Shiyou shiyan dizhi (Jan 2022)
Carbon isotopic evolution of hydrocarbon gases generated from carbonate source rocks via different thermal simulation methods
Abstract
Chinese marine source rocks are generally at the stage of high- to over-mature, there are still some uncertainties about whether the existing hydrocarbon isotopic indexes can be directly employed to distinguish the genetic types of marine carbonate natural gas and consequently, gas-oil source correlation can be carried out. In this paper, a series of pyrolysis experiments for hydrocarbon generation with different conditions were carried out using the low-maturity marine carbonate source rocks of Luquan Ⅱ1 type in Yunnan province to analyze the carbon isotope of the collected gas products. The results showed that: ①During the entire thermal evolution stage, the carbon isotope values of kerogen did not vary significantly with the increasing of maturity, while the carbon isotope values of methane and ethane both decreased and then increased with the increase of maturity, showing similar evolution characteristics. In the main oil and gas generation stage, it is significantly smaller than the isotope value of its parent material kerogen. In the over-mature stage (VRo ≥2.2%), the tendency of carbon isotope of ethane to become heavier is obviously accelerated, even greater than the carbon isotope value of its parent material kerogen, showing the characteristics of "coal-type gas". This suggested that we need to be cautious when simply using the carbon isotope values of methane and ethane to identify the type of natural gas. ②At the same maturity, the carbon isotope values obtained from the pyrolysis experiment of semi-closed and semi-open system are lighter than those obtained from the pyrolysis experiment of closed system. This indicates that conventional natural gas reservoirs formed by the discharge of hydrocarbon gas from the same source rock have a certain difference between the carbon isotope values of hydrocarbon gas and the carbon isotope values of shale gas retained in the source, showing the characteristics of "different sources". Attention should also be exercised to the use of carbon isotope templates or regression formulas to carry out gas-source comparisons. ③With the two pyrolysis methods, the carbon isotope value of methane is always lighter than that of ethane in the full evolution stage of the same source rock. This indicated that conventional natural gas reservoirs formed by direct gas supply from single set of source rock do not have "inversion" phenomenon of methane and ethane carbon isotopes.
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