Origin of the hydrate bound gases in the Juhugeng Sag, Muli Basin, Tibetan Plateau

Abstract

The Juhugeng Sag, located in northwest of the Muli Basin, Tibetan Plateau, has been investigated for coal and petroleum resources during the past several decades. There have been successful recoveries of gas hydrates during recent years from the Middle Jurassic Yaojie Formation that offer insight into the origin of the hydrocarbon gases from the complex sag feature. This study examines the organic geochemical and stable carbon isotopic characteristics of shale and coal samples from the Middle Jurassic Yaojie Formation of the Juhugeng Sag, as well as compares with carbon isotopes, gas amounts and components of hydrate-bound gas. A total of 19 samples from surface mining, including 12 samples of black shale and 7 samples of coal, were analysed using a micro-photometer, a gas chromatograph, Rock-Eval and isotope methods. All the shale samples contained 100% type I kerogen, and the random vitrinite reflectance values vary from 0.65% to 1.32% and achieve thermal pyrolysis phase. Isotope values of methane (d 13 C ranging from-52.6% to-39.5% and dD ranging from-285% to-227%) in the hydrate bound gases suggest that the methane originates mainly from thermogenic contributions. It is proposed that ethane from the gas hydrate is thermogenic-produced, and this conjecture is supported by the fact that most of the gas hydrate also contains more than 30% of thermogenic C 2? hydrocarbons and is similar to structure II hydrate. Carbon isotope data from the gas hydrates show a positive carbon isotope series (d 13 C 1-\ d 13 C 2 \ d 13 C 3), with ethane d 13 C values being lighter than-28.5%, as high consistency with source rocks from the Jurassic period indicate thermal oil-prone gas. A model of the accumulation of gas hydrate is plotted. However, the gaseous sources of gas hydrates may be a subject for more research.

Influence of Gas Supply Changes on the Formation Process of Complex Mixed Gas Hydrates

“…In addition, it is characterized by more complicated gas components compared to other well-documented permafrost hydrate reservoirs [9][10][11]. The composition and carbon isotopes of gases released from gas hydrates suggest that the feed gas source for the hydrate reservoir is composed mainly of deep or crude oil-concomitant gases originating from the coal layers of the Middle Jurassic period, occasionally mixed with microbial sources in shallower depths [12][13][14][15]. Fracture-filling gas hydrates are the main reservoir type and manifest as thin layers, flakes, or blocks on the fractured surface of siltstone, mudstone, and oil shales.…”

Genesis of Coalbed Methane and Its Storage and Seepage Space in Baode Block, Eastern Ordos Basin

“…Energies 2022, 15, x FOR PEER REVIEW 2 of 18 reservoirs often contain a certain proportion of CO2 and N2, which can be used as important objects for studying the genesis of CBM [10].…”

“…However, there is a lack of a systematic understanding of the origin of CBM in Baode block. In addition to methane, low-rank coal reservoirs often contain a certain proportion of CO 2 and N 2 , which can be used as important objects for studying the genesis of CBM [10].…”

Assessment of the Ecological Impacts of Coal Mining and Restoration in Alpine Areas: A Case Study of the Muli Coalfield on the Qinghai-Tibet Plateau

“…The coalfield is 50 km long from east to the west and 8 km wide from north to south, with a total area of approximately 400 km 2 . The average annual temperature here is -5.1°C [25], and the annual precipitation and evaporation are 477.1 mm and 1,049.9 mm, respectively [26].…”