RESERVOIR CHARACTERISTICS AND EVALUATION METHODS OF LACUSTRINE BLACK SHALES IN ZHANHUA SAG, CHINA

The potential of lacustrine shale oil and gas is huge and has a broad prospect in China. However, there has been little research on the formation of lacustrine shale reservoir and the knowledge and effective evaluation methods are insufficient. On the basis of the core, thin section and SEM analysis, combined with X-ray diffraction and organic geochemistry tests, this article focuses on reservoir space characteristics and controlling factors of the Eogene lacustrine black shales of Luojia area in Zhanghua Sag. Seven lithofacies were identified based on mineral compositions, sedimentary textures and structures in the black shales, i.e., (1) laminated boundstone, (2) laminated marl, (3) massive marl, (4) laminated calcareous shale, (5) massive calcareous mudstone, (6) silty mudstone, and (7) massive mudstone. Three types of pores and four kinds of fractures are present in the black shale in the study area. The pores include organopores, intercrystalline pores of carbonate, and intercrystalline pores of clay minerals. The fractures consist of structural fractures, abnormal pressure fractures, mineral shrinkage fractures, and interlayer microfractures. The reservoir property is mainly controlled by lithology and mineral compositions, the occurrence of organic matters, tectonism, and diagenesis. The variability in lithologies results in a variety of reservoir space types. The density of structural fractures has a positive relation with the calcite content. Mineral shrinkage fractures are mainly concentrated in the clay minerals. The laminated structures have large effects on interlayer microfractures. The organic matters occurring in forms of continuous, discontinuous and isolated, determine organoporosity and connectivity. Tectonism controls the formation of tectonic fractures. And diagenesis plays a certain role in the reservoir reconstruction.