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Lithologic heterogeneity of lacustrine shale and its geological significance for shale hydrocarbon-a case study of Zhangjiatan Shale

Abstract The Zhangjiatan shale of the Southeastern Ordos Basin, which deposits in deep lake facies, has strong lithologic heterogeneity; it represents as shale, sandy laminae shale and thin sandstone. Shale with mm- to cm-scale sandy laminae is defined as Sandy Laminae Shale (SLS). However, the relationship between lithologic heterogeneity and hydrocarbon accumulation has never been studied. This study shows that lithologic heterogeneity, especially the occurrence of SLS will influence the accumulation of hydrocarbons within the shale system. SLS commonly has a larger pore size, higher porosity and better connectivity than thick and homogeneous shale. SLS commonly contains fewer residual hydrocarbons than homogeneous thick shale, indicating comparative ease in hydrocarbon expulsion. SLS commonly generates more bedding fractures. More fractures can be incurred when fracture extending into SLS. The occurrence of SLS can provide more storage spaces for fluids and gases, including oil, water, adsorbed gas, dissolved gas and free gas. SLS has been the pilot exploration and development target for shale hydrocarbon in the southeastern Ordos Basin. Homogeneous and thick shales, which have a large content of residual hydrocarbons, would be the targets for future exploration.