Determination of in-situ stress orientation and subsurface fracture analysis from image-core integration: an example from ultra-deep tight sandstone (BSJQK Formation) in the Kelasu Belt, Tarim Basin

Abstract

Present-day stress state is significant for petroleum industry and prediction of stress-related geohazards. The Kelasu Belt, located between Southern Tianshan and the bank-land of the Tarim Basin, is a main gas source in China and is extremely active during Himalayan movement. Predicting the in-situ stress of the BSJQK Formation in the Kelasu Belt is difficult due to the ultra-deep tight sandstone reservoir (5500–8500m depth), with pervasively developed fractures, surrounded by multi-detachment layers and complicated structural association. In this study, we first analyzed the present-day maximum horizontal stress in the BSJQK Formation using imaging logs (FMI-HD, OBMI, EI and CBIL), then carried out the fracture interpretation based on core-image calibration, finally evaluated their hydraulic conductivity integrating well testing, petrophysical experiments and logging interpretation. The average SHmax orientation in wells B, C, D and E in the Kelasu Belt correspond to 124.76°N (±8.5°), 92.86°N (±4.3°), 143.28°N (±7.1°) and 180.89°N (±4.5°), respectively, which coincides well with the result derived from the focal mechanism solution. The stratigraphic attitude and boundary structure may control the variation of SHmax orientation. Fracture attitude in wells B, C, D and E correspond to 70°/26°N, 54°/346°N, 81°/239°N and 76°/253°N, respectively and intersect SHmax at an angle less than 29°. Fracture could increase formation permeability with 1–3 order of magnitudes and provides the main channel for gas migration. Furthermore, the calculation of “Gas Productivity Index per Meter” indicates that fracture density is closely related with gas productivity, which will guide the next fracturing intervals. The N-S or NEE-SWW could be the preferable orientations for drilling trajectory design of horizontal wells.