Earth-Science Reviews 190 (2019) 247–272 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Carbon and hydrogen isotopes of methane, ethane, and propane: A review of T genetic identifcation of natural gas ⁎ Quanyou Liua,b, , Xiaoqi Wua,b, Xiaofeng Wangc, Zhijun Jina,b, Dongya Zhua,b, Qingqiang Menga,b, Shipeng Huangd, Jiayi Liua,b, Qi Fue a State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Efective Development, SINOPEC, Beijing 100083, China b Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 100083, China c Lanzhou Institute of Geology, Chinese Academy of Sciences, Lanzhou 730000, China d Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China e Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA ARTICLE INFO ABSTRACT Keywords: The genetic identifcation of diferent types of natural gas is notably important for assessment of its sources and Natural gas exploration potential. The chemical and isotopic (C and H, in particular) compositions of natural gas vary sig- Alkane nifcantly due to the complexity of its generation, migration, and accumulation processes. The “coal-type” gas Carbon isotope generated from humic matter is generally enriched in 13C as compared to "oil-type" gas generated from sapro- Hydrogen isotope pelic organic matter. However, gas originating from fresh-brackish water environments is depleted in 13C Genetic identifcation whereas gas from saline environments is enriched in 13C. Notwithstanding organic precusors and sedimentary Secondary process environments, both isotope compositions of alkanes tend to become enriched both in 13C and 2H with prograde thermal evolution. Therefore, in addition to thermal maturity, source material is the major controlling factor of carbon isotope compositions, whereas sedimentary environment is predominant in governing hydrogen isotopes. Secondary processes, including thermochemical sulfate reduction (TSR) and difusion, result in an enrichment of the gases in 13C and 2H due to mass-dependent kinetic isotope efect. Microbial degradation causes a decrease in propane content and an enrichment in 12C and 2H of the residual propane. The abiogenic gases may include methane from deep mantle and high molecular weight hydrocarbons through Fischer-Tropsch type (FTT) synthesis. Methane of mantle origin possesses a narrow range of isotope compositions, although it is still a tall task to determine the exact values. In contrast, isotopes of alkane gases synthesized from FTT processes are in a wide range. In sedimentary basins, the mixing of gases from multiple sources and/or through diferent secondary processes may pose a challenage to identifcation of their origins. The detailed assessment is provided here with case studies from major oil and gas basins in China. This review provides identifcation of misconceptions in genetic types of natural gas using carbon and hydrogen isotopes of alkanes, and sheds insights into using isotope geochemistry as an important diagnostic tool for energy exploration as well. 1. Introduction organic matter, biogenic gas can be further divided into “coal-type” gas and “oil-type” gas (Dai et al., 1985, 1992; Xu, 1994). Coal-type gas, also Owing to the complexity of formation processes of natural gas, the called coal-formed gas, is formed by thermal decomposition of humic classifcation scheme varies with diferent criteria, including original organic matter (type III kerogen), and coal seams are included in the source and thermal maturity. On the basis of sources, there are two source rocks of this type. Oil-type gas is generated from decomposition broad categories: biogenic and abiogenic gas. Biogenic gas mainly re- of sapropelic organic matter (type I and II kerogen), including gases fers to the natural gas formed by organic matter, while abiogenic gas from direct decomposition of sapropelic rocks and secondary cracking includes gas both directly from mantle (mantle-derived gas) and from of crude oil. On the basis of thermal maturity, natural gas can be abiogenic formation (Fischer-Tropsch type synthesis) in the crust classifed into microbial gas, low-mature or immature gas, bio-ther- (Welhan and Craig, 1979; Gold and Soter, 1980; Dai et al., 2005a; mocatalytic transitional zone gas (Liu et al., 1997), and thermal Jenden et al., 1993; Sherwood Lollar et al., 2002). Based on the type of cracking gas (Xu, 1994). Among them, thermal cracking gas includes ⁎ Corresponding author at: State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Efective Development, SINOPEC, Beijing 100083, China. E-mail address: [email protected] (Q. Liu). https://doi.org/10.1016/j.earscirev.2018.11.017 Received 6 July 2018; Received in revised form 27 October 