Abstract With the increasing global demand for oil and gas, exploration efforts have extended to technically challenging ultra-deep reservoirs. The Jurassic to Lower Cretaceous strata along the southern margin of the Junggar Basin have been verified as containing substantial hydrocarbon resources. However, the development of these reservoirs presents significant technical obstacles. While traditional logging indicates very low porosity, initial exploration phases have demonstrated notable production rates, suggesting that conventional evaluation techniques may underestimate their potential. Accurate reservoir assessment and a comprehensive understanding of geological features are therefore critical to the effective evaluation and utilization of these ultra-deep formations. This study presents a systematic reservoir evaluation framework that integrates a range of advanced logging technologies, including conventional logging, nuclear magnetic resonance (NMR), borehole micro-resistivity imaging, and array sonic imaging. This integrated methodology is designed to deliver a robust assessment of reservoir quality and to address the unique challenges associated with evaluating ultra-deep reservoirs. Borehole micro-resistivity and sonic imaging techniques facilitate the detection and analysis of fractures extending from the borehole into the surrounding formation. Fracture effectiveness is assessed using natural fracture classification, quantitative determination of fracture attributes, correlation analysis between fracture orientation and principal stress directions, and evaluation of fracture propagation distances. These processes collectively enhance the understanding of fracture contributions to reservoir fluid conductivity. In addition, analyses of porosity and pore structure provide valuable insights into the reservoir matrix. NMR T2 spectrum interpretation reveals high-porosity features associated with large-pore distributions within the formation, enabling quantitative assessments of matrix properties. Non-electrical methods for fluid identification are also incorporated to overcome limitations inherent in traditional resistivity logging. Comparative density-NMR analysis and two-dimensional NMR response characterization identify gas as the dominant component in free fluid pores, thereby improving the accuracy of hydrocarbon potential evaluations. The findings demonstrate that borehole micro-resistivity imaging enables effective detection of fractures within the borehole, while sonic imaging verifies fracture presence beyond the wellbore. This comprehensive approach allows for the integrated characterization of fracture attributes in conjunction with petrophysical property evaluation, substantiating the development prospects of these ultra-deep reservoirs. Well testing data demonstrate increased gas production rates, thereby reinforcing the relevance and economic feasibility of this approach for ultra-deep tight reservoir development. Additionally, the outlined evaluation framework offers essential guidance for fracturing and perforation procedures through comprehensive analysis of fracture propagation and efficiency, facilitating the optimization of strategies for improved reservoir development. This methodology ultimately reduces economic risks associated with suboptimal fracturing, enhances gas recovery, and contributes to superior reservoir performance, supporting the ongoing advancement of ultra-deep hydrocarbon reservoirs.
Gan et al. (Mon,) studied this question.