High Quality Reservoirs Characteristics and Forming Mechanisms of Mixed Siliciclastic-Carbonate Sediments in the Bozhong Sag, Bohai Bay Basin

Mixed siliciclastic-carbonate reservoir is considered as one of new important high-quality reservoirs because giant hydrocarbon accumulations have continually been discovered in the continental rifting basin, which have attracted great interest of sedimentologist. This study investigates the mixed siliciclastic-carbonate sediments in the first and second Member of the Shahejie Formation in the Bozhong Sag, Bohai Bay Basin. A combination of reservoir geological and geochemical research was used to reveal the characteristics and formation mechanisms of mixed siliciclastic-carbonate reservoirs. The results show that and bioclastic-dominated mixed rocks represent the high-quality property of reservoirs since they do not only preserve a great amount of primary pores, but also preserved an amount of primary pores with organic matter fabrics or associated secondary pores. The main diagenesis recognized in the mixed sediment reservoirs are micritization, cementation, dissolution and compaction. The micritization and precipitation of ctenoid dolomite during penecontemporaneous stage, together with meteoric dissolution during the early diagenetic period are the two key elements to form high-quality reservoirs. The micritic crusts effectively resisted the compaction, preserving high amount of primary porosity. Moreover, secondary pores were well generated by meteoric dissolution. This study on the formation conditions and controlling factors of excellent reservoirs of mixed sediment would not only provide an effective technique for the prediction of high-quality reservoirs, but also facilitate the hydrocarbon exploration at the medium-deep depth in the continental rifting basin.