Abstract:

Mixed siliciclastic-carbonate sediments （MSCSs） occur extensively in the 1^st to 2^nd members of the Shahejie Formation （Es_1-2）， Zhuanghai area， Bohai Bay Basin. Over recent years， breakthroughs have been constantly achieved in hydrocarbon exploration in the MSCSs in the Zhuanghai area， making these rocks gradually become significant exploration targets of the Paleogene strata. Using techniques such as core observation， thin section observation， scanning electron microscopy （SEM）， and porosity and permeability analyses， we investigate the characteristics of MSCS reservoirs in the Zhuanghai area and the factors controlling the formation of high-quality reservoirs within. The results indicate that terrigenous clastic grains in the MSCSs in the Zhuanghai area are dominated by proximal sediment and exhibit lowmaturity in rock component. Rock debris originating from the Changdi uplift is dominated by intermediate-acidic extrusive rock detritus， while that from the Chengdao uplift consists primarily of the detritus of metamorphic rocks such as quartzites. Using terrigenous clasts， chemogenic micritic carbonates， and biogenic carbonate grains as three end-members， the MSCSs in the Zhuanghai area are categorized into four types： terrigenous clast-dominated MSCSs （terrigenous clast content： 50 % to 85 %）， chemically precipitated carbonate-dominated MSCSs （chemically precipitated carbonate content： 50 % to 90 %）， ortho-MSCSs （contents of the three components： all less than 50 %）， and bioclast-dominated MSCSs. The reservoirs of bioclast-dominated MSCSs exhibit the most favorable physical properties among others， with an average porosity ranging from 15 % to 25 % and storage spaces composed largely of primary intergranular and bioclastic pores. Micritization and the sparry cementation on the rims are the most developed in the bioclast-dominated MSCSs， with the rigid framework formed by both allowing for the preservation of primary pores. Meteoric water leaching and organic acid charging are conducive to the formation of secondary pores in the MSCS reservoirs. Carbonate cementation with poikilitic textures is the most developed in the ortho-MSCSs， while continuous compaction produces the most significant impacts on the chemically precipitated carbonate-dominated MSCSs. Favorable conditions for the formation of high-quality MSCS reservoirs include （1） the high rock-texture maturity and bioclastic content in the depositional period； （2） the development of micrite coats， meteoric water leaching， and the formation of the sparry cements around the rims during the penecontemporaneous period， and （3） strong organic acid dissolution during the burial and diagenetic period.

Key words: diagenesis, Es_1-2, reservoir of mixed siliciclastic-carbonate sediments (MSCSs), Paleogene, Zhuanghai area, Bohai Bay Basin