Paleogene shale fabric and typical lithofacies reservoir characteristics in Dongying Sag

Abstract

Based on image analysis methods such as rock thin sections， scanning electron microscopy， micro fluorescence， and cathodoluminescence， combined with testing techniques such as nuclear magnetic resonance and nitrogen adsorption， two typical lithofacies of the Paleogene carbonate mineral shale and mixed shale in Dongying Sag were studied. According to the mineral composition and occurrence， shale was divided into two types of structural units： carbonate laminas and mudstone laminas. By analyzing the mineral types and related pore development characteristics of them， two typical lithofacies reservoir characteristics were revealed. In view of the relative changes in geological elements during burial， the development conditions of shale reservoir space were discussed. The research results indicate that the Paleogene shale in Dongying Sag has diverse mineral components， characterized by the widespread development of multi-component carbonate minerals， local concentration of multiple sources of felsic detrital particle， and plastic filling of rheological components such as clay minerals and organic matter. The skeletal particles limit the distribution boundaries of organic components， and the organic components contain some autogenous minerals. The pore filling materials are influenced by factors such as the high content of crude oil asphaltene and wax， as well as high surface solidification points. The organic components and inorganic minerals exhibit multiple chimeric characteristics. The shale fabric and complex evolution process jointly determine the diversity of pore types and development mechanisms. The carbonate mineral shale develops nanoscale intracrystalline pores and micrometer-scale intergranular （solution） pores， characterized by large and well-connected pores， with relatively low porosity. Mixed shale develops nano-sub micron-sized interlamellar pores of clay minerals， which are small and poorly connected， with relatively high porosity. The interlayer fracture development zone has good reservoir properties， and the reservoir space is composed of multi-scale networks rich in matrix pores. In the overall background of rich organic matter， the Paleogene rich carbonate shale has the characteristics of the oil enrichment in matrix pores and at the laminae planes.

Keywords

• shale
• reservoir property
• microscopic characteristic
• pore structure
• dongying sag