Abstract
In order to understand the influence of source rock types and maturity on hydrocarbon gases carbon isotope change more objectively, a closed-system pyrolysis experiment was carried out on six samples from 250 to 550°C. The values of δ13C1, δ13C2, and δ13C3 were ranged from −73.3 to −29.8%, from −36.3 to −9.5%, and from −38.5 to −12.4%, respectively. The range of δ13C1 was the largest, reaching a top value of 43.5%. The results showed that the temperature has an effect on the carbon isotope value of pyrolysis gas. With the increase of the degree of thermal evolution, the carbon isotope value of methane in all samples, except for huangxian gangue, had a change trend from heavy to light firstly, then got heavier. In addition, the carbon isotope values of methane, ethane and propane had the features of δ13C1 < δ13C2 < δ13C3 when the temperatures were under 550°C, which were made up of a series of positive carbon isotopes. However, when the temperature increased above 550°C, there was an inversion of the simulated gas carbon isotope values in Huangxian coal gangue, Minqin oil shale and Huaan carbonaceous shale, i.e., δ13C2 > δ13C3 and δ13C2 > δ13C1. It indicates that the secondary cracking has occurred at high maturity or over maturity stage.
Highlights
The distribution and evolution of stable carbon isotopes contain important information of natural gas
As an important means to understand the natural evolution of geochemical characteristics, hydrothermal pyrolysis simulation experiment makes multi-parameter analysis of the controlled system possible, and the analysis results can be applied to practical production
The methane carbon isotope value from different types of hydrocarbon source products were first light and heavy with the increase of thermal simulation temperature, and an inflection point occurred after reaching the oil window
Summary
The distribution and evolution of stable carbon isotopes contain important information of natural gas. Carbon stable isotope has an important role in the study of natural gas genesis, oil and gas filling history, migration and oil source correlation (Cramer, 2004; Duan et al, 2011; Tian et al, 2012; Hao and Zou, 2013; Wu et al, 2015; Han et al, 2018). In order to meet the needs of oil and gas exploration, many scholars have studied the relationship between methane stable carbon isotopes and temperature and maturity by simulating hydrocarbon generation in immature and low-mature sediments (Dieckmann et al, 2006; Lewan and Roy, 2011; Duan et al, 2011; Li et al, 2018).
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