Geochemistry and origins of natural gases in the central Junggar Basin, northwest China

► Comprehensive gas geochemistry in the central Junggar Basin was reported. ► Gases are mostly middle mature and have complex origins with four genetic types. ► Gases derived from sapropelic and humic kerogens were found with a certain mixing. ► Gases most likely sourced from oil degradation were fo...

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Published in:Organic geochemistry 2012-12, Vol.53, p.166-176
Main Authors: Cao, Jian, Wang, Xulong, Sun, Ping’an, Zhang, Yueqian, Tang, Yong, Xiang, Baoli, Lan, Wenfang, Wu, Ming
Format: Article
Language:English
Subjects:
biomarkers
carbon
Coal
Earth sciences
Earth, ocean, space
Exact sciences and technology
gases
Geochemistry
hydrocarbons
Isotope geochemistry
Isotope geochemistry. Geochronology
isotopes
oils
petroleum
Sedimentary rocks
Soil and rock geochemistry
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Summary:► Comprehensive gas geochemistry in the central Junggar Basin was reported. ► Gases are mostly middle mature and have complex origins with four genetic types. ► Gases derived from sapropelic and humic kerogens were found with a certain mixing. ► Gases most likely sourced from oil degradation were found. The petroliferous central Junggar Basin in northwest China is predominantly an oil exploration region. However, its gas exploration also might have good prospects. Thus to assist in gas exploration, the geochemistry and origins of gases are discussed in this paper based on relatively comprehensive analyses of compositions, carbon isotopes and light hydrocarbons of gases. Based on the results, the gas genetic types are grouped into families and combined with the geological setting (e.g., biomarkers of retrograde condensates and source rock characteristics). We show that there are four representative genetic types of gases. The first consists of gases derived from Permian lacustrine mudstones with type I–II kerogen and type III kerogen sources in the Penyijingxi sag. Their representative geochemical feature is δ13C2 ranging from −31.4‰ to −24.7‰. The second is gas sourced from Carboniferous tufaceous mudstones of type III kerogen in the Dishuiquan sag, whose representative geochemical feature is the heaviest values of δ13C1 in the studied samples, ranging from −32.0‰ to −30.4‰. The third consists of gases sourced from Jurassic coals and mudstones in the Shawan–Fukang sag. The light hydrocarbon fingerprints of these gases are similar to those of gases and oils typically derived from Jurassic source rocks in the southern Junggar Basin. The fourth is gas most likely generated from the degradation of crude oil. It is mainly found in the Luliang area and has dryness values as much as 0.999 and δ13C1 ranging from −54.8‰ to −43.2‰. Among these four types of gases, the first (mainly sourced from the Permian lacustrine mudstones in the Penyijingxi sag) is the predominant type.
ISSN:0146-6380
1873-5290
DOI:10.1016/j.orggeochem.2012.06.009