The lithofacies and reservoir characteristics of the fine-grained sedimentary rocks of the Permian Lucaogou Formation at the northern foot of Bogda Mountains, Junggar Basin (NW China)

Abstract

The northern foot of Bogda Mountains in Junggar Basin, Northwest China hosts thick organic-rich fine-grained sedimentary rocks in the Permian Lucaogou Formation consisting of the lower member and upper member. The lithofacies and reservoir characteristics of the fine-grained sedimentary rocks of Lucaogou Formation here are investigated. Analytical results show that the major lithofacies of lower member and upper member are carbonate fine-grained sedimentary rocks and felsic fine-grained sedimentary rocks, respectively. The study areas were the deep lake environments with weak hydrodynamic energy, and the distance from source areas is long, the terrestrial clastics input is low during the deposition of lower member period. However, the study areas were the semi-deep lake environments with strong hydrodynamic energy, the distance from source areas is short and the terrestrial clastics input is high during the deposition of upper member period. The reservoir space types of Lucaogou Formation can be divided into the effective pore reservoir space (primary intergranular pore, intercrystalline pore and dissolved pore), effective fracture resrvoir space (structure fracture, lamellation fracture and abnormal pressure fracture) and ineffective reservoir space (intracrystalline pore and organic pore), respectively. Dissolved pore, structure fracture, lamellation fracture and abnormal pressure fracture are the major effective reservoir space types of No. 1 tight oil layer. Furthermore, the major effective reservoir space types of No. 2 tight oil layer are primary intergranular pore, intercdrystalline pore, lamellation fractures and structure fracture. However, the major effective reservoir space types of No. 3 tight oil layer are structure fracture and lamellation fracture.