Diagenesis and reservoir quality of Paleocene tight sandstones, Lishui Sag, East China Sea Shelf Basin

The Lishui Sag in the East China Sea Shelf Basin (ECSSB) has significant potential hydrocarbon resources, with Paleocene sandstones as its main oil and gas-bearing reservoir. The sandstones in the Paleocene formations are mainly composed of medium-grained, moderately sorted litharenite. In this study, distributions of the porosity and permeability reveal that the Paleocene sandstones have the characteristics of medium porosity and low permeability. The sedimentary microfacies, primary texture, diagenesis, and bioturbation jointly control the reservoir quality. Sandstones with distributary channel and sand bar sedimentary microfacies as well as better sorting tend to exhibit a higher quality. Compaction and carbonate cementation are the major determinants to reductions in porosity and permeability. Grain-coating chlorites within a certain content range may play a positive role in the preservation of porosity in eodiagenesis. Early dissolution has a constructive effect on the reservoir quality. In contrast, the dissolution products such as kaolinite and dawsonite cannot move over a long distance and precipitate nearby to block the intergranular pores when reservoir closure increases. Therefore, dissolution in a relatively closed geochemical reservoir system may contribute little to porosity. Contrastingly, the permeability of the Paleocene reservoir may be reduced by the conversion of the pore network. In the Lishui Sag, the bioturbation plays a role in the deterioration of reservoir quality in a limited scale by decreasing reservoir sorting and increasing compaction.