Abstract
Organic macerals are the basic components of organic matter and play an important role in determining the hydrocarbon generation capacity of source rock. In this paper, organic geochemical analysis of shale in the Chang 7 member of the Yanchang Formation was carried out to evaluate the availability of source rock. The different organic macerals were effectively identified, and the differences in hydrocarbon generation and pore-forming capacities were discussed from two perspectives: microscopic pore development and macroscopic hydrocarbon generation through field emission scanning electron microscopy (FE-SEM) and energy-dispersive spectrum (EDS) analyses, methane isotherm adsorption, and on-site analysis of gas-bearing properties. The results show that the source rock of the Chang 7 member has a high abundance of organic matter and moderate thermal evolution and that the organic matter type is mainly type I. Based on the morphology of the organic matter and the element and pore development, four types of hydrogen-rich macerals, including sapropelite and exinite, and hydrogen-poor macerals, including vitrinite and inertinite, as well as the submacerals, algae, mineral asphalt matrix, sporophyte, resin, semifusinite, inertodetrinite, provitrinite, euvitrinite, and vitrodetrinite, can be identified through FE-SEM and EDS. A large number of honeycomb-shaped pores develop in sapropelite, and round-elliptical stomata develop in exinite, while vitrinite and inertinite do not develop organic matter pores. The hydrogen-rich maceral is the main component of organic macerals in the Chang 7 member of the Yanchang Formation. The weight percentage of carbon is low, so it has good hydrocarbon generation capacity, and the organic matter pores are developed and contribute 97% of the organic matter porosity, which is conducive to hydrocarbon generation and storage. The amount of hydrogen-poor maceral is low, and the weight percentage of carbon is low, and the organic matter pores are not developed, which is not conducive to hydrocarbon generation and storage.
Highlights
Organic maceral is a component of organic matter and has an important effect on hydrocarbon generation capacity [1, 2]
The total gas content was measured by on-site analysis using the gasbearing experiment and the KD-III shale gas field analyzer developed by China University of Petroleum, Beijing
The hydrogen-rich macerals sapropelite and exinite developed large organic matter pores, while the organic matter pores of hydrogenpoor macerals vitrinite and inertinite were relatively underdeveloped, and the content of each maceral was different, which contributed to the different degrees of organic matter porosity
Summary
Organic maceral is a component of organic matter and has an important effect on hydrocarbon generation capacity [1, 2]. Previous research used FE-SEM to effectively identify the organic maceral of the Shahezi Formation in the Songliao Basin, China, but did not detect the sapropelite and exinite, and the identification methods were not systematic [6]. The research focused on Chang 7 shale samples, and FE-SEM combined with EDS methods was used to identify each organic maceral. The contribution of each maceral to the pore-forming capacity of organic matter and the relationship between macerals and hydrocarbon generation potential and gas content were studied. This research provides theoretical support for the in-depth understanding of the hydrocarbon generation capacity of organic matter and has a positive effect on the exploration and development of continental shale gas
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