Source-reservoir assemblages and four-property sweet spots of continental shale oil in the Jurassic Lianggaoshan Formation,eastern Sichuan Basin

【Objective】The lower submember of the second member of the Jurassic Lianggaoshan Formation (lower Liang-2 submember) in the eastern Sichuan Basin is rich in continental shale oil, but the frequent interbedding of sandstone, mudstone and shell limestone produces vertically variable source-reservoir configurations. The differences in the four key properties, namely oil-bearing capacity, reservoir quality, oil mobility and fracability, and their controls remain unclear, which hampers the accurate selection of sweet-spot intervals. 【Methods】Based on core analysis and wireline logs from two cored wells, lithofacies were classified by a three-level coupled scheme of total organic carbon (TOC) content, sedimentary structure and mineral composition, and were grouped into source-reservoir assemblages according to the lithology and scale of the interlayers and their contact relationships with the shale. Free hydrocarbon (S₁), porosity, the oil saturation index (OSI) and brittle-mineral content were used to characterize the four properties and were plotted by assemblage. A sweet-spot index (SSI) was then constructed with combination weights obtained by multiplicatively coupling an improved analytic hierarchy process with the entropy weight method, and was used to rank the sublayers and to delineate sweet-spot intervals along a typical column. 【Results】Eight lithofacies were identified and grouped into four assemblages, namely the continuous-shale, sand-interbedded, shell-interbedded and sand-mud interbedded types. The four properties vary systematically with configuration. The continuous-shale type has the highest S₁ (1.48 mg/g), OSI (84 mg/g) and porosity (3.09%) but the lowest brittle-mineral content (39%). The sand-mud interbedded type has comparable porosity, but frequent thin-sand interfaces disperse the free oil so that its S₁ is only 0.95 mg/g. The sand-interbedded type has the highest brittle-mineral content (52%), whereas the shell-interbedded type is the poorest in oil-bearing capacity and mobility. Sublayers 8, 6 and 2 are all of class I, and class-I sweet-spot intervals total 36.8 m. 【Conclusion】The four properties are controlled at the first order by the source-reservoir configuration: 77% of the class-I sample points belong to the continuous-shale type, far more than in the other three assemblages (33%-40%), so sweet-spot boundaries should be defined by the source-reservoir configuration rather than by stratigraphic surfaces. The sweet spots can be summarized as two vertical intervals, sublayers 6-8 and sublayers 2-4, both spanning 33-36 m, comparable to the local hydraulic-fracture height, with the tight sublayer 5 between them serving as a natural barrier; a vertically staggered well pattern with layered volumetric fracturing is therefore recommended.