hot fluid flows around a major fault identified by paleothermometric studies (Tim Mersoï basin, Niger)[END_REF], or from thermally driven transformations of precursors [START_REF] Aagaard | Formation of graincoating chlorite in sandstones. Laboratory synthesized vs. natural occurrences[END_REF]. Well-crystalized clay coatings begin to develop during low-temperature diagenesis (30-70 °C) from the transformation of clay precursors such as berthierine or odinite for chlorite coatings [START_REF] Wilson | Authigenic clays in sandstones; recognition and influence on reservoir properties and paleoenvironmental analysis[END_REF][START_REF] Hillier | Pore-lining chlorites in siliciclastic reservoir sandstones: Electron microprobe, SEM and XRD data, and implications for their origin[END_REF]Aagard et al., 2000;Worden and Morad, 2003;[START_REF] Morad | The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy[END_REF][START_REF] Beaufort | Chlorite and chloritization processes through mixed-layer mineral series in low-temperature geological systems: A review[END_REF]. This transformation might be enhanced by feldspar dissolution and the availability of iron in the sedimentary system [START_REF] Morad | The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy[END_REF][START_REF] Khalifa | Impact of structural setting on diagenesis of fluvial and tidal sandstones: The Bahi Formation, Upper Cretaceous, NW Sirt Basin, North Central Libya[END_REF][START_REF] Haile | Experimental nucleation and growth of smectite and chlorite coatings on clean feldspar and quartz grain surfaces[END_REF][START_REF] Daneshvar | Feldspar alteration and Fe minerals: Origin, distribution and implications for sandstone reservoir quality in estuarine sediments[END_REF].

In the initial depositional environment, clays may be incorporated within the sediments during or shortly after deposition [START_REF] Griffiths | Detrital clay coats, clay minerals, and pyrite: a modern shallow-core analogue for ancient and deeply buried estuarine sandstones[END_REF], Virolle et al., 2019). Detrital clay minerals can form coats around grains [START_REF] Wooldridge | The origin of clay-coated sand grains and sediment heterogeneity in tidal flats[END_REF], Griffiths et al., 2018;Virolle et al., 2019). They may thus contribute to forming the precursors required for the chloritization process during burial diagenesis [START_REF] Gould | Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin[END_REF][START_REF] Dowey | Sedimentary controls on modern sand grain coat formation[END_REF]. In the estuarine Permian Cape Hay Formation, densely packed clay mineral aggregates are observed beneath or close to the well-crystallized coating with more euhedral and more widely spaced chlorite crystals [START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF]. Those aggregates might be the relic of clay precursors, suggesting chlorite coating nucleation in clastic sand during burial [START_REF] Gould | Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin[END_REF][START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF]. Various processes have been described to explain the incorporation of clays within sand grains during or just after deposition. For example, through sediment mixing during bioturbation, mechanical infiltration of muddy waters, tidal pumping in coastal environments, or even through interactions with exopolymeric substances produced by diatoms acting as glue [START_REF] Herringshaw | Bioinfiltration: Irrigation-driven transport of clay particles through bioturbated sediments[END_REF][START_REF] Pittman | Clay coats: Occurrence and relevance to preservation of porosity in sandstones. Origin, diagenesis, and petrophysics of clay minerals in sandstones[END_REF][START_REF] Walker | Diagenetic Origin of Continental Red Beds[END_REF][START_REF] Walker | Diagenesis in first-cycle desert alluvium of Cenozoic age, southwestern United States and northwestern Mexico[END_REF]Worden and Morad, 2003;Wooldridge et al., 2017aWooldridge et al., , 2017b)). Such detrital coats have been described in the modern Gironde and Ravenglass estuaries in sand bar sequences or on sand flats [START_REF] Dowey | Sedimentary controls on modern sand grain coat formation[END_REF]Woolridge et al., 2017aWoolridge et al., , 2018;;[START_REF] Griffiths | Detrital clay coats, clay minerals, and pyrite: a modern shallow-core analogue for ancient and deeply buried estuarine sandstones[END_REF][START_REF] Griffiths | Estuarine clay mineral distribution: Modern analogue for ancient sandstone reservoir quality prediction[END_REF]Virolle et al. 2019). Uncertainties exist as to the influence of sedimentation on chloritized sandstone reservoir qualities. Besides, it remains unclear how to associate the occurrence and proportion of coats with a specific sedimentary facies type such as intertidal, supratidal, lower or upper sand bars, proximal or distal bay-head deltas (Wooldridge et al., 2017a;[START_REF] Griffiths | Detrital clay coats, clay minerals, and pyrite: a modern shallow-core analogue for ancient and deeply buried estuarine sandstones[END_REF]Virolle et al., 2019). A recent study of a modern analogue in the United Kingdom (Ravenglass) shows that chloriteenriched tidal bars and dunes of an inner estuary might experience a low cementation rate during burial with their well-developed detrital clay coats [START_REF] Griffiths | Estuarine clay mineral distribution: Modern analogue for ancient sandstone reservoir quality prediction[END_REF]. Another uncertainty arises over coat development in coastal environments within the stratigraphic cycle. Linking tidal facies types with the presence of clay in optimum proportions within sequence stratigraphy is fundamental to improving the prediction of clay coating and reservoir location in coastal sandstones.

This study proposes to compare a Permian estuary, and more specifically the Cape Hay Formation (Australia) buried at great depth (3600 m), with the Gironde estuary (South-West France) as a modern analogue to link sedimentary facies with clay coats and reservoir quality. The ultimate aim is to identify and precisely locate the tidal sedimentary facies in an estuarine environment and/or in sequence stratigraphy that may have the best reservoir qualities. This prediction must integrate the presence of clays as coats and clay heterogeneities in sand estuarine facies.

The Upper Cape Hay Formation (Petrel sub-basin, part of the Bonaparte Basin in Australia) is revisited to detail very precisely the sedimentary facies and the occurrence of coating. This sedimentary formation was deposited during the Permian and during a third-order transgressive period within a tide-dominated environment with tidal sand bar deposits [START_REF] Kloss | A revised depositional model for the Cape Hay Formation, Petrel Field, northern Australia. Timor Sea Pet[END_REF][START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF]. The link between the preservation of high porosity and permeability values observed in this geological formation and the development of clay coatings (mostly Fe-rich chlorite) during early diagenesis has been clearly demonstrated by Saiag et al. (2016) because coatings prevent quartz overgrowth during burial.

The Gironde estuary is a mud-rich macrotidal estuary [START_REF] Sottolichio | Observations of suspended sediment dynamics in a highly turbid macrotidal estuary, derived from continuous monitoring[END_REF]Jalon-Rojas et al., 2016). Tidal bars forming the bay-head delta observed within the Gironde estuary are large, elongate and asymmetrical (Féniès and Tastet, 1998;Virolle et al., 2019). They may be several kilometres wide, tens of kilometres long and tens of metres high (25 to 50 m; [START_REF] Dalrymple | Estuarine dunes and bars[END_REF]Berne et al., 1998;Féniès and Tastet, 1998;Dyer and Huntley, 1999;Reynaud et al., 1999;Wood, 2003;Dalrymple and Choi, 2007;[START_REF] Leuven | Tidal sand bar dimensions and shapes in estuaries[END_REF][START_REF] Aschoff | Recognition and significance of bayhead delta deposits in the rock record: A comparison of modern and ancient systems[END_REF]. Two tidal bars have been studied in detail, the Plassac and the Richard tidal bars located in the inner and outer estuary funnel respectively. The comparison is based on the study of six cores from the Gironde estuary tidal bars (for a total of 31 m) and detailed descriptions of cores (28 m) from one well in a set of six wells from the Permian Upper Cape Hay Formation (P.U.C.H.F.) within the Petrel sub-basin. The analogy will provide a better understanding of the distribution and prediction of well-preserved porosity and permeability layers within deeply buried sandstone reservoirs.

