--> Origin and Spatial Temporal Distribution of Clay Coatings in Shallow Marine Clastic Deposits: Insights From a Modern Estuarine Reservoir Analog (Gironde Estuary, France)

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Origin and Spatial Temporal Distribution of Clay Coatings in Shallow Marine Clastic Deposits: Insights From a Modern Estuarine Reservoir Analog (Gironde Estuary, France)

Abstract

Abstract

The development of clay (mainly chlorite) coatings around detrital quartz can be a major factor controlling reservoir quality in deeply buried sandstone reservoirs (> 3500m). These coatings prevent quartz overgrowth and thus preserve porosity and permeability during burial. Although the link between clay coatings and good permeability has been established in numbers of ancient estuarine sandstone reservoirs (e.g. North Sea, Bonaparte Basin…), questions remain about the origin of clay coatings (sedimentary, diagenetic or both) and about the prediction of their spatial distribution. These questions increase the uncertainty during the exploration of such reservoirs, for deep hydrocarbons or geothermal purposes.

In order to minimize those uncertainties, the objective of the scientific project CLAYCOAT “CLAY COATing in shallow marine clastic deposits to improve reservoir quality prediction” is to establish the factors controlling the initiation and early diagenetic evolution of clay coatings. The first phase of this project is the study a modern mud-rich estuary, the Gironde estuary (SW France) in order to determine the mineralogy, proportion and origin of clay coating: 1) through surface sampling campaigns and 2) in 2D/3D through coring.

All tidal point bars and mid-channel tidal bars have been sampled (surface sampling) from the Bay-line to the estuary mouth, during low-river and high river stages, forming a 130 km long transect. In addition, three cores have been sampled in the Plassac estuarine tidal bar in order to have a complete vertical facies. Preliminary results show that: (1) Clay minerals within the sand bars are composed of kaolinite, chlorite, illite and mixed layers illite/smectite. (2) Thin sections analyses demonstrate the existence of clay coatings around detrital quartz grains and 10% to 30 % of detrital quartz observed on thin-sections present clay coats. (3) SEM observations reveals that diatoms are present within clay coats, suggesting that diatoms and associated biofilms may have a role in the binding of clay on quartz grains.

More generally, all results, based on the study of a modern analog, would increase the prediction of good reservoirs properties in deep fossils reservoirs.