Fracture characteristics and hydrocarbon significance in the Chang 7 to Chang 9 members of Triassic Yanchang Formation of Gufengzhuang area, western Ordos Basin

Natural fractures play an important role for the exploration and development of low-permeability reservoirs in the Triassic Yanchang Formation of the Ordos Basin. Abundant hydrocarbon resources have been found in the Yanchang Formation of the Gufengzhuang area, showing good exploration prospects. However, only limited research results on fractures have been obtained, and the distribution law and active period are both unclear. Based on the analyses of core, imaging logging, carbon-oxygen isotope, homogenization temperature of fluid inclusion, acoustic emission, tectonic stress and fracture activity, the characteristics, formation stages and causes of natural fractures in the Chang 7 to Chang 9 members of the Yanchang Formation in the Gufengzhuang area were studied, and corresponding indications for petroleum accumulation were discussed. Natural fractures are well-developed in the Chang 7 to Chang 9 members of the Yanchang Formation, especially in the Chang 7 member. High-angle and vertical fractures are ubiquitously developed, mainly of NEE orientation, followed by NE, and finally NW and NWW. The fractures are generally unfilled or semi-filled by calcites, providing effective pathways for hydrocarbon migration. The fracture surface is flat, and obvious traces of oil migration such as oil invasion, oil stain and oil spot can be observed. The average values of oxygen isotopes of the Chang 7 to Chang 9 members are -15.8‰, -17.5‰ and -18.7‰, indicating that the factures were formed at depth of 1 675, 2 338 and 2 785 m. According to the homogenization temperature peaks of fluid inclusions of 90-100 ℃ and 120-130 ℃, the formation depth of fractures is 1 750-2 000 m and 2 500-2 750 m. The fractures in the Chang 7 to Chang 9 members were active during multiple stages, including the Indosinian, Yanshanian and Himalayan stages, with the Yanshanian and Himalayan as the main active stages. Effective fractures developed in multiple stages have improved the physical properties of low-permeability reservoirs in the study area. They provided important migration channels and storage spaces for the formation of multi-layer and deep-layer reservoirs such as Chang 7, Chang 8 and Chang 9, and affected the productivity of oil wells.