Shale petrological and reservoir characteristics of the Upper Permian Linxi Formation in the northern Songliao Basin

The Upper Paleozoic shale represents a critical potential target for unconventional oil and gas exploration in the Songliao Basin. However, existing research has predominantly focused on the peripheral regions of the basin, while evaluation of the basin's interior has been severely hindered by a lack of key data. To address this gap, this study focuses on Well Minggu-1 in the Central Depression and provides a comprehensive assessment of the shale gas potential of the Linxi Formation from four key aspects: hydrocarbon generation potential, reservoir properties, gas-bearing characteristics, and fracability, based on systematic mineralogical, geochemical, rock mechanical, and gas content data. The results indicate that the shale was deposited in a freshwater, shallow-water, reducing environment within a continental arc setting, with felsic igneous rocks as the primary provenance. With a thickness exceeding 200 m, it possesses a fair to good source rock foundation (TOC: 0.37%–1.46%; average 0.88%) and overmature gas-generation conditions (Ro: 2.42%–3.32%). In terms of reservoir quality, the shale exhibits moderate brittle mineral content (26.1%–55.3%) and relatively high clay content (29.5%–48.0%), resulting in an overall moderate-to-poor fracability; however, certain deeper intervals demonstrate superior mechanical properties. Gas logging data reveal persistently high total hydrocarbon content below 2000 m, stabilizing above 0.1% (against a background value below 0.05%) and peaking at 0.23%, confirming significant gas-bearing potential. The methane adsorption capacity (average excess adsorption capacity: 2.67 cm 3 /g) is comparable to that of renowned shale gas systems such as the Longmaxi Shale and Barnett Shale, indicating favorable gas storage capability. Comprehensive analysis suggests that the Linxi Formation shale holds significant exploration potential. Notably, the interval from 2065 to 2095 m is identified as the most valuable "sweet spot", characterized by high organic matter abundance (peak TOC: 1.46%), high brittle mineral content (>60%), high thermal maturity, and favorable gas content, making it a priority target for further exploration.