Development characteristics and controlling factors of fractures in deep-buried tight oil reservoirs of the 3rd member of Paleogene Hetaoyuan Formation in southeast An'peng area, Nanxiang Basin

Abstract

To elucidate the development patterns and influencing factors of natural fractures in deep-buried tight oil reservoirs, a comprehensive evaluation was conducted using a large amount of core samples, thin section, physical property data, imaging and conventional logging, water injection pressure testing and other data. The focus was on the tight oil reservoirs within the Ⅱ-Ⅵ oil layers of the third member of the Paleogene Hetaoyuan Formation in the southeastern An'peng area of the Biyang Depression, Nanxiang Basin. These oil formations, deposited in fan-delta front environment, are characterized by a high content of rock debris, indicating proximal deposition. A strong positive correlation between reservoir porosity and permeability was observed. Among the various sandstone lithologies, fractures predominantly developed in fine sandstone, followed by siltstone, while gravelly sandstone generally lacked fractures. High-angle and vertical fractures were predominant, constituting 87.8% of the total, while low-angle oblique and horizontal fractures accounted for 7.3% and 4.9%, respectively. The main controlling factors for fracture development in these tight reservoirs included lithology, depositional microfacies, and local structures. Thin and fine-grained single or composite sand bodies typically had more deve-loped fractures, particularly in front channel, channel flank, mouth bar, and outer edge of distal bars. Conversely, fractures were less developed in sheet sands or delta front microfacies. Moreover, fractures primarily formed at structural inflection points, predominantly at the tops and wings of forward structures and were primarily oriented along the WE and NE directions, followed by the NW direction. These fractures predominantly formed during the Neogene depression period (late Himalayan). Fractures significantly influence water channeling in tight oil reservoirs, necessitating enhanced dynamic and static monitoring of the degree, extent, and orientation of fracture development.

Keywords

• fracture
• tight oil
• hetaoyuan formation
• paleogene
• southeast an'peng area
• biyang sag
• nanxiang basin