Differentiation and effect on ultradeep clastic reservoirs of early carbonate cements: Insights from the Cretaceous in Kuqa depression, northwestern China

Early carbonate cement is common and widely distributed in clastic rocks and has a complex impact on the quality of clastic reservoirs. At present, although there are studies indicating that early formation of carbonate cement can protect reservoirs to a certain extent, there is no empirical evidence to prove it. Early carbonate is the most abundant cement in the Cretaceous continental ultradeep clastic sandstone in the Kuqa depression. The petrographic characteristics, spatial distribution, and geochemical characteristics of early carbonate cements in the Cretaceous sandstone reservoir are systematically investigated to discuss the origin, distinct distribution, and multifaceted impact on ultradeep clastic reservoirs. During the early Paleogene, meteoric water leached the Paleogene carbonate rocks, providing material for early carbonate cements. Affected by lateral recharge of meteoric water, the vertical recharge of seawater, and the normal-graded sequence, early calcite and dolomite cements with significantly spatial different distribution characteristics were precipitated in the ultradeep clastic reservoirs. Due to the differences in the content and compaction resistance of early calcite and dolomite, their impact on reservoir quality is opposite. Early calcite occupies the reservoir space to varying degrees and has weak compaction resistance, seriously damaging the reservoir quality. Early dolomite has a relatively low content and strong compaction resistance, which can effectively protect the reservoir during deep burial and strong lateral compression processes. This study provides a good example of the positive effect of early carbonate cements on the formation and development of high-quality reservoirs.