Estuaries provide an excellent depositional environment to study the interaction between minerals (clays, quartz sands…) and biofilms. The estuary bottom is largely covered by biofilms that impact sediment stability, the mud and clay-coat content in sands, and sedimentary-structure stability, thus influencing sandstone properties during burial. Although numerous oil, gas, and geothermal reservoirs are exploited in estuarine heterolithic point bars, many questions remain about the origin of reservoir properties and heterogeneities in these sedimentary bodies. In order to better understand the sedimentary and microbiologic processes in estuarine systems and to better predict the reservoir quality of estuarine sandstones, this study characterizes a modern heterolithic point bar located in the Garonne estuarine channel at various scales, ranging from the microscopic (thin section) to the macroscopic (core) scale. Three piston cores 4.5 to 6.8 m long were drilled in the Bordeaux North Point Bar. Three main facies were identified in these cores: 1) sandy gravel, 2) heterolithic, medium-grained sand dunes, and 3) thin heterolithic, fine-grained sand beds with mud drapes. The sands are classified as lithic arkoses to feldspathic litharenites. Detrital clay grain coats, which at deep burial depths are transformed to permeability preserving authigenic chlorite coatings, are observed from the base to the top of the point bar. These detrital clay grain coats are mainly composed of smectite, illite, kaolinite, and chlorite, intermixed with other components, such as diatoms or pyrite. Biofilms of exopolymeric substances (EPSs), mostly produced by diatoms, are believed to control the adhesion of the clay coats to the surface of sand grains. Quantification by thin section shows that on average about 30% of the sands are coated in the point bar. The proportion of clay-coated grains appears to be independent of facies. Radiocarbon age dating measured on organic matter points to significant vertical mixing, highlighting the significance of erosion and redeposition. The activities of 137Cs and 210Pb indicate a vertical sedimentation rate of ca. 0.02 m.yr–1 in the muddy chute channel. These ages, coupled to historical maps, suggest that the present-day point bar has developed over the last 300 years with a vertical sedimentation rate ranging from 0.015 to 0.036 m.y–1 and a lateral migration rate of about 1 m.y–1. The combination of sedimentary geology, thin-section petrography, and mineralogy at high spatial and temporal resolutions highlights the potential of this study area as a modern analogue for ancient tidally influenced point-bar deposits associated with clay coatings.

中文翻译：

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吉伦特河口（法国波尔多）现代河口异质岩尖坝中碎屑粘土层的保存和分布

河口为研究矿物质（粘土、石英砂……）和生物膜之间的相互作用提供了极好的沉积环境。河口底部大部分被生物膜覆盖，这些生物膜会影响沉积物稳定性、砂中的泥浆和粘土涂层含量以及沉积结构的稳定性，从而影响埋藏过程中的砂岩性质。尽管在河口异质岩点坝开发了大量的石油、天然气和地热储层，但关于这些沉积体中储层性质和非均质性的起源仍存在许多问题。为了更好地了解河口系统的沉积和微生物过程，更好地预测河口砂岩的储层质量，本研究对位于加龙河河口河道的不同尺度的现代异岩点坝进行了表征，从微观（薄片）到宏观（核心）尺度。在波尔多北角酒吧钻了三个 4.5 至 6.8 m 长的活塞芯。在这些岩心中确定了三个主要相：1) 砂砾岩，2) 杂岩、中粒沙丘，和 3) 具有泥披覆的薄杂岩、细粒砂床。砂被归类为岩屑长石岩到长石岩屑岩。从底部到顶部可以观察到碎屑粘土颗粒涂层，在深埋深度处转变为保持渗透性的自生绿泥石涂层。这些碎屑粘土颗粒涂层主要由蒙脱石、伊利石、高岭石和绿泥石组成，并混有其他成分，如硅藻或黄铁矿。外聚合物物质 (EPS) 的生物膜，主要由硅藻产生，相信控制粘土涂层对砂粒表面的粘附。薄截面量化表明，平均约 30% 的砂被包覆在尖棒上。粘土包覆颗粒的比例似乎与相无关。在有机质上测量的放射性碳测年表明显着的垂直混合，突出了侵蚀和再沉积的重要性。137Cs 和 210Pb 的活性表明垂直沉降速率约为 0.02 m.yr–1 在泥泞的溜槽通道中。这些年代与历史地图相结合，表明现今的点坝在过去 300 年中发展起来，垂直沉积速率范围为 0.015 至 0.036 my-1，横向迁移速率约为 1 my-1。沉积地质学、薄层岩相学、