Geological settings

Petrel sub-basin

The Petrel gas field is located offshore (95 m below sea level), 260 km west-southwest of Darwin city in northern Australia, on the border between Western Australia and the Northern Territory (Fig. 1A; [START_REF] Earl | Bonaparte Basin Petroleum Systems[END_REF]. It is located in the Joseph Bonaparte Basin, in the centre of the Petrel sub-basin [START_REF] Earl | Bonaparte Basin Petroleum Systems[END_REF][START_REF] Zhixin | An oil and gas resource reassessment of the Bonaparte Basin, northwest shelf of Australia[END_REF][START_REF] Seebeck | Polygonal faulting and seal integrity in the Bonaparte Basin, Australia[END_REF]. The Bonaparte Basin covers an area of 270,000 km² and forms a province of the hydrocarbonrich north-western Australian shelf. The inferred resources are: 938 million barrels of oil, 1901 million barrels of condensate and 975 trillion cubic metres of natural gas [START_REF] Zhixin | An oil and gas resource reassessment of the Bonaparte Basin, northwest shelf of Australia[END_REF].

Most of the hydrocarbon reservoirs of the Petrel sub-basin are in the Permian formations.

Lithostratigraphically, they are composed of the Treachery Shale, Keyling Formation, Fossil Head Formation, and the Hyland Bay subgroup (Fig. 1B; [START_REF] Bhatia | Depositional framework and diagenesis of the late Permian gas reservoirs of the Bonaparte Basin[END_REF]. The Hyland Bay subgroup consists of five geological formations [START_REF] Gorter | Revised Upper Permian stratigraphy of the Bonaparte Basin[END_REF]: Pearce, Cape Hay, Dombey, Tern and Penguin (Fig. 1B). The Petrel gas field reservoir is located in the Upper Cape Hay Formation (Fig. 1B). This formation is a succession of shallow-marine, coastal to fluvial environments within a second-order regression-transgression cycle (Saiag et al., 2016). The source rocks are continental in nature and were also deposited during the Permian (Pearce Formation, Keyling Formation). The Petrel gas field is an extended anticline (30 km by 15 km) oriented NW-SE, and 60 m thick. A total of seven boreholes were drilled including 275 m of cores in the Cape Hay Formation [START_REF] Kloss | A revised depositional model for the Cape Hay Formation, Petrel Field, northern Australia. Timor Sea Pet[END_REF][START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF].

The sandstones studied belong to the upper part of the Cape Hay Formation composed of deeply buried tidal deposits (>3500 m; pressures of 350 bars and temperatures of 130 °C).

They exhibit wide ranges of porosity (2-26%) and permeability (0.001-2500 mD) [START_REF] Bhatia | Depositional framework and diagenesis of the late Permian gas reservoirs of the Bonaparte Basin[END_REF][START_REF] Kloss | A revised depositional model for the Cape Hay Formation, Petrel Field, northern Australia. Timor Sea Pet[END_REF][START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF]. Saiag et al. (2016) have demonstrated that the preservation of high porosity and permeability values are linked to clay coatings (mostly Fe-rich chlorite). The depositional environment of the Upper Cape Hay Formation, the gas reservoir target, is described as a tide-dominated estuary with tidal sand bar deposits (Fig. 1C; [START_REF] Kloss | A revised depositional model for the Cape Hay Formation, Petrel Field, northern Australia. Timor Sea Pet[END_REF][START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF].

The Gironde estuary

The Gironde estuary in south-west France is one of the largest estuaries in Europe (Fig. 2A; [START_REF] Dalrymple | Estuarine dunes and bars[END_REF]. The estuary funnel is 75 km long and covers an area of 630 km 2 (Fig. 2A). The estuarine tidal channels (Dordogne and Garonne channels) extend over 85 km inland from the estuary funnel (Fig. 2A). The estuary is divided into three main sections differentiated by their morphological and hydrodynamic characteristics (Fig. 2A; [START_REF] Allen | Etude des processus sédimentaires dans l'estuaire de la Gironde[END_REF][START_REF] Allen | Sedimentary processes and facies in the Gironde estuary: a recent model for macrotidal estuarine systems[END_REF].

(1) The estuarine channels are composed by the Garonne and Dordogne. The Garonne estuary channel extends inland from the estuary funnel as far as the upstream tidal limit near La Réole some 160 km from the estuary mouth (Fig. 2A). Estuarine heterolithic point bars can be observed along a large part of these channels.

(2) The landward part of the Gironde inner estuary funnel is occupied by the bay-head delta [START_REF] Allen | Sedimentary processes and facies in the Gironde estuary: a recent model for macrotidal estuarine systems[END_REF]Féniès and Tastet, 1998), composed of several tidal bars including the Plassac tidal bar (Fig. 2A-B). The estuary funnel is mainly composed of tidal bars and estuarine mud (Fig. 2A).

(3) The Gironde outer estuary funnel consists of a tidal inlet channel and a muddy central basin located further inland (Fig. 2A). The Richard tidal bar is located in this area (Fig. 2A andC).

The Gironde is a mud-rich macrotidal estuary with a well-developed Turbidity Maximum Zone (TMZ) that corresponds to highly turbid waters [START_REF] Allen | Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf[END_REF][START_REF] Doxaran | Dynamics of the turbidity maximum zone in a macrotidal estuary (the Gironde, France): Observations from field and MODIS satellite data[END_REF][START_REF] Sottolichio | Observations of suspended sediment dynamics in a highly turbid macrotidal estuary, derived from continuous monitoring[END_REF]. It is a zone with a high concentration of suspended matter, extending 70 to 80 km along the estuary. Its concentration fluctuates from 1 to 10 g.l at the water surface to hundreds of g.l on the estuary bottom [START_REF] Allen | Suspended sediment transport from the Gironde estuary (France) onto the adjacent continental shelf[END_REF][START_REF] Allen | Transport and deposition of suspended sediment in the Gironde Estuary, France[END_REF]. The formation of the TMZ results from two mechanisms: density-driven circulation (saline and fresh water) and the hypersynchronous nature of the estuary [START_REF] Allen | Sedimentary processes and facies in the Gironde estuary: a recent model for macrotidal estuarine systems[END_REF]. Both mechanisms contribute to the trapping of fine particles and to the formation of the TMZ. The TMZ position shifts in the estuary due to the seasonal cycle of fluvial discharge [START_REF] Doxaran | Dynamics of the turbidity maximum zone in a macrotidal estuary (the Gironde, France): Observations from field and MODIS satellite data[END_REF]. Clay minerals in the suspended matter are, on average, composed of illite (49%), smectite (22%), kaolinite (14%) and chlorite (15%) [START_REF] Jouanneau | The Gironde Estuary[END_REF].

