The joint controls of tectonism, sedimentation and diagenesis on reservoir quality in the East China Sea Basin

Abstract

Variation in reservoir quality is especially important in low-permeability and tight sandstone reservoirs. By using log, seismic and core test data, we developed a method that uses tectonism, sedimentation and diagenesis to evaluate sweet spot distribution of the third (Mbr 3), fourth (Mbr 4) and fifth (Mbr 5) members of the Oligocene Huagang Formation in the Xihu Depression, East China Sea Basin. Sweet spot maps of the Mbr 3, Mbr 4 and Mbr 5 sandstone reservoirs were compiled by superposing tectonic locations, depositional microfacies and diagenetic facies. Sweet spots were classified into three types. Mbr 3 exhibits type I and type II sweet spots, Mbr 4 exhibits type I, type II and type III sweet spots, and Mbr 5 exhibits type II and type III sweet spots. In the Oligocene, tectonism and climatic controlled the depositional microfacies, sandstone grain size and clastic composition of the Mbr 5, Mbr 4 and Mbr 3 members, which were deposited in a braided river delta setting that ranged from delta-front distal part to proximal part. From late Oligocene to present, along with depositional microfacies, diagenetic stage, diagenetic facies and reservoir properties, the joint control of tectonism, sedimentation and diagenesis determined the distribution of diagenetic facies, and was used to develop a synthetic pattern for reservoir quality variation.