Limits of critical parameters for sweet-spot interval evaluation of lacustrine shale oil
Determining the limits of critical parameters for sweet-spot intercal evaluation of lacustrine shale oil is the key to commercial shale oil exploitation.Based on the on-site observations and lab analysis of lacustrine shale oil exploratory wells,supplemented by previous research results and achievements in exploration and development practices,we try to determine the limits of critical parameters for sweet-spot intervals of diverse shale oil types,including the total organic carbon(TOC)content,vitrinite reflectance(Ro),pyrolytic hydrocarbon content(S1),porosity and permeability,oil saturation index(OSI),and brittle mineral content.Findings suggest that for the shale oil sweet-spot intervals of the mixed type with source rock-reservoir integrated and the pure shale type,the lower limit of TOC content is greater than 1.0%or 2.0%,and the upper limit should not exceed 6.0%.For sweet-spot intervals in the source rock measures of brine-saline,saline-brackish,and brackish-fresh lacustrine basins,the lower limits of Ro are 0.50%,0.60%,and 0.80%,respectively.Regarding S1,two scenarios are recommended considering factors such as sample preparation:for low limit of TOC content at 1.0%,the lower limit of S1 is 1.0 mg/g(conventional pyrolysis)or 2.0 mg/g(pyrolysis of sealed and frozen crushed samples);for low limit of TOC content at 2.0%,the lower limit of S1 is 2.0 mg/g(conventional pyrolysis)or 4.0 mg/g(pyrolysis of sealed and frozen crushed samples).The lower limits of porosity and permeability are 5.0%and 0.01×10-3 μm2,respectively for the intercalated-type sweet-spot intervals of shale oil,and are 4.0%and 0.01×10-3 μm2,respectively for the pure shale-type and mixed-type sweet-spot intervals with laminated and layered textures and well-developed fractures.The lower limit of OSI is 100 mg/g or 200 mg/g for the pure shale-type and mixed-type and is 300 mg/g or 400 mg/g for intercalated-type.The lower limit of the brittle mineral content is 65.0%.These findings can lay the foundation for genuine shale oil sweet-spot interval development to achieve commercial shale oil exploration and exploitation in low-oil-price environment.
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