Yinbo Xu

The production of gas and condensate from liquid-rich shale reservoirs, particularly within heterogeneous lacustrine systems, remains a critical challenge in unconventional hydrocarbon exploration due to intricate multiphase hydrocarbon partitioning, including gases (C₁–C₂), volatile liquids (C₃–C₇), and heavier liquids (C₇₊). This study investigates a 120-meter-thick interval dominated by lacustrine deposits from the Lower Cretaceous Shahezi Formation (K₁sh) in the Songliao Basin. This interval, characterized by high clay mineral content and silicate–pyrite laminations, was examined to identify the factors controlling hybrid shale gas condensate systems. We proposed the Hybrid Shale Condensate Index (HSCI), defined as the molar ratios of (C₁–C₇)/C₇₊, to categorize fluid phases and address shortcomings in traditional GOR/API ratios. Over 1000 samples were treated by geochemical pyrolysis logging, X-ray fluorescence (XRF) spectrum element logging, SEM-based automated mineralogy, and in situ gas desorption, revealing four primary controls: (1) Thermal maturity thresholds. Mature to highly mature shales exhibit peak condensate production and the highest total gas content (TGC), with maximum gaseous and liquid hydrocarbons at T_max = 490 °C. (2) Lithofacies assemblage. Argillaceous shales rich in mixed carbonate and clay minerals exhibit an intergranular porosity of 4.8 ± 1.2% and store 83 ± 7% of gas in intercrystalline pore spaces. (3) Paleoenvironmental settings. Conditions such as humid climate, saline water geochemistry, anoxic bottom waters, and significant input of volcanic materials promoted organic carbon accumulation (TOC reaching up to 5.2 wt%) and the preservation of organic-rich lamination. (4) Laminae and fracture systems. Silicate laminae account for 78% of total pore space, and pyrite laminations form interconnected pore networks conducive to gas storage. These findings delineate the “sweet spots” for unconventional hydrocarbon reservoirs, thereby enhancing exploration for gas condensate in lacustrine shale systems.

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.