Abstract
The characteristics and formation mechanisms of the mixed siliciclastic–carbonate reservoirs of the Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and scanning electron microscopy observations, X-ray diffractometry, carbon and oxygen stable isotope geochemistry, and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type I and Type II mixed siliciclastic–carbonate sediments, and the dominant pore types include residual primary intergranular pores and intrafossil pores, feldspar dissolution pores mainly developed in Type II sediments. Type I mixed sediments are characterized by precipitation of early pore-lining dolomite, relatively weak mechanical compaction during deep burial, and the occurrence of abundant oil inclusions in high-quality reservoirs. Microfacies played a critical role in the formation of the mixed reservoirs, and high-quality reservoirs are commonly found in high-energy environments, such as fan delta underwater distributary channels, mouth bars, and submarine uplift beach bars. Abundant intrafossil pores were formed by bioclastic decay, and secondary pores due to feldspar dissolution further enhance reservoir porosity. Mechanical compaction was inhibited by the precipitation of pore-lining dolomite formed during early stage, and oil emplacement has further led to the preservation of good reservoir quality.
Highlights
In addition to carbonate and clastic reservoir rock types, magmatic, metamorphic, shale and mixed siliciclastic– carbonate sedimentary reservoirs can be considered as important targets for oil and gas exploration and development (Ge et al 2011; Tong et al 2012; Xiao et al 2015; Palermo et al 2008)
The E2s high-quality mixed siliciclastic–carbonate sedimentary reservoirs in the central Bohai Sea were deposited in a fan delta-lacustrine environment
The development of the highquality reservoirs of Class I siliciclastic–carbonate sediments was mainly controlled by a high-energy depositional environment, high bioclastic content and pore-lining dolomite and hydrocarbon charging in the early stage
Summary
In addition to carbonate and clastic reservoir rock types, magmatic, metamorphic, shale and mixed siliciclastic– carbonate sedimentary reservoirs can be considered as important targets for oil and gas exploration and development (Ge et al 2011; Tong et al 2012; Xiao et al 2015; Palermo et al 2008). Many Chinese and foreign scholars have made in-depth studies of the formation mechanisms of this type of sediment and suggested that it can be developed in both marine and lacustrine environments. Influenced by sea (lake)-level fluctuations, structural changes, storm, current and tidal actions, mixed siliciclastic–carbonate sediments are widely distributed in transitional marine-terrestrial, continental shelf, and slope environments (Garcıa-Hidalgo et al 2007; Zonneveld et al 2012). Under certain conditions, mixed siliciclastic–carbonate sediments may be rich in oil and gas.
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