Influence of fractures in tight sandstone oil reservoir on hydrocarbon accumulation: A case study of Yanchang Formation in southeastern Ordos Basin

The southwestern Ordos Basin is located at the junction of several stable plates, where faults and fractures are relatively developed, and the influence of fractures on the distribution of tight oil reservoirs in the Yanchang Formation is not clear. To solve this problem, the characteristics of fracture development of the Yanchang Formation, southwestern Ordos Basin and its control on hydrocarbon accumulation have been systematically studied using the core, thin section, well logging, productivity, fault and sand body distribution data. The results show that vertical and horizontal bedding fractures are much more developed in the Chang 8 Member compared with the Chang 6 and 7 Members. For horizontal bedding fractures, they are mainly developed in fine sandstone, followed by siltstone, while no horizontal bedding fractures are observed in medium sandstone. This is because horizontal bedding fractures are more common in fine-grained sediments. For vertical fractures, they are also mainly developed in fine sandstone. The controlling factors of fractures include lithology, sand body thickness, sedimentary microfacies and fault–fracture coupling relationship. Fractures are well developed in the fine-grained sandstone of the wing parts of the main river channel due to small compacted space. In the area where multiple river channels intersect, the sand is pure and easy to break. Based on the comprehensive study of sedimentation, structure and fracture, the classification criteria of sweet point reservoir of the Yanchang Formation are determined: sedimentary microfacies of the main river channel and its wing, tectonic location within 1.5 km from the main fault, plane arrangement of right-lateral and right-order faults and developed fractures. The sweet point reservoir can be identified effectively using the developed sweet point screening criteria.