Material and Methods

Facies descriptions

Facies descriptions for the P.U.C.H.F. were made on high-resolution pictures provided by Neptune Energy. The study focused on the detailed description of the Petrel E well for which 28 m of cores have been described. Descriptions were made at one-tenth scale. For the Gironde estuary tidal bars, facies were described on the same scale on cores extracted from the Plassac and the Richard tidal bars (Fig. 2B-C). Cores were extracted from an east-west transect, perpendicular to the long axis of the tidal bars (Fig. 2B-C). On the Plassac tidal bar (45°06'24.49''N 0°39'37.87''W) three cores (PLA-2015-E, PLA-2015W and PLA-2015C for East, West and Centre) were extracted at about 150 m intervals (Fig. 2B). Core lengths vary from 4.44 m to 6.70 m for a total length of about 16 m. The three cores (Ri-2016-E, Ri-2016-W and Ri-2016-C) of the Richard tidal bar were extracted along a 90 m long transect, with depths ranging from 4.60 m to 6.50 m and for a total length about 15 m (Fig. 2C).

Petrographic descriptions

A total of 38 plugs from the P.U.C.H.F. (27 were used for petrographic analyses and 33 for mineralogical analyses), and 85 plugs (diameter: 4 cm) from the Gironde estuary (53 from the Plassac tidal bar and 32 from the Richard tidal bar) were analysed. The sampling in the Gironde estuary was carried out while the sediments were still wet and cohesive so as to preserve sedimentary structures. The plugs from the Gironde were dried in a heat chamber at 40 °C for 24 hours and then indurated with blue epoxy resin under a low vacuum to preserve the sedimentary structures and rock fabric. Standard polished thin sections (30 µm thick) were then made from the indurated plugs.

Thin sections of resin-impregnated samples were observed under a Nikon Eclipse Ci-POL (Nikon, Tokyo, Japan) and a Leica DM 750P (Leica Microsystems, Wetzlar, Germany) polarizing microscope to characterize the microfacies. In each thin section, the relative surface area (%) of the pores (macroporosity) and the sediment components were quantified using random grid point counting with JmicroVision Image analysis software [START_REF] Roduit | JMicroVision: un logiciel d'analyse d'images pétrographiques polyvalent[END_REF]. A minimum of 300 counts were performed on 20 different images (2.5 mm x 2.5 mm) in each thin-section. Several categories were distinguished based on whether the study involved the modern environment or the deeply buried sandstone reservoir: detrital clay grain coats, quartz (coated and non-coated), feldspars (coated and non-coated), lithics (coated and non-coated), clay matrix, biotite, muscovite, bioclasts, pyrite, quartz overgrowth, macroporosity (primary and secondary porosity), pore filling, clay cement and carbonated cement.

In the Gironde estuary samples, grain coat coverage was quantified by determining the proportion of the outer perimeter of the grains that was coated relative to the total surface area (coated and non-coated). The coated grains category was divided into four subcategories depending on the grain coat coverage of the grain surface: (i) 1 to 5%; (ii) 5 to 15%; (iii) 15 to 30%; and (iv) more than 30%.

In the P.U.C.H.F., coat coverage was analysed by considering surfaces where quartz overgrowths could nucleate and surfaces with coatings. Surfaces were excluded if they were in contact with neighbouring grains, the detrital matrix, or pore-filling cement formed before quartz cementation.

Scanning electron microscopy (SEM) was performed on thin sections and on individual sand grains mounted on 1 cm diameter stubs. Samples were gold and palladium coated.

Observations and pictures were made in backscattered electron mode (BSE) with a Phenom X Pro SEM (Phenom-World B.V., Eindhoven, The Netherlands), at 10 keV in imaging conditions and 15 keV for elementary analyses.

Grain size measurements were performed using a Mastersizer 2000 laser granulometer (Malvern, Worcestershire, UK) for the Gironde estuary samples, and using J-Microvision (100 measurements per thin section in point counting) for the P.U.C.H.F. samples. Mean grain size was calculated using [START_REF] Folk | Petrology of Sedimentary Rocks[END_REF] equation: Mz =(ø16 + ø50 + ø84)/3, where ø16, ø50 and ø84 correspond to particle diameter (mm) in ø units (ø = -log2) at 16, 50 and 84% of the grain size distribution.

Sorting was calculated using standard deviation øi defined by [START_REF] Folk | Brazos river bar: A study in the significance of grain size parameters[END_REF]: øi=(ø84 -ø16)/4+(ø95 -ø5)/6.6, where ø5, ø 16, ø84 and ø95 correspond to particle diameter (mm) in ø units (ø = -log2) at 16, 50 and 84% of the grain size distribution. Mud was used to characterize the grain-size fraction less than 63 (µm), i.e. silts plus clays.

Clay fraction analysis

As 'clay' can refer to grain size as well as mineralogy, this study uses the term clay fraction to define the fine-grained fraction of less than 2 µm including clay minerals but also other minerals such as quartz grains [START_REF] Grim | Modern concepts of clay materials[END_REF]. The relative weight percentage of the clay fraction of homogenized sediment subsamples was measured on 33 samples from the P.U.C.H.F., 67 samples from Plassac and 41 samples from Richard. The clay-size fraction was detached from the sand grains using a sonicator probe. Samples were weighed in order to obtain the same dry mass of 5 g. They were then suspended in distilled (DI) water before being decarbonated using 20% acetic acid to remove the fine carbonate fraction, and the fine organic-matter fraction was also removed using 35% H2O2. After agitation, the suspension was allowed to rest for 1 hour 35 minutes. Then, the top 2 cm of the supernatant water (containing the clay grain-size particles) was sampled. The operation was repeated until the top 2 cm of the supernatant water was clear. The sampled fraction (< 2 μm) was dried and weighed in order to calculate the weight percentage of the clay fraction in the sediment. It was expressed as a weight percentage of the sample (wt. %). Values obtained for the P.U.C.H.F. are underestimated because not all the denser and thicker authigenic clay minerals were included in this measure. Chlorite coatings and dickite crystals in intergranular space are greater than 10 μm in some samples [START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF].

Composition of the clay fraction (< 2 μm) was then determined by X-ray diffraction analysis (XRD) with PANalytical X'Pert Pro (PANalytical, Almelo, The Netherlands) with a Cu anticathode (Kα1 1/4 1.540598, 45 kV; 40 mA) from 2 theta: 3-30° (step: 300 s and 0.0334°).

Determination of clay mineral associations was based on the study of air-dried, ethyleneglycol saturated, and 550 °C-heated oriented powders [START_REF] Brindley | The X-ray identification and crystal structures of clay minerals[END_REF]. Semi-quantitative estimations of smectite, illite, kaolinite and chlorite were made with Macdiff software based on the peak area of the ethylene-glycol diffractograms summed to 100%, the relative error for the peak area being ±5% [START_REF] Petschick | Macdiff-a programme for analysis and display of X-ray powder diffractogrammes on Apple Macintosh platforms[END_REF].

