Characteristics and main controlling factors of Chang 7 shale oil in Triassic Yanchang Formation, Fuxian area, Ordos Basin

Abstract

The shale in the 7th member (Chang 7) of Triassic Yanchang Formation on the southeastern margin of the Ordos Basin is characterized by shallow burial depth, light oil quality, large variability in oil well production capacity, and significant resource potential. Identifying the main controlling factors of reservoir enrichment is key to efficient exploration. Based on analysis results of cast thin sections, physical properties, and scanning electron microscope (SEM) tests of the Chang 7 shale member in the Fuxian area, combined with core, well logging, and seismic data, its layer characteristics are described and the main controlling factors are discussed. The results show that the Chang 7 shale in the research area extensively develops gray, dark gray, and gray black source rocks. The lithology is mainly mudstone and mud shale, with organic matter types mainly classified as type Ⅰ-Ⅱ2. The vitrinite reflectance (Ro) values of source rocks range from 0.81% to 1.10%, indicating strong hydrocarbon generation potential. The sandstone reservoirs are mainly fine-grained feldspathic sandstone, with pore types primarily consisting of intragranular pores, residual intergranular pores, dissolution pores, and primary intergranular pores. Porosity ranges from 2.0% to 16.0%, and permeability ranges from 0.01×10-3 to 1.20×10-3 μm2, indicating tight reservoirs. The oil-bearing properties of the reservoir are impacted by the physical properties of the sandstone interlayers and proximity to faults: coarser-grained reservoirs with better physical properties exhibit better oil and gas bearing potential. Reservoir properties are controlled by two factors, sedimentary microfacies and diagenesis. Subaqueous distributary channel microfacies have the best properties, followed by mouth bar microfacies. Strong early diagenetic chlorite cementation and weak calcite cementation contribute to the formation of sweet spots in the physical properties. The development and nature of faults play a crucial role in the Chang 7 shale oil enrichment. Statistical analysis shows that it is difficult to obtain industrial oil flow when the fault displacement exceeds 10 m and the wellbore is within 1 km of the fault. However, when the fault displacement is less than 7 m or when the wellbore is more than 1 km away from a large fault, industrial oil flow is more easily obtained.

Keywords

• reservoir physical properties
• diagenesis
• fault
• 7th member of yanchang formation (chang 7)
• triassic
• ordos basin