Burial evolution of evaporites with implications for sublacustrine fan reservoir quality: A case study from the Eocene Es4x interval, Dongying depression, Bohai Bay Basin, China

Abstract

Eocene, sublacustrine-fan, sandstones that developed in a rift basin are important tight gas reservoirs in the Dongying Depression, Bohai Bay Basin, northeastern China. Two units of evaporites, developed at the top and bottom of the lower unit of the Es4 interval (Es4x), consist predominantly of anhydrite with subordinate gypsum. Evaporite and related diagenetic processes greatly influenced reservoir quality. δ13C values for micritic dolomite cements in Es4x are depleted (−7.45 to −2.57‰) due to microbial sulfate reduction (MSR) under shallow burial conditions and this interpretation is supported by large δ34S fractionation between anhydrite and framboidal pyrite. Precipitation temperatures for micritic dolomite are calculated as 57.5–72.8 °C. Anhydritization of gypsum probably occurred at 100.5–145.2 °C during progressive burial as evidenced by homogenization temperatures of aqueous inclusions within anhydrite cements. This process resulted in dehydration fractures within anhydrite cements that increased reservoir permeability by connecting isolated pores. Thermochemical sulfate reduction (TSR) probably resulted in dissolution of gypsum and anhydrite cements under relatively deep burial conditions. Ankerite cements are replaced by anhydrite cements and are enclosed by solid bitumen in Es4x. Ankerite cements likely were derived from TSR as reflected in negative δ13C values (−7.12 to −3.70‰) and high calculated temperatures (121.3–185.1 °C). Dissolution by-products (e.g. saddle dolomite, ankerite, nodular pyrite) related to TSR precipitated in adjacent pores. A lack of significant δ34S fractionation between parent sulfate and nodular pyrite indicates that TSR occurred in a relatively closed system. Therefore, dissolution of gypsum and anhydrite related to TSR contributed little to reservoir quality. Middle-fan lithofacies with better sorting, porosity and permeability than inner- and outer-fan lithofacies constitute high-quality reservoirs in Es4x.