Results

Facies description and vertical facies succession

Six lithofacies were defined from core descriptions based on lithology, dominant sedimentary process and depositional context. Facies associations and their interpretations are described in detail in Table 1, 4). The same terminology was used to describe facies from the Gironde estuary and facies from the P.U.C.H.F. as similar facies are identifiable and comparable within both tidal settings (Table 1). Cores from well E are composed of tidal bars with similar sedimentary structures to those observed in the Gironde (Virolle et al., in prep) Comparisons between modern and ancient deposits indicate that at least five tidal bars are stacked within the P.U.C.H.F. well E (Fig. 4). Tidal bar thickness varies from 3.5 m to more than 7 m (not decompacted) within this formation. A synthetic vertical facies association can be reconstructed within this deeply buried sedimentary succession of the Petrel sub-basin (Fig. 4). Tidal bars from the Gironde consist of 8-10 m thick vertical successions (Fig. 3). The two vertical successions (Richard and Plassac tidal bars) display coarsening-upward facies sequences (Fig. 3).

For the entire clastic deposits under study, facies are typical of depositional environments ranging from outer estuary funnel (muddy central basin) to inner estuary funnel (bay-head delta, Table 1 and Fig. 4).

Sand and sandstone petrography

Based on the study of 27 samples from the P.U.C.H.F. well E, the original feldspar content ranges from 16 to 33% and the present-day feldspar content ranges from 4 to 14% corresponding to an evolution from subarkose to quartzarenite during diagenesis (Fig. 5A).

Samples from the Gironde estuary vary from litharenite to feldspathic litharenite (Fig. 5A).

Grain composition differs slightly between tidal bars from the P.U.C.H.F. and those from the Gironde estuary. On average, grain content (> 63 µm) represents ca. 76% of the total rock composition within the P.U.C.H.F. cores, while it represents ca. 67% and ca. 73% within the Plassac and Richard Gironde estuary tidal bars (Table 2).

For the Plassac tidal bar of the Gironde estuary, on average for 52 thin sections, samples are mostly composed of quartz (34%), feldspar (8%), lithics (16%), clay (30%) and porosity (14%) (Table 2, Fig. 5B-C). Samples vary from feldspathic litharenite to litharenite, grains are moderately well sorted overeall, and the mean grain size is about 228 µm (Table 2, Fig. 5A-C).

For the Richard tidal bar, sand grain composition varies slightly. This bar is mostly composed, on average for the 32 thin sections, of quartz (36%), feldspar (6%), lithics (15%), clay (24%) and porosity (14%) (Fig. 5D, Table 2). The main difference with the Plassac tidal bar is the non-negligible content of bioclast fragments (ca. 5%) in the Richard tidal bar (Fig. 5D).

Samples belong mostly to the litharenite class as the proportion of carbonated lithics is higher than within the other tidal bars in the Gironde estuary (Table 2, Fig. 5A andD). Grains are mostly fine (average of 182 µm) and moderately sorted (Table 2). Detrital grains from both tidal bars frequently have clay minerals attached to their surfaces forming detrital clay grain coats (Fig. 5B-D).

For the P.U.C.H.F., grains are, on average for 27 samples, mostly composed of quartz (66%), feldspar (present day ratio of about 7%), lithics (2%), clay fraction (5%), cement (14%) and porosity (10% for primary plus secondary porosities) (Table 2, Fig. 5E-G). It corresponds to subarkose to quartzarenite sandstones (Fig. 5A). Grains are globally fine, with a mean grain size ca. 150 µm, which is close to the Richard tidal bar. Grains are on average moderately to well sorted (Fig. 5E-G). Diagenetic features are observed such as feldspar dissolution, quartz overgrowth, pseudomatrix development or plastic deformation of micas squeezed between resistant quartz and framework grains (Fig. 5E-G).

Clay minerals within sand and sandstones

Clay fraction content varies between modern and buried deposits. On average, all facies included, it is very low within the P.U.C.H.F. (average of 5 wt% on 37 samples) while it is higher in the Gironde estuary tidal bars (average of 30 wt% on 71 samples at Plassac and 24 wt% on 42 samples at Richard; Table 2). Variations are found along the vertical facies associations of each deposit (Figs 4 and 7A). On average within the P.U.C.H.F., the Fluid mud facies (F1) displays the highest clay fraction content (8 wt% and 5 wt% in F1b and F1a respectively), while the others facies range between 2 and 4 wt% (Table 2, Figs 4 and7A).

Tidal bars from the Gironde estuary also have the highest clay fraction content within mudrich facies (F1b, F2, F4 and F6) with clay fractions ranging from 25 to 58 wt% (Fig. 7A, Table 2).

Within the Gironde estuary tidal bars, clay mineral analysis by XRD allows us to identify four clay minerals (Figs 7B-E and 8A-B): (i) chlorite, characterized by diffraction peaks (001) at 14.10Å, (002) at 7.05Å, (003) at 4.73Å and (004) at 3.54Å; (ii) illite diffraction patterns show diffraction peaks (001) around 9.99Å, (002) at 4.99Å and (003) at 3.33Å; (iii) kaolinite displays diffraction peaks (001) at 7.16Å and (002) at 3.57Å; (iv) smectite is identifiable after ethylene-glycol saturation, with a diffraction peak (001) close to 17 Å. Within the P.U.C.H.F., clay mineral composition differs with the presence of an illite-rich illite/smectite (I/S) mixed layer in all samples analysed by XRD (Fig. 8C). The EDX spectra under SEM indicate that the chlorites are Fe-rich within the P.U.C.H.F. The IR spectra of diagenetic kaolins are typical of dickite with shoulders at 3710 cm -1 and 3655 cm -1 (Beaufort et al., 1998;Saiag et al., 2016). G, 6D and9E-H).

Within the P.U.C.H.F., grain coating varies slightly within the different facies, ranging from 60% to 73%, for an average of 66% (Fig. 6D, Table 2). Clay coatings in the P.U.C.H.F. cover on average 61% of the outer sand perimeter (Table 2). Coat coverage (on the available surface as previously specified) is higher in facies F2 where ca. 71% of the available surface is coated on average, and lower in facies F1a (ca. 48 %), while it is roughly equivalent in the other facies (Table 2).

In the Plassac tidal bar, coated grain content ranges on average from 22% (F6) to 29% (F5a), while this content is lower in the Richard tidal bar, ranging from 14% (F5b) to 19% (F1b; Fig. 6D; Table 2). In facies F3, ca. 26% of grains in the Plassac tidal bar are coated while ca. 17% are coated in the Richard tidal bar. Coats cover on average 11% of grain surfaces in the Richard tidal bar and 19% in the Plassac tidal bar (Fig. 9A-D, Table 2). Coat coverage is low in facies F2 of the Richard tidal bar (ca. 5%), while it is higher in facies F2 of the Plassac tidal bar (ca. 25%, Table 2). In each modern and ancient deposit, no correlation is found between coated grain content and increasing depth or porosity variations. 9D). Within the P.U.C.H.F., coatings are composed of well-crystallized Fe-rich chlorites that are mostly oriented perpendicular to the grain surface (Fig. 9E-H). Thickness varies from 2 to 10 µm approximately. Coatings are mostly continuous around sand grains (Fig. 9E-F).

As observed by Saiag et al. (2016), densely packed clay mineral aggregates composed of chlorite are sometimes located beneath or close to the well-crystallized coating with more euhedral and more widely spaced chlorite crystals (Fig. 10).

Finally, similar coating textures can be observed in both tidal settings with clays that drape the surface of sand grains (Fig. 9A, 9E and 9F), or with aggregates at the surface of framework grains (Fig. 9B-D and9G-H). In the Gironde estuary, other textures can be observed as bridges between detritic grains or even ridged textures (Fig. 9B).

