Discussion on the Genesis and Exploration Potential of Different Microscopic Forms of Organic Matters in the Longmaxi Formation Shale

Abstract

BACKGROUND Organic matter is the most important component in shale, and organic pore is an important reservoir space of shale gas. Scanning electron microscope (SEM), the mainstream microscopic analysis and detection technology, has become an important means of shale research. The comprehensive study of organic matter is gradually becoming an important part of shale gas exploration and development. OBJECTIVES To summarize the microscopic morphology of organic matter in organic shale of the Longmaxi Formation and discuss its hydrocarbon generation and reservoir ability according to its internal structure and composition characteristics. METHODS The organic shale of the Longmaxi Formation in Sichuan Basin and its periphery were polished with argon ion, and the samples were examined by high resolution field emission scanning electron microscope under the condition of low voltage and close working distance. RESULTS There were three main microscopic forms of organic matter in the organic shale of the Longmaxi Formation, which were structure-type sedimentary organic matter, differentiated symbiotic organic matter and interstitial migration organic matter. The structural sedimentary organic matter formed in the stage of sedimentation and early diagenetic evolution, and retained some biological structure characteristics or sedimentary directional structure. Differentiated symbiotic organic matter interacted with clay minerals and underwent evolutionary differentiation, forming a 'granitic structure' similar to that in magmatic rocks. On the other hand, the interstitial migration organic matter was formed in the high evolution stage of late diagenetic organic matter, and the liquid hydrocarbon filled in the intergranular of authigenic minerals, and the surrounding minerals had euhedral characteristics. The shale of the Longmaxi Formation was dominated by organic matter pores-rich interstitial migration organic matter and differentiated symbiotic organic matter, which were the high-quality gas parent materials of the Longmaxi Formation shale, with high reservoir performance. CONCLUSIONS The research results provide deep-level micro-information for the theory of shale gas formation and the evaluation of exploration and development in the Longmaxi Formation in China.

Keywords

• the longmaxi formation shale
• high resolution field emission scanning electron microscope
• different microscopic forms of organic matter
• discussion on the cause and source of formation
• contribution of hydrocarbon generation and reservoir performance