Depositional and diagenetic controls on deeply buried Cambrian carbonate reservoirs: Longwangmiao Formation in the Moxi–Gaoshiti area, Sichuan Basin, southwestern China

Abstract

This study is about factors controlling dolostone reservoir quality, and the material studied is cores, mainly from two wells, in deeply buried Early Cambrian Longwangmiao Formation in a recently discovered gas field in the Sichuan Basin. The vertical distribution of porosity in the formation was highly related to the presence of grainy lithofacies in manifold cycles and dissolution vugs, which in turn were controlled by multiple-order of sea-level fluctuations and paleotopography. The transgressive sections have very low porosity values, whereas the regressive sections demonstrate significantly better porosities. Pore networks are dominated by vugs, intercrystalline pores, and residual intergranular pores. The major diagenetic processes that affect reservoir quality include cementation, dissolution, dolomitization, and emplacement of bitumen. Primary intergranular pores were substantially decreased by marine and burial cementation. Early freshwater-related dissolution produced predominant vugs and locally enhanced intergranular pores, a key factor controlling the distribution of porosity. Intercrystalline pores are interpreted to be inherited from precursor rocks by rearrangement during dolomitization. Most pores were occluded partly, largely or completely by bitumen in the latest diagenetic stage, resulting substantial reduction in porosity. This work contributes to a better understanding of processes and controlling factors that have affected the Longwangmiao reservoir property, which may serve as an analog for many dolostone reservoirs worldwide, especially other Infra-Cambrian (Neoproterozoic–Early Cambrian) carbonate reservoirs.