Discussion

Influence of detrital materials on chloritization

In low-temperature geological systems such as burial diagenesis, chlorite is more likely to arise from chloritization processes through mixed-layer mineral series than from direct precipitation from solution (Beaufort et al., 2015 and references therein). The formation of chlorite from the transformation of a clay-mineral precursor through a series of intermediate mixed-layer minerals is promoted as it requires low activation energy and is kinetically more likely [START_REF] Putnis | An Introduction to Mineral Sciences[END_REF]. This mean that the diagenetic chlorites observed in deeply buried sandstone reservoirs such as the P.U.C.H.F. are derived from specific clay precursors [START_REF] Pagel | Bassins sédimentaires-les marqueurs de leur histoire thermique[END_REF][START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF]. Moreover, depending on the geochemical conditions, chloritization does not involve the transformation of just one mineral precursor via a single reaction mechanism (Aagard et al., 2000;Worden & Morad, 2003;[START_REF] Beaufort | Chlorite and chloritization processes through mixed-layer mineral series in low-temperature geological systems: A review[END_REF]. Several reaction paths including several clay precursors lead to the formation of chlorite during burial (Worden and Morad, 2003).

Serpentine-like minerals such as berthierine and odinite are the most common clay precursors specified in the chloritization process of diagenetic Fe-chlorite (Worden and Morad, 2003;[START_REF] Gould | Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin[END_REF][START_REF] Beaufort | Chlorite and chloritization processes through mixed-layer mineral series in low-temperature geological systems: A review[END_REF]. Berthierine is an Fe-rich phyllosilicate that is formed from the interactions of the most reactive detrital Al-Fe minerals with sedimentary solutions under reducing conditions and/or from bioturbation with the ingestion of freshly deposited sediments by macro-organisms [START_REF] Velde | Clay Minerals: A Physico-Chemical Explanation of Their Occurrence[END_REF]Peyaud and Worden., 2007). It can also be formed during early burial (eodiagenesis) from the transformation of odinite or other reactive Al-bearing clay minerals (such as smectite) as long as ferrous iron is available in solution (Fig. 10, Pe-Piper et al., 2008;[START_REF] Morad | The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy[END_REF].

Then, berthierine can be transformed into Fe-rich chlorite such as chamosite in a temperature range of 40-120°C (Fig. 10, [START_REF] Beaufort | Chlorite and chloritization processes through mixed-layer mineral series in low-temperature geological systems: A review[END_REF].

In the Gironde, the detrital clay minerals composing the clay fraction are illite, smectite, chlorite and kaolinite [START_REF] Jouanneau | The Gironde Estuary[END_REF]Virolle et al., 2019). These clay minerals forming detrital clay grain coats could be the pre-requisite sedimentary material that is essential for chlorite-precursor formation (Fig. 10).

Densely-packed clay mineral aggregates observed in the P.U.C.H.F., just beneath or close to the well-crystallized Fe-rich chlorite coating suggest that those aggregates might be the relics of initial detrital clay coats (Fig. 11). These aggregates might be the remnants from transformation of detrital clay grain coats or eogenetic smectite into berthierine if Fe 2+ was available in solution, and after burial into well-ordered chlorite-smectite mixed layers and finally to Fe-rich chlorite (Fig. 10). These aggregates may also reveal the transformation of detrital kaolinite into berthierine as long as ferrous iron is available during eogenesis before forming Fe-rich chlorite through a dissolution and recrystallization process during burial (Fig. 10, Morad et al., 2000[START_REF] Beaufort | Chlorite and chloritization processes through mixed-layer mineral series in low-temperature geological systems: A review[END_REF]. Kaolinite is also transformed into dickite during burial. Besides, the absence of coatings on grain sutures suggests that the Fe-rich chlorite crystallization occurs after mechanical compaction of the sediment [START_REF] Saïag | Sedimentological control on the diagenesis and reservoir quality of tidal sandstones of the Upper Cape Hay Formation (Permian, Bonaparte Basin, Australia)[END_REF].

Finally, formation of Fe-rich clay minerals requires labile Fe [START_REF] Gould | Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin[END_REF]. Alteration of Mg and Fe-rich materials within the Gironde such as volcanic lithic, biotite and/or oxides and hydroxides (such as goethite or ferrihydrite) may release Fe 2+ and Mg 2+ which are then available for the chlorite formation process during diagenesis (Morad et al., 2000;[START_REF] Mckinley | Smectite in sandstones: A review of the controls on occurrence and behavior during diagenesis[END_REF]. Moreover, dissolution of detrital clay minerals (such as smectite) mediated by microorganisms may release iron and silica into the surrounding pore waters [START_REF] Vorhies | Microbial dissolution of clay minerals as a source of iron and silica in marine sediments[END_REF]Fang et al., 2017). This release of iron within ground water could improve the subsequent precipitation of clay minerals (such as berthierine) that may act as the precursors to Fe-rich chlorite leading to the chloritization process in estuarine sandstones.

Therefore, the detrital minerals and detrital clay grain coats observed in the Gironde estuary could initiate the generation of Fe-rich chlorite during burial.

Tidal bar vertical facies succession

Tidal bars observed within P.U.C.H.F. and within the modern Gironde estuary can be quite similar in thickness despite the erosion and compaction that occurred in the ancient deposits (Fig. 3). In general, tidal bar dimensions are on the scale of estuary dimensions, in particular in width and depth [START_REF] Leuven | Tidal sand bar dimensions and shapes in estuaries[END_REF]. It is likely, then, that the local estuary depth in the P.U.C.H.F. was similar to the depth of the Gironde estuary.

Each tidal bar from modern and ancient deposits presents a global coarsening-upward sequence (Figs 3 and4). In the P.U.C.H.F. well, the vertical association is composed of several tidal bars stacked one on top of the other. A total of five tidal bars can be identified in well E (Fig. 4). Each displays similarities with the modern tidal bars as they exhibit sedimentary structures of tidal environments: (1) bidirectional cross-strata, (2) reactivation surfaces and

(3) tidal bundles with clay drapes and sometimes mud-pebbles (Figs 3 and4, Table 1).

Comparisons with the Gironde deposits show that facies deposited near the bottom of the tidal bar (facies F1-F2) are mud-rich and display many clayey sedimentary structures (thick clay drapes, mud pebbles or fluid mud layers) that will affect reservoir qualities (Figs 3 and4, Table 1). Above, facies (F3), ( F5a) and ( F5b) are sandier but still with some clay drapes and mud pebbles (Fig. 4). In addition to muddy facies at the bottom of tidal bars, large clay baffles are also settled within the facies associations of tidal bars in P.U.C.H.F. deposits, as observed in tidal bar 2, and in the Gironde tidal bars (facies F4; Figs 3 and4; Féniès and Tastet, 1998;Chaumillon et al., 2013).

