--> Burial Evolution of Evaporites With Implications for Sublacustrine Fan Reservoir Quality: A Case Study From the Eocene Es4x Interval, Northern Minfeng Sag, Dongying Depression, Bohai Bay Basin, China

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Burial Evolution of Evaporites With Implications for Sublacustrine Fan Reservoir Quality: A Case Study From the Eocene Es4x Interval, Northern Minfeng Sag, Dongying Depression, Bohai Bay Basin, China

Abstract

Evaporites, dark mudstones and deep-water sublacustrine fan facies are present at burial depths of 4000~5000m in the Eocene Es4x interval in Bohai Bay Basin. Evaporitic lithologies, including gypsum, anhydrite and gypsiferous mudstone, are developed in cumulative thicknesses of more than 1600m. Burial evolution of the evaporites critically influenced sublacustrine fan reservoir quality. In the Eocene Es4x interval, anhydritization of gypsum is partial to complete. Anhydrite contents in the reservoirs range from 2% to 18% whereas gypsum contents typically are less than 1%. Dolomite and ankerite are the dominant carbonate cements; contents reach up to 28% where the reservoirs are close to evaporites and are less than 5% away from evaporites. Petrographic observations show that gypsum, anhydrite and feldspar were dissolved whereas euhedral dolomite and ankerite crystals show no evidence of dissolution. Water-rock simulation experiments demonstrate that, when subjected to high temperatures and pressures, gypsum reacts with acetic acids to provide Ca2+ for precipitation of carbonates. This is confirmed by anhydrite engulfed in dolomite and ankerite. The isotopic composition of dolomite and ankerite display a narrow range of δ13C values (-4.28‰~-7.23‰) and δ18O values (-15.89‰~-18.38‰). The δ13C VPDB and δ18O VPDB of the carbonates cements reveal that carbon was probably derived from decarboxylation of organic acid as well as from the dissolution of carbonate clasts. According to the fractionation equation of Matthews (1977), precipitation of the carbonates would have occurred at temperature of 99.8~122.9 degrees centigrade, which are in agreement with the inference that dolomite and ankerite were derived from the dissolution of anhydrite and gypsum. Dissolution pores of feldspars and overpressure fractures are the dominant reservoir spaces. Implications for sandstone reservoir quality are: 1) large volumes of water were released into the reservoirs when gypsum transformed to anhydrite resulting in generation of overpressures and protection for pore spaces; 2) gypsum and anhydrite reacted preferentially with organic acids and provided Ca2+ for precipitation of carbonate cements in adjacent reservoirs; 3) abundant intercrystalline microfractures and dissolution pores coupled with the presence of hydrocarbon inclusions indicate that evaporitic rocks acted not only as seals but were once the pathways for hydrocarbon migration or formed hydrocarbon reservoirs.