Characteristics and distribution of sweet spots in ultra-deep basement reservoirs in compressional tectonic settings: insights from the Northern Qaidam Basin

Ultra-deep basement gas reservoirs typically exhibit complex lithology, strong heterogeneity, and a lack of unified classification criteria, posing significant challenges for high-quality reservoir prediction. This study focuses on the Devonian basement of the T Block in the Kunteyi Gas Field, Qaidam Basin, at depths exceeding 6000 m. An integrated approach combining thin-section petrography, petrophysical measurements, image logging, gas productivity tests, principal component analysis, wave impedance inversion, and co-kriging modeling was employed to systematically characterize the reservoir and predict sweet spot distribution. The basement is dominated by low-potassium granitic gneiss and plagioclase gneiss, with reservoir space comprising a dual-porosity system of tectonic fractures and dissolution pores. Matrix porosity averages ~ 4% (1–6%), and fracture density typically ranges from 3 to 4 fractures/m (0.5–6 fractures/m), with a dominant NE 30° strike nearly orthogonal to the regional NW-trending major faults. A three-tier reservoir classification scheme (Types I-III) based on lithology, porosity, and fracture intensity correlates strongly with gas productivity. Sweet spots were quantitatively predicted using thresholds of porosity ≥ 3%, fracture density ≥ 4 fractures/m, and gas saturation ≥ 50%. The sweet spots exhibit distinct vertical zonation: they form large, NE-trending continuous zones in the 0–100 m interval; migrate to structural flanks as arcuate belts in the 100–200 m interval; disintegrate into isolated patches in the 200–300 m interval; and diminish to scattered remnants below 300 m. Notably, sweet spots are predominantly distributed within 200–500 m of major faults, representing a critical equilibrium window where fracture intensity is sufficient for storage and flow, yet fault damage is not severe enough to compromise gas preservation. This study provides a quantitative evaluation framework and a practical fault-offset criterion for sweet spot prediction in ultra-deep basement gas reservoirs in compressional tectonic settings.