Besides, facies F4 could originate from similar processes in both of the modern and ancient tidal settings under study. Tidal bar 2 from the P.U.C.H.F. displays a similar arrangement to the Richard tidal bar in the Gironde with the superimposition of two sand units displaying cross-bedded sand facies (F3), separated by a muddy layer exhibiting muddy-sand tidal rhythmite facies (F4; Figs 3 and4). As observed in another Gironde tidal bar, facies (F4) in the Richard tidal bar corresponds to a long period of low river discharge, causing sandstarvation and allowing the decantation of many fine particles (Féniès and Tastet, 1998) 2, Fig. 6C). Tidal bars from the bay-head delta, such as the Plassac tidal bar, tend to be slightly coarser (ca. 228 µm; Table 2, Fig. 6C). Within the clay assemblages of modern tidal bars, smectite is probably more common in the Plassac tidal bar (with coarser grains) than in the Richard tidal bar, which is richer in illite, while chlorite and kaolinite are relatively homogeneous in both tidal bars (Fig. 7, Table 2). This distribution is similar in other modern estuaries such as the Ravenglass estuary where illite is most abundant in the fine-grained sediments of the inner estuary and mud from the central basin [START_REF] Griffiths | Detrital clay coats, clay minerals, and pyrite: a modern shallow-core analogue for ancient and deeply buried estuarine sandstones[END_REF].

Besides, each tidal bar is heterolithic, but to various degrees (Fig. 3). In the Gironde, tidal bars from the bay-head delta (such as Plassac) seem more heterolithic, with more clayey heterogeneities at various scales, from large clay layers such as facies F1a and F4, to the higher concentration of clay drapes and mud pebbles as are found within facies F3 or F5a (Fig. 12, Table 1). The clay fraction content is also higher in the Plassac than the Richard tidal bar (Table 2, Fig. 7A), and facies (F3) and (F5a) from the Plassac tidal bar have more and thicker clay drapes and mud pebbles than (F3) and (F5b) from the Richard tidal bar. In the Gironde, this can be explained by the TMZ position that is more regularly centred in the vicinity of the Plassac than the Richard tidal bar [START_REF] Allen | Transport and deposition of suspended sediment in the Gironde Estuary, France[END_REF]Doxaran et al., 2008). In the P.U.C.H.F., tidal bars 3 to 5 display less clayey sedimentary structures than tidal bars 1 and 2.

All these clues combined may suggest the tidal bars in the P.U.C.H.F. vertical succession were mainly deposited in an outer (tidal bars 3 to 5) rather than an inner (tidal bars 1 and 2) estuary funnel (Fig. 4).

Origin of unstratified mud deposits

Fluid muds are a relatively new criterion for identifying tidal conditions within distributary channel deposits [START_REF] Longhitano | Tidal depositional systems in the rock record: A review and new insights[END_REF]. A fluid mud deposit is defined as a bottomhugging mobile subaqueous body composed of fine-grained sediment with a concentration of tens to hundreds g.l -1 [START_REF] Parker | Sources and transport patterns of sediment in the inner Bristol Channel and Severn Estuary[END_REF]Jalón-Rojas et al., 2015). They can be recognized in the rock record as centimetre-to decimetre-thick unbioturbated and unlaminated mud layers, and in the distributary channels they occur in the lower parts of the channel fill [START_REF] Longhitano | Tidal depositional systems in the rock record: A review and new insights[END_REF]. Fluid mud appears to be especially common in areas beneath the Turbidity Maximum Zone (TMZ) as within the Gironde estuary [START_REF] Parker | Sources and transport patterns of sediment in the inner Bristol Channel and Severn Estuary[END_REF][START_REF] Wright | Marine dispersal and deposition of Yellow River silts by gravity-driven underflows[END_REF][START_REF] Uncles | Turbidity maximum in the macrotidal, highly turbid Humber Estuary, UK: Flocs, fluid mud, stationary suspensions and tidal bores[END_REF][START_REF] Ichaso | Tide-and wave-generated fluid mud deposits in the Tilje Formation (Jurassic), offshore Norway[END_REF][START_REF] Sottolichio | Observations of suspended sediment dynamics in a highly turbid macrotidal estuary, derived from continuous monitoring[END_REF]. Fluid mud deposits and thick clay drapes observed within the P.U.C.H.F. well may indicate that the estuary was mud-rich with a well-developed TMZ as observed in the Gironde estuary (Figs 1 and 12). All these muddy layers, as well as facies F4, will act as large clay baffles affecting connectivity between sandy reservoir units (Fig. 12).

Multi-scale reservoirs characterization

Reservoir characterization requires a multi-scale approach. On a large scale (x100 m), heterogeneities must be considered, and notably clay layers that will act as baffles [START_REF] Wood | Predicting tidal sand reservoir architecture using data from modern and ancient depositional systems. In: Integration of outcrop and modern analogs in reservoir modeling[END_REF][START_REF] Massart | Improved characterisation and modelling of heterolithic tidal sandstone reservoirs[END_REF]. Comparisons with modern estuarine deposits may allow us to determine how clay baffles can affect fluid flow within sandy reservoir units. Since facies (F1) and (F4) are continuous over several tens to hundreds of metres in the Gironde estuary deposits, it is possible that these same facies within the P.U.C.H.F. deposits constitute largescale discontinuities (Figs 3 and12).

On smaller scales within the P.U.C.H.F., the best reservoir properties are observed in facies F3 with notably five samples that stand out (14A, 17A, 18A, 19B and 51B; Fig. 13A). These samples also correspond to a coated grain content of more than 60% (Fig. 13B), a clay fraction content that ranges from 3 wt% to 10 wt% (Fig. 13C) and chlorite content among this clay fraction mostly exceeding 1% (Fig. 13D). Despite a certain percentage of coated grains, the thickness of the coating can affect the variations in porosity and permeability for the same percentage of coated grains (thick coatings block pore spaces), just as a clay content that is too low or too high can affect these parameters (forming pore filling or allowing quartz overgrowth).

Although well E deposits display abundant and sometimes thick clayey sedimentary structures (such as fluid mud or clay drapes), the clay fraction (< 2 µm) within samples is very low (Table 2). It can be partly explained by diagenetic events with dissolution and recrystallization of detrital clay minerals but also by particle transport due to fluid flow during burial. Therefore, the final content of the fraction lower than 2 µm will be lower than the initial content.

As coarser-grained sandstones have a smaller surface area relative to bulk volume and thus require less clay content to achieve full surface coverage, the optimum grain coat coverage to preserve porosity might be influenced by mean grain size [START_REF] Bloch | Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability[END_REF]. [START_REF] Bloch | Anomalously high porosity and permeability in deeply buried sandstone reservoirs: Origin and predictability[END_REF] report that a relatively minor amount of clay content (even as little as 1-2% of the rock volume) can form extensive coats on individual sand grains. Other authors such as [START_REF] Pittman | Clay coats: Occurrence and relevance to preservation of porosity in sandstones. Origin, diagenesis, and petrophysics of clay minerals in sandstones[END_REF] suggest an optimum range of 4-7% sediment volume as in clays for the Berea Sandstone and 5-12% in the Tuscaloosa Formation.

Sandy facies (F3 and F5b) from tidal bars located in the outer estuary funnel of the Gironde, such as the Richard tidal bar, display fewer clayey sedimentary structures (clay drapes and mud pebbles) than the tidal bars located in the inner estuary funnel, such as the Plassac tidal bar (facies F5a; Figs 7 and 12, Table 2). The latter may experience pseudomatrix formation during eodiagenesis owing to pseudoplastic deformation and compaction of ductile grains between rigid grains [START_REF] Morad | The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy[END_REF]. Besides, samples from the Richard tidal bar have, on average, a lower coat coverage (11%), a lower coated grain (17%) and a lower clay fraction (24 wt%) content than the Plassac tidal bar (25% of coated grain, 19% of coat coverage and clay fraction of 30 wt%, Figs 6 and 7, Table 2). However, it could be sufficient to form thin and continuous clay coating during burial, whereas in the Plassac tidal bar, too many and more developed coatings associated with a high clay fraction content can partially obstruct porosity and permeability during diagenesis.