2018; Accepted 24 November 2018 Available online 26 November 2018 0012-8252/ © 2018 Elsevier B.V. All rights reserved. Q. Liu et al. Earth-Science Reviews 190 (2019) 247–272 natural gas (also known as kerogen-cracking gas) formed by primary gas in the Xujiaweizi depression in Songliao Basin is classifed as a cracking of humic (coal-bearing) and sapropelic source rocks, and mixture of deep-sourced abiogenic gas and lacustrine oil-type gas (Li secondary cracking gas through thermal decomposition of crude oil (Liu et al., 2005a; Liu et al., 2016). et al., 2007). The latter can be further divided into three diferent gases, This paper collects the chemical composition, carbon and hydrogen depending on burial and thermal history in the basin. They are: (1) oil- isotope compositions (methane, ethane and propane) of 431 gas sam- cracking gas, which is generated from secondary cracking of oil; (2) oil- ples from commercial gas reservoirs in China and northern Appalachian and-gas-cracking gas, generated from secondary cracking of both oil basin in U.S., in combination with data from water-rock reaction ex- and gas; and (3) gas-cracking gas, which is from secondary cracking of periments (Fu et al., 2007). By assessing isotope compositions of alkane previously formed gas. gases from diferent sources, the authors intend to overview the current Water plays an important role in hydrocarbon generation and evo- knowledge on the impact of diferent geologic backgrounds (sedimen- lution, and participates in chemical reactions as source of hydrogen tary environment and source rocks) on isotope compositions of alkane (Lewan, 1993, 1997; Schimmelmann et al., 2001; Wang et al., 2015; gases, and re-examine this relationship with newly collected data. Yoshinaga and Morita, 1997). Hydrogen atoms in water can undergo Building on this knowledge base, a new approach for identifying al- reversible isotope exchanges with hydrogen in kerogen, most of which kanes of diferent origins is established, taking all potential environ- are linked to heteroatoms (for example, NeH, SeH, and OeH) mental factors into consideration. It would shed insights into deci- (Hoering, 1984; Seewald et al., 1998). After alkane gases are generated, phering the formation of natural gas with isotope geochemistry as a however, their hydrogen atoms barely participate exchange reactions diagnostic tool and facilitate future energy exploration. with hydrogen in water (Schimmelmann et al., 2001, 2004). For gas from sapropelic organic material deposited in marine or 2. Geochemical characteristics of source rocks and natural gas lacustrine environments, distinct chemical and isotope compositions are observed. Secondary processes during migration and accumulation 2.1. Source rocks may cause changes in the original gas composition. For example, with increasing thermal maturity, liquid hydrocarbons generated earlier may At present, natural gas discovered with potential for commercial be subject to molecular breakdown and thermochemical sulfate re- exploration is mainly formed by biodegradation or thermal cracking. duction (TSR) to form natural gas. The involvement of H2S in TSR al- Although abiogenic natural gas has been observed/observed, commer- teration processes causes changes in geochemical characteristics as cial abiogenic gas felds have yet been discovered. Therefore, natural compared to typical oil-type gas (Cai et al., 2005; Hao et al., 2008; Liu gas in major gas reservoirs was generated from source rocks with dif- et al., 2013; Machel, 2001; Worden and Smalley, 1996). Similarly, with ferent sedimentary environments, organic matter, and thermal matu- participation of deep fuids, methane becomes not only enriched in 13C rities. The geological characteristics of source rocks, carbon and hy- forming carbon isotopic reversal of alkanes (Dai et al., 2005a), but also drogen isotope values of alkane gases (C1–3) from 15 large-scale natural depleted in deuterium (Liu et al., 2016; Sherwood Lollar et al., 2002, gas reservoirs in China and northern Appalachian basin in U.S. have 2006). been summarized in Table 1. Their source rocks are made up of a Thus, the key scientifc question for assessing natural gas resource variety of sedimentary facies, including marine, lacustrine, lagoon, and potential and fnding favorable areas for exploration is: How can the transitional facies. The lithology includes carbonate rocks, mudstone or genetic types and sources of natural gas be efectively identifed in carbonaceous mudstone, and coal seams. The organic matter displays diferent geological backgrounds and secondary alteration processes, all types of kerogen (I, II, and III), and thermal maturity (Ro) ranges