The Sedimentation And Reservoir Formation Mechanism Of Hydrocarbon-bearing Fine-grained Sedimentary Rocks

The deposition and diagenesis of fine-grained sediments remains relatively weak research field in sedimentology. With the exploration and development of shale oil and gas, the study of fine-grained sedimentary rocks becomes increasingly urgent. The Longmaxi shale in the SE Sichuan Basin and the Es4s-Es3 x shale in the Dongying Depression are selected for the study. The former is silicon-rich marine fine-grained sedimentary rocks and the later is carbonate-rich lacustrine fine-grained sedimentary rocks. These two study objects taking account differences in sedimentary background and rock characteristics, and both two are representative.The proposed classification is based on mineral composition of the rocks, which takes TOC content, silica(quartz plus feldspars), clay minerals and carbonate minerals as parameters, therefore divides fine-grained sedimentary rocks into four categories based on the dominant minerals, and within every category further classification is refined depending on subordinate mineral.Based on the petrology and the lithofacies association, combined with the analysis of sedimentary environment, laminas features and association, paleontology and minerals crystal texture, the depositional process of the Longmaxi shale rich in silicon includes susupension, upwelling, turbidite and gravity slumping. The calcite of lacustrine fine-grained sedimentary rocks in the Dongying Depression is mainly formed by bio-chemistry and chemistry effect. In the organic poor shale interval, especially the LST of Es4 s, the calcite is mainly chemical precipitation. While in the organic rich interval, the bio-chemical effect is dominant, the laminas are formed associated with cyclical seasonal changes.In this study, the reservoir space characteristics are detailed characterized,and their origin are analyzed. What’s more, the liquid hydrocarbon occurrence of different reservoir spaces is characterized in different observation scales.the hydrocarbon are mainly free in the laminated fractures, tectonic fractures, abnormal pressure fractures, and bigsize pores,such as the recrystallization intercrystal pores and dissolved pores,wile are adsorpted in the organic pores, intracrystalline pores in the pyrite framboidal and clay floccules.The shale reservoir is mainly controlled by the lithofacies, the TOC content, tectonism and diagenesis, and the lithofacies and organic matters are the key controlling factor. The reservoir space models of different lthofacies are established and the relationship of hydrocarbon occurrence manner and lithofacies and burial diagenesis. Focus on the shale oil reservoir, an effective reservoir evaluation method is proposed, in which the lithofacies and TOC content are the key parameters, and the thickness of organic rich shale, the development of fractures, and physical property are should considered.