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 (δ¹³C ranging from − 52.6‰ to − 39.5‰ and δD 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 C₂₊ hydrocarbons and is similar to structure II hydrate. Carbon isotope data from the gas hydrates show a positive carbon isotope series (δ¹³C₁ < δ¹³C₂ < δ¹³C₃), with ethane δ¹³C 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.