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RESEARCH PRODUCT

Sedimentological and diagenetic influences on low permeability properties of the Middle Jurassic limestones (Paris Basin, France)

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A detailed sedimentological, geochemical and mineralogical study is carried out on the Early Bajocian to Early Callovian (Middle Jurassic) limestones of the Paris Basin. Deposits include 18 lithofacies which are stacked into 10 third-order depositional sequences. Our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern. A cooling from the latest Early Bajocian to the Late Bajocian and a subsequent warming from the earliest Callovian to the Early/Middle Callovian transition are described using a proxy of sea surface temperatures. Together with δ13C and mineralogical data, our new δ18O values suggest a palaeo-climatic/palaeo-environmental control of facies in this shallow carbonate ramp environment. Using petrographic and geochemical analyses (stable O and C isotopes, Sr isotopes, major elements), we identify six calcitic spar stages, two dolomite stages, and several episodes of fracturing and stylolitization ordered in paragenetic sequence, reducing significantly the porosity during the burial diagenesis. Cement quantification shows the predominance of two blocky calcite cement stages (75% of total cementation). Lateral meteoric recharge may have occurred as a result of two separate uplift events (Late Cimmerian Unconformity and Late Aptian Unconformity) and the related exposures of carbonates to the north of the study area during the Early Cretaceous. This palaeohydrological circulation brought about a significant reduction of porosity (from 40% to 10%) through calcite cement precipitation. Two porous levels (PL1 and PL2; Φ=15-20%) are detected at the top of the Dogger formation (Calcaire de Chaumont –Bathonian) in the study area. They form the carbonate reservoir levels. Their permeability is variable: high for PL1 (k=100-700mD) corresponding of macroporous grainstones (deposited in a ooid shoal), and low for PL2 (k=0,1-1mD) within muddy facies dominated by intercrystalline microporosity (deposited in protected lagoon). The sedimentological and diagenetic study shows the existence of PL1 and PL2 can be summarized in 3 stages: (1) lithification and early mineralogical stabilization (aragonite and high magnesium calcite to low magnesium calcite) below exposure surfaces (SB7 and SB8) which rigidify the sediment (grainstone or micrite), prevent compaction and favor the preservation of the primary porosity; (2) low meteoric recharges and cementation (Bc1 and Bc2) during the Cretaceous due to the Marnes à Rhynchonelles, acted as a lateral permeability barrier, which prevent lateral phreatic circulations and cementation at the top of the Dogger formation; (3) very low hydrothermal circulations and cementations (dolomite, Bc3 and fuorite) during the Oligocene. These 3 factors were combined only in PL1 and PL2, on a very low thickness (20 m) compare to the 210 m completely compacted and/or cemented forming the the Dogger.