Carbonate Depositional Models, Rock Fabrics and Porosities for a Prograding Oolitic Wedge: the Bathonian of Western France (Aquitaine Basin)

Abstract

Abstract

Oolitic wedges form high quality hydrocarbon reservoirs for which a better analyse of the geometry and the spatial/temporal dynamic of outcrop analogues may reduce the uncertainty during the exploration. A detailed outcrop study is presented on the bathonian carbonate platform in western France. Microfacies and porosity types have been studied on 38 thin-sections. Four depositional environments are distinguished on a shallow (paleodepth < 10 m) to deeper platform (∼50 m) 15 km-long cross-section: foreshore, lagoon, platform slope and offshore. Offshore facies correspond to marl/mud-supported bioclastic limestone alternations. Eight rock fabrics (A to H) with various grain types, early cements and porosity types are identified. Large-scale high angle clinobeds, developing on a thickness of 40 m after compaction, indicate a paleoslope of ∼20°. Two rock fabrics can be recognized downslope: oolitic grainstones with no early cementation and interparticular porosity of ∼10% (rock fabric A), and lithoclast rudstones affected by dedolomitisation (porosity: 5 to 10%; rock fabric C). The top of the wedge is characterized by lithoclast rudstones displaying meniscus and microstalactitic early cements (D). Landward facies are oolitic grainstones with porosity near zero (B) or ooid/oncoid rudstones rich in lithoclasts (E to G). Ooid/oncoid rudstones have usually a very low porosity which can be moldic or interparticular, and can exhibit perforated hardgrounds with fibrous isopachous cements. Foreshore environments display pellet/lithoclast facies with birdeyes and vadose cements (H). A dynamic depositional model is proposed for three 3^rd order sequences, where the geometries and rock fabrics are precisely located. During transgressive system tracts, lagoonal facies are replaced by onlapping oolitic deposits (B). Downslope, oolitic clinobedded lithosomes (A) prograde on marl/limestone alternations. Over the regressive system tract, ooid/oncoid facies (E to G) and pellet/lithoclast facies (H) are deposited landward. Downslope, prograding lithoclast rudstones (C) overly oolitic grainstones (A). Maximum regressive surfaces are marked by perforated hardgrounds in the lagoon (G), while exposure surface displaying vadose cements occurs on the shoal preceding the slope break (D). On this depositional model, the highest porosities (about 10%) are located in oolitic and lithoclast prograding wedge facies (A and C). The lateral extension of the slope does not exceed 200 m.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016