Classification and reservoir characteristics of lacustrine carbonate conglomerate in the Shulu Sag of the Bohai Bay Basin

Abstract

Carbonate conglomerate in the lower part of the third member of the Eocene Shahejie Formation comprises one of the most important petroleum exploration targets in the Bohai Bay Basin. Based on core observations and thin-section identifications, the carbonate conglomerate is divided into five types in the central area of the Shulu Sag. Reservoir space types are studied by observation through casting thin sections, fluorescent thin sections and scanning electron microscopes. The category of carbonate conglomerate is grouped into five classes: clast-supported extraformational calcirudite (LF1), clast-supported mixed-source calcirudite (LF2), clast-supported intraformational calcirudite (LF3), matrix-supported extraformational calcirudite (LF4) and matrix-supported mixed-source calcirudite (LF5). The mixed-source calcirudites and intraformational calcirudites are relatively rare and consist of gravel-sized micritic intraclasts formed from the intra-basin soft lime mud sediments and terrigenous clasts from the surrounding provenance areas. The conglomerate rocks in the Shulu Sag can be primarily attributed to two genetic types: ‘fan delta formed by alluvial fan and lacustrine’ and ‘seismic-induced slump fan’. The rock types formed by fan deltas mainly include LF1 and LF4. The rock types formed by seismic-induced slump fans are more complex and include LF1, LF2, LF3, LF4 and LF5. The reservoir space consists of various types, including intra-granular pores (e.g. dissolution pores in pyrite and intercrystalline dissolution pores in dolomite), intergranular pores, fractures (tectonic fractures, fractures around grains, diagenetic shrinkage fractures and fractures in the allogenic grains) and organic matter pores. The reservoir spatial distribution is mainly affected by the content of terrigenous grain and matrix, grain composition and fracture density. LF1 exhibits the best reservoir properties. LF2 and LF4 have the second best reservoir quality. They are all excellent tight oil reservoirs in the Shulu Sag.