In the Gironde, facies F3 and F5b deposited in the outer estuary funnel might therefore be the best reservoir units if preserved after burial (Fig. 12, Table 2).

Therefore, taking into account petrographic observations and facies descriptions in both modern and ancient tidal settings, better reservoir properties within an estuarine depositional environment might be preferentially located in tidal bars deposited in the outer estuary funnel than in tidal bars deposited in the inner estuary funnel. Sandy facies from the Middle sand bar facies (F3) to the Upper sand bar facies (F5b) with clay content of 20-25 wt% maximum, a coated grain of 15-20% and coat coverage of about 10% could be the ideal initial conditions for reservoir formation after burial (Figs 3 and 12). 5.6 Tidal bar sequence in a retrogradational sequence Within the P.U.C.H.F., inter-well scale correlations can be made from gamma-ray and core descriptions [START_REF] Kloss | A revised depositional model for the Cape Hay Formation, Petrel Field, northern Australia. Timor Sea Pet[END_REF][START_REF] Robinson | Permo-Triassic reservoir fairways of the Petrel Sub-basin, Timor Sea[END_REF]. Tidal sand bars facies that have been observed in the upper part of the Cape Hay Formation are the sandstone reservoir units targeted (Fig. 14). The Upper Cape Hay Formation within the Petrel sub-basin was deposited in shallow-marine to fluvial environments with a basin opening towards the north-northwest (Bhatia et al., 1984, Robinson andMcInerney, 2003, Figs 1 and14).

Differentiating between deposits laid down in the inner or outer estuary funnel can help to explain the stratigraphic evolution of the study area. Within the P.U.C.H.F. well, the transition from tidal bar 2 to tidal bar 4 may record a shift from tidal bars that belong to an inner part of a mud-rich palaeo-estuary to a more external area subject to the influence of higher tidal currents and probably of waves (Figs 4 and 12). It might therefore record the transgression period that occurred at this period with outer estuarine tidal bars that might overlie muddier inner estuarine tidal bars (Fig. 14).

Facies F3 has good petrographic properties in terms of coated grain content, coat coverage and porosity preservation (Figs 3,6,7 and 13,Table 2). Samples with better reservoir properties are located at the top of the transgressive cycle (Figs 3,13 and 14). Other examples can be found around the world displaying well-preserved reservoir qualities within transgressive cycles and linked to chlorite coatings as within the Lower to Mid-Jurassic series of the Halten Banken Area (Norwegian Sea; [START_REF] Lafont | Porosity preservation in deeply buried coated sandstones of the Norwegian Sea: Understanding early diagenetic processes in a high-resolution sequence stratigraphic framework[END_REF]. Besides, [START_REF] Morad | The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy[END_REF] show that eogenetic grain coatings composed of berthierine, odinite and smectite are mostly formed in the Transgressive Systems Tract (TST) and early Highstand Systems Tract (HST) in deltaic and estuarine sandstones. These coatings can be transformed into ferrous chlorite during mesodiagenesis helping preservation of good sandstone reservoir properties.

In the Gironde, the highest concentration of smectite is found in the inner estuary tidal bars, but the latter are also associated with the highest clay fraction content that can obstruct pore throats (Fig. 7, Table 2).

Genetic stratigraphic sequence cyclicity (second-through fifth-order) might be considered when comparing modern and ancient deposits. In the Gironde, the TST comprises the bulk of the incised valley fill and forms a landward-thinning wedge of tidal-estuarine sands and muds [START_REF] Allen | Sequence stratigraphy and facies model of an incised valley fill: The Gironde Estuary, France[END_REF]. The HST, initiated at about 4000 BP, forms a seawardprograding, tide-dominated estuary head delta that has been gradually filling the estuary since the post-Holocene stillstand [START_REF] Allen | Sequence stratigraphy and facies model of an incised valley fill: The Gironde Estuary, France[END_REF]. In buried and ancient analogues, the same time scales are not necessarily observable, notably because of erosion.

The trend observed in the P.U.C.H.F. belongs to a third-order transgressive period (5-10 Ma) and only the transgressive trend can now be seen.

However, integrating all the results, the best reservoir facies are located at the top of transgressive sequence, preferentially in outer estuarine funnel tidal bars in sand rich facies located from the Middle sand bar facies (F3) to the Upper sand bar facies (F5b) if preserved, which is not the case for the P.U.C.H.F. concerning the facies (F5b), with fewer multi-scale clay heterogeneities and optimal petrographic and mineralogical characteristics (Figs 3,12 and 13, Table 2).

Conclusions

Comparisons between estuarine sands cored within two tidal bars from the Gironde estuary and sandstones from P.U.C.H.F. show that sediments of P.U.C.H.F. were deposited in a palaeo-estuary very similar to the present-day Gironde estuary.

Vertical facies associations of tidal bars are very similar with many common facies between ancient and modern deposits such as a muddy bottom, deposited near the estuary channel floor, overlain by sandier facies indicative of an increase in hydrodynamic conditions.

Extensive clay layers may be deposited in the middle sand bar areas owing to hydrodynamic variations or to specific processes (e.g. lateral accretion processes). Fe 2+ is available in the pore water. Then, this mineral may serve as a precursor to Fe-rich chlorite transformation during burial. The transition from detrital clays into Fe-rich chlorite during eogenesis and burial diagenesis through berthierine should be more thoroughly explored. Several processes can dissolve detrital minerals and release Fe, such as microbial activity, promoting berthierine formation and then Fe-rich chlorite crystallization.

Sandy facies forming sand bars during transgressive cycles, and mostly those deposited at the top of the sequence before the settlement of the clay seal, might be good candidates for finding clay coatings and well-preserved sandstone reservoir qualities. From petrographic observations of sand bar facies in the Gironde estuary, sandy facies with a maximum clay content of 20-25 wt%, associated with coated grain content of 15-20% and coat coverage of about 10% could be the ideal initial conditions for reservoir formation after burial. In ancient estuarine sandstones, good reservoir qualities must be sought vertically in the last sequences of third-order cycles below the clay seal and horizontally in sediments deposited in outer estuary funnels. part of the estuary. This will affect the petrographic properties with potentially more or less clay pore filling despite clay coatings. The best reservoir qualities can be found in the sandier outer estuarine tidal bars. Sandy facies with a maximum clay content of 20-25 wt%, associated with coated grain content of 15-20% and coat coverage of about 10% could provide the ideal initial conditions for reservoir formation after burial. et al. (2004). Petrographic observations are replaced so as to determine locations of the best reservoirs properties.

