Abstract
Abstract The marine shale in southern China has undergone complex tectonic evolution with a high thermal evolution degree. Excessive thermal evolution brings certain risks to shale gas exploration and development. With the advancement of experimental methods, the evolution process of shale reservoirs can be better understood from the micro-nanoscale. This work takes the Ordovician-Silurian Wufeng and the first member of Longmaxi Formation in the Sichuan Basin and Lower Cambrian Niutitang Formation in Outer Margin of the Sichuan Basin to study the impact of maturity upon the genesis of shale gas and development features of the reservoir. A series of geochemical research methods, including TOC, gas component and gas isotope, were adopted to study the impact of different thermal evolution stages of organic matter upon the genesis of shale gas. The nanoscale micro-imaging technique, such as FIB-SEM and FIB-HIM, was used to analyze the development of OM-hosted pores. As shown from the results, when R o = 1.2–3.5%, the marine shale gas is dominated by methane and other hydrocarbon gases, since the mixture of cracking gas from liquid hydrocarbons and kerogen-cracking gas cause the carbon isotope reversal. Besides, the pyrobitumen pores characterized by the strong connectivity and storage capacity were primarily developed. When R o > 3.5%, the organic matter is at the graphitization stage. The shale gas is mainly composed of nitrogen at this stage. The nitrogen is originated from the atmosphere and the thermal evolution process, and the OM-hosted pores (pyrobitumen and kerogen pores) characterized by the bad connectivity and storage capacity are developed. Finally, the main component of shale gas, the genesis of shale gas and the pattern of OM-hosted pores under different thermal evolution stages of organic matter are summarized, which provide technical support for the exploration and development of shale gas.
Highlights
After the breakthrough of shale gas in the United States, the application and promotion of horizontal well hydraulic fracturing, micro-seismic monitoring, multi-well industrialized exploitation and other technologies have set off a “shale gas revolution” in the world, and shale gas is becoming an important domain for oil and gas exploration to be replaced [1,2,3,4]
During the period of mass development of shale organic matter pores, the formation was not uplifted in time, which led to the further enhancement of the thermal evolution of the reservoir, the number of OM-hosted pores decreased sharply, and shale gas lost the effective storage space of organic matter pores after it was generated
The Youyang block located at southeast Chongqing, on the outer margin of the Sichuan Basin, does not have good commercial shale gas flow
Summary
Large-scale transgression occurred in the Yangtze area in the early Cambrian, and the study area was in the sedimentary environment of the deep-water continental. During the late Ordovician and the early Silurian, due to the convergent collision between the Yangtze plate and Cathaysia plate, the upper Yangtze region is in the deep-water shelf environment surrounded by ancient land, which led to strong sealing ability and appears stratified phenomenon of the water body. It has deposited the Ordovician-Silurian Wufeng and Longmaxi Formation shale with high-organic matter content, large thickness and wide distribution [32,33,34]. The underlying strata of Wufeng Formation are gray nodular limestone of Lin Xiang formation of the upper Ordovician, and the overlying strata of the first section of Longmaxi Formation are gray argillaceous siltstone of the second section of Longmaxi Formation
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