Characterization of lacustrine carbonate reservoirs in the Eocene Sikou Sag, Bohai Bay Basin, East China

Abstract

With the recent discovery of new oil fields around the Bohai Bay Basin, a better understanding of the origin of its high-quality lacustrine carbonate reservoirs has become important. This paper investigates marginal lacustrine carbonates of the upper fourth member of the Eocene Shahejie Formation (Es4s) around the Sikou Sag, using core analysis, thin section petrology and geochemical data. The δ13C and δ18O values of the Sikou carbonates differ significantly from Eocene marine values, the 87Sr/86Sr ratio of carbonate minerals (0.710210–0.710844) is higher than Eocene marine water and same to the surrounding Palaeozoic carbonate bedrock and the distribution of the carbonates is controlled by the surrounding Palaeozoic carbonate bedrock, all of which indicate their lacustrine origin. The main facies associations are reef mound, shoal facies (including low-energy and high-energy shoals), higher salinity central-lake deposits, and fan delta-related deposits. The pore types are classified into four categories: depositional, fabric-selective, non-fabric selective, and fracture. The diagenetic processes improving reservoir quality are dissolution, dolomitization and fracture development. Reservoir quality of the carbonates was decreased by burial calcite cementation and compaction. High quality reservoirs in the basin are mostly distributed in dolomitzed bindstones and grainstones, deposited in the reef mound and shoal facies of shallow lakes. Owing to karstification, a high quality reservoir zone also occurs in the studied section beneath the top unconformity. Variation in reservoir quality in the Sikou Sag is due to a combination of lithofacies type, dissolution intensity, early dolomitization and tectonic related deformations. Proper lithofacies type controlled the primary porosity. Restricted depositional facies and a high-frequency cyclic depositional architecture of the strata caused lateral and vertical reservoir heterogeneity. Dissolution produced secondary pores and caused occlusion of some porosities in the rocks. In addition, tectonic deformation produced some fractures, as well as karstification is associated with tectonic uplift.