Figure captions

2-column fitting image

2-column fitting image

2-column fitting image

  Figure 3. Detailed facies descriptions and interpretations and the vertical facies associations

  . The bottom is made up of muddy deposits with the Fluid mud facies (F1). The Lower sand bar facies (facies F2) is composed of small tidal dunes with frequent and thick clayey sedimentary structures such as mud pebbles and clay drapes. Above, the middle part of tidal bars is composed by the Middle sand bar facies (F3), sand dominated, or by the Middle bar muddy facies (F4), mud-dominated. In the Gironde, facies (F3) within the Plassac tidal bar is richer in clayey sedimentary structures (mud pebbles and clay drapes) than the Richard tidal bar. Within the Gironde, these facies are capped by the Upper sand bar facies (F5), sand dominated. It can be rich in mud pebbles (F5a) with abundant mud pebbles on dune foresets (Plassac tidal bar), or on the contrary it can be poor in clays with rare clay drapes or mud pebbles (F5b, Richard tidal bar). The Tidal flat facies (F6), observed in the Plassac tidal bar, cap the underlying facies and consists of three types of bedding (flaser, wavy and lenticular) that are vertically stacked.

  Chlorite has the highest surface area in XRD diffractograms and is the main clay mineral within the clay assemblage observed in the P.U.C.H.F. followed by the illite-rich I/S mixed layer (Figs4 and 7; Table 2). Chlorite is observed in almost every sample in SEM within the P.U.C.H.F. Within the Gironde estuary tidal bars, dominant detrital clay minerals within the clay fraction are smectite and illite, with more smectite in the inner estuary funnel (Plassac) and more illite in the outer estuary funnel (Richard; Fig. 7; Table 2). 4.4 Clay coating in sands and sandstones Sand grains from the P.U.C.H.F. and from the Gironde estuary share the particularity of having clay minerals around their surfaces forming clay coatings (Figs 5B-G and 9). Of the total grain content measured within samples, ca. 25% at Plassac and 17% at Richard are coated (Figs 5B-D, 6D and 9A-D,

  Within the Gironde estuary, coatings are detritic and are essentially composed of detrital clay minerals (smectite and illite essentially) associated with other components such as carbonates, pyrite, diatoms or silt-sized quartz (Figs 5B-D and 9A-D). The Richard tidal bar has the particularity of displaying coated carbonated components such as gastropods, bivalves or echinoderms (Figs 5D and

  . The same process may have occurred in the Permian deposits for the settlement of this facies in tidal bar 2. The depositional environment of the P.U.C.H.F. might therefore have been very similar to the Gironde estuary in terms of hydrodynamic conditions and turbidity, producing a very similar vertical facies succession within the tidal bars. The top of the Petrel tidal bars, normally marked by tidal flat or tidal marsh facies, may have been eroded before the deposition of the following tidal bar. This is why they are not observed. 5.3 Tidal bar location within the palaeo-estuary Differentiating between tidal bars deposited in the inner estuary funnel and those deposited in the outer part can prove difficult. Comparison with a modern estuary can provide some clues. First of all, in terms of grain size. The mean grain sizes measured in P.U.C.H.F. tidal bars are almost similar to those measured in the Richard tidal bar within the Gironde (ca. 150 µm in Petrel and 182 µm in the Richard tidal bar; Table

  Vertical facies associations and sedimentary structures observed in tidal bars from the P.U.C.H.F might have been deposited in a mud-rich palaeo-estuary with a well-developed TMZ as indicated by fluid mud deposits and clay drape thickness. Facies comparisons indicate that facies from the middle to the upper sand bar areas deposited in the outer estuarine tidal bars may be the best reservoir facies. They have a mean grain size about 150-180 µm with fewer and thinner clayey sedimentary structures than are observed in the inner estuary tidal bars. Detrital clay grain coats are observed in the modern Gironde tidal bars. About 20% of grains are coated and coatings display various textures as aggregates. Fe-rich chlorite coatings are observed in every sample of the Upper Cape Hay Formation (Petrel). The absence of Fe-rich clay coating between grains suggests that crystallization occurred after mechanical compaction. Observations of densely-packed aggregates within the P.U.C.H.F., similar to detrital clay coats observed in the modern Gironde estuary, suggest that transformation of detrital clay might be at the origin of well-crystallized Fe-rich chlorite during burial. In the Gironde, detrital clay minerals within detrital clay grain coats can initiate chloritization in sandstone that is done in at least two steps. The first occurs during eogenesis, with very shallow burial, where detrital minerals such as kaolinite or smectite evolve into berthierine if

Figure 1 :Figure 3 :Figure 4 :

 134 Figure 1: (A) Location of the Petrel sub-basin and simplified geological map modified after Earl (2004). (B) Permian lithostratigraphy of the Bonaparte Basin and reservoir arrangement, modified from Bhatia et al. (1984); Formation nomenclature after Gorter (1998). (C) Proposed palaeo-environmental reconstruction for the Petrel sub-basin during the Permian. 2-column fitting image

Figure 8 :Figure 9 :Figure 10 :Figure 12 :

 891012 Figure 8: X-Ray diffractograms showing clay minerals identifiable within (A) the Plassac tidal bar, (B) the Richard tidal bar and (C) the Permian Upper Cape Hay Formation. The

Figure 13 :Figure 14 :

 1314 Figure 13: Porosity-permeability relation in samples from the Permian Upper Cape Hay Formation as a function of (A) facies, (B) coated grain content, (C) clay fraction content and (D) chlorite content within samples.2-column fitting image

Table 2 )

 2 . Within the P.U.C.H.F. well, grain content within samples is ca. 76% and ca. 67% of grains are coated (average of 26 samples; Table 2, Figs 5E-

Table 1 :

 1 Table of facies recognized in this study

Table 2 :

 2 Data table for each facies from the Gironde estuary tidal bars and from the Permian Upper Cape Hay Formation tidal bars sampled in this study

		Facies		Tidal bars	Petrographi c Samples	Mineralogic al samples				Grains (%)				Coat coverage (%)	Cement (%)	Porosity (%)	Mean grain size (µm)	Clay fraction (wt%)		Clay minerals content (%)
			F1a
	F1
			F1b
				Richard	3	6	36	7	12	1	10	66	19	15	/	16	183	29	4	2	14	2
				Petrel	1	2	70	7	0	1	0	78	73	71	4	14	176	4	0.0	2.4	1.0	0.2
	F2		F2	Plassac	8	12	33	6	18	0	8	66	24	25	/	15	231	25	11.0	2.0	9.0	3.0
				Richard	8	10	39	7	18	1	12	76	18	5	/	14	185	21	2.0	2.0	20.0	2.0
				Petrel	23	28	65	7	2	1	0	76	68	64	10	12	156	3	0.0	1.1	0.6	0.4
	F6 F3 F4 F5	Average	F6 F5a F5b F3 F4	Plassac	16	20	40 15 34 9 15 0 8 72 26 20 / 14 220 8 16 0 8 67 25 19 / 14 228 30	14	3	10	3
				Richard	/	/	36	6	15	1	14	73	17	11	/	14	182	24	3	2	16	3

Acknowledgements

This work is the result of collaborative project No. P04980 CLAYCOAT 'CLAY COATing in shallow marine clastic deposits to improve reservoir quality prediction' between University Paris-Sud, Bordeaux INP, University of Bordeaux Montaigne, University of Poitiers and Neptune Energy. This study has benefitted greatly from Neptune Energy funding. The authors would also like to thank Serge Miska (GEOPS) for assistance with XRD analysis. We are grateful to Valérie Godard (GEOPS) for high-quality thin sections.