Characteristics of rock types and exploration significance of the shale strata in the Chang 73 sub-member of Yanchang Formation, Ordos Basin, China

Abstract

For a long time, the Chang 73 sub-member of the Ordos Basin was mostly investigated as a single source rock, with hardly any investigation into the characteristics of various rock types or the integration of source and reservoir. Based on correlation analysis of lithology and physical properties combined with a fine interpretation of logging, the characteristics of the Chang 73 sub-member are discussed in detail, and the reservoir characteristics and exploration significance of different types of rocks are analyzed. The results show that: (1) There are five types of lithology in the Chang 73 sub-member: black shale, dark mudstone, fine sandstone, siltstone, and tuff. (2)The storage space of sandy rocks is primarily made up of feldspar dissolution pores and residual intergranular pores, which are held together by rigid grains. The sandstone with high feldspar and quartz mineral content, low carbonate cementation, low clay mineral content, and a large accumulated thickness is the optimal shale oil reservoirs of Chang 73 sub-member. (3) The Chang 73 sub-member has the most organic-rich shale deposits. Clay mineral intercrystalline pores, pyrite intercrystalline pores, and clastic grain intergranular pores make up the majority of the reservoir space. Although organic matter pores are largely undeveloped, organic matter contraction fractures are prevalent. Despite the porosity and pore size of shale in the Chang 73 sub-member being microscopic, it possesses specific reservoir performance and hydrocarbon fluid mobility. It is estimated that the total amount of retained hydrocarbon resources in Chang 7 Member shale is about 15 × 109 t, with movable hydrocarbon resources of about 6 × 109 t; (4)The oil saturation of Chang 73 sub-member is generally higher than that in Chang 71 and Chang 72 sub-members. Several factors play a key role in rapidly increasing shale oil production, including further optimizing the vertical and plane distribution characteristics of sand bodies, improving the drilling encounter rate of sandstone in horizontal wells, strengthening the research of pilot production scheme, and ensuring sufficient oil-water infiltration and replacement. (5) Optimizing black shale with high brittleness index and oil content, developing high-efficiency permeability improvers, adopting pressure drive production integrated design, increasing fracture width and height, and improving the conductivity of artificial fractures are all important technical means to realize shale oil production.

Keywords

• Chang 73 sub-member
• Shale strata
• Rock type
• Exploration significance
• Ordos Basin