Development model of intrusive rocks and its influence on shale oil exploration in Gaoyou Sag, Subei Basin

The study takes the widely developed Paleogene diabase of the Shanian to Huazhuang area in the central part of the northern slope in the Gaoyou Sag, Subei Basin, as the research object. Based on core observations and experimental analysis, integrated with comparisons of logging curves and seismic response characteristics, and combined with shale oil exploration practices in the second member of the Paleogene Funing Formation (E1f2) in the Huazhuang area, the study systematically analyzed the development model of intrusive rocks and discussed its influence on the enrichment and accumulation of shale oil. The results showed that: (1) Multiple phases of magmatic emplacement developed during the Paleogene in the Gaoyou Sag, and large-scale magmatic activities mainly occurred under the strong extensional background of the Eocene. (2) Magma mainly rose into the basin in the form of dikes through weak zones between the deep sag and the slope zone. Subsequently, part of the magma expanded laterally within weak layers of the Funing Formation, forming large, continuous sills through multi-phase superposition. Another portion intruded upward to the vicinity of the unconformity resulting from the Wubao Movement and into the overlying Paleogene Dainan and Sanduo formations, forming several smaller, lobate sills. Together, these features constituted a vertical dike, multi-level sill, andmulti-phase emplacement development model. (3) The main period of magmatic activity was consistent with the hydrocarbon-generation stage of E1f2. Magmatic emplacement exerted complex effects on hydrocarbon generation and evolution, reservoir space development, and shale oil preservation in organic-rich shales. (4) Intrusive rocks significantly increased the maturity of the intersected shales and generally increased shale porosity by more than 5% within a range of about 10 to 15 m around the intrusions, collectively lowering the lower limit of recoverable shale oil. (5) Low-angle sills confined within E1f2 had little impact on shale oil preservation conditions, whereas inclined sills cutting through E1f2 and large-scale dikes associated with shallow sills disrupted shale oil enrichment. Small, isolated dikes with widths less than 10 m did not cause obvious damage to shale oil accumulation, but during fracturing transformation, they tended to induce fractures to extend over long distances along the dikes.