Why is it easy to form high-quality reservoirs in a mixed siliciclastic-carbonate system? Evidence from diagenetic characteristics

Mixed siliciclastic-carbonate reservoirs are considered as new important high-quality exploration targets because giant hydrocarbon accumulations have continually been discovered in these reservoirs in continental rift basins. This study used a combination of reservoir geological and geochemical evidence to investigate the characteristics and formation mechanisms of mixed siliciclastic-carbonate reservoirs in the first and second members of the Shahejie Formation in the Bozhong Sag, Bohai Bay Basin, China. The results show that bioclastic-dominated mixed rocks represent high-quality reservoirs because they did not only preserve a amount number of primary pores but also preserved a fair proportion of secondary pores that contain organic matter fabrics or associated secondary pores. The main diagenetic processes recognized in the mixed sediment reservoirs are micritization, cementation, dissolution and compaction. Micritization in the contemporaneous stage, dolomitization in the penecontemporaneous stage and leaching of meteoric freshwater in the early diagenetic process are the main controlling factors leading to the formation of high-quality reservoirs. The micritic envelope effectively resists compaction, preserving a high amount of primary porosity, and the early leaching of meteoric freshwater can produce a large amount of secondary dissolution pores. This study does not only reveal the genetic mechanism of high-quality reservoirs of mixed rocks in continental basins but also provide significant guides that can be utilized to search and produce from new types of high-quality reservoirs.

Automated summary

This study investigates the characteristics and formation mechanisms of mixed siliciclastic-carbonate reservoirs in the Bozhong Sag, Bohai Bay Basin, China, using a combination of reservoir geological and geochemical evidence. The results show that bioclastic-dominated mixed rocks represent high-quality reservoirs due to the presence of primary and secondary pores. The main diagenetic processes include micritization, cementation, dissolution, and compaction.