Controlling Factors of Drilling-Associated Hydrocarbon Content in Shale Reservoirs with Hybrid Condensates in Terrigenous Basins

The production of gas and condensate from liquid-rich shale reservoirs, particularly within hybrid shale condensate systems, has become a central focus in hydrocarbon exploration. However, quantifying the hydrocarbon content in these reservoirs remains challenging due to the in situ occurrence of multiphase liquids—comprising gas (C1–C2), volatile light hydrocarbons (VLH, C3–C7), and the liquid phase (C7+). This complexity is pronounced in terrigenous basins characterized by high clay contents and numerous lacustrine laminae. To investigate the controlling factors governing hybrid shale condensate and gas content in lacustrine settings, the China Geological Survey (CGS) extracted nearly 120 meters of core from the Cretaceous Shahezi Formation (K1sh Fm.) in the Songliao Basin, China. Over 1,000 samples underwent analyses—including rock pyrolysis, desorption gas analysis, elemental analysis, SEM, and Roqscan—to assess shale composition, lithofacies, thermal maturity, hydrocarbon phases, paleo-sedimentary conditions, and lamina-induced fracturing. A novel Hybrid Shale Condensate Index (HSCI) was introduced to delineate the maturity levels of both gaseous and liquid hydrocarbon components. The study identifies four critical factors: (1) in mature to highly mature shales, condensate and gas predominate; (2) argillaceous shales with clay content ranging from 50% to 75%, especially those containing mixed carbonate clays (MC(C)), are pivotal; (3) favorable paleo-sedimentary conditions—static water, volcanic input, humid climate, saline conditions, and reducible lacustrine settings—are essential; and (4) the presence of siliciclastic and pyrite laminae is associated with organic-rich clayey shales. Collectively, these factors delineate the “sweet spot” for hybrid shale condensate reservoirs, thereby enhancing hydrocarbon supply potential.