Diagenetic controls on pore structure variations in fan deltaic sandstones of the Eocene Shahejie Formation, Bohai Bay Basin, China: Implications for reservoir quality and oiliness heterogeneities

Abstract

The fan deltaic sandstones in the third section of the third member of the Eocene Shahejie Formation (Es33) are important hydrocarbon exploration targets in the northern Nanpu sag, Bohai Bay Basin, China. The Es33 sandstones are characterized by highly variable pore structures. A total of 364 Es33 sandstone samples were measured by a series of petrological and petrophysical analyzes to investigate these variations and their controls on reservoir quality and oiliness heterogeneities. The heterogeneity of Es33 sandstone can be manifested by type I, type II and type III pore structures. Primary intergranular pores and dissolution pores were the main storage spaces for type I sandstones, while dissolution pores and intercrystalline pores were the main storage spaces for type II and type III sandstones, respectively. These results indicate that compaction is the main factor in the loss of primary intergranular porosity, with an average loss of 78%. The grain size controlled the preservation of primary pores in Es33 sandstones with a carbonate cement content of less than 6%. The different types of clay minerals have various effects on the pore structure. Chlorite plays an active role in the preservation of intergranular pores and the improvement of reservoir quality, while mixed layers of illite and smectite greatly reduced the pore size and connectivity and lowered the reservoir quality. The Es33 sandstones have undergone a more open diagenetic system. Dissolution significantly increased the porosity of type I and type II sandstones with more primary intergranular pores, but exerted a minor role in contributing to the permeability. Furthermore, a pore‐throat radius of approximately 0.4 μm is considered to be the threshold for oil accumulation in Es33 fan deltaic sandstones.

Pore Structure Characteristics and Main Control Factors of Sandstone in the Jurassic Zhiluo Formation in the Northern Ordos Basin

“…The main controlling factors of sandstone pore structure have been studied by scholars. Wu et al [33] believe that diagenesis is the main factor controlling the pore structure of Shahejie Formation sandstone in the Bohaiwan Basin, Wang et al [34] believe that lithology and diagenetic fluid jointly control the development of secondary pores in Benxi Formation sandstone, and Feng et al [35] believe that compaction is the main factor controlling the pore structure of sandstone reservoir.…”