Geochemical characteristics and possible origin of natural gas in the Taibei Depression, Turpan-Hami Basin, China

Abstract

292 chemical composition data and 82 isotopic composition data of gas samples collected from the Taibei Depression of the Turpan-Hami Basin, West China, were used in the study of their origin. Non-hydrocarbon gas is poor in most samples whereas abundant nitrogen in some samples is positively correlated with δ 13 C 1 . Although methane is the main constituent, higher molecular gaseous hydrocarbons, from ethane to pentane, are detected in most samples, in accordance with the distribution of oil reservoirs. The stable carbon isotope ratios of methane, ethane and propane are defined as δ 13 C 1 : -45.5‰ to -33.5‰, δ 13 C 2 : -30.2‰ to -10.5‰, and δ 13 C 3 : 27.6‰ to -11.2‰, respectively. According to the distribution of carbon isotope ratios, 2 families of gas can be grouped, most showing normal distribution of carbon isotopes, and others having obvious heavier carbon isotopes and being of abnormal distribution. Based on the isotopic composition, the disagreement between the relationship of Δ(δ 13 C 1 -δ 13 C 2 ) and δ 13 C 2 and that of Δ(δ 13 C 1 -δ 13 C 2 ) and δ 13 C 2 , and the calculated Ro, there are oil-associated gas, coal-derived gas and mixture of them. Other samples with obviously heavier isotopic compositions from the Yanmuxi oilfield of the Taibei Depression have been degraded by organisms.

Petroleum System Analysis and Burial History of Middle Permian Source Rock in Turpan–Hami Basin, NW China

“…Structurally, it lies in the Southwestern Central Asian Orogenic Belt, sandwiched by the Bogeda Mountain and Jueluotage Mountain. Since the Jurassic Period, the basin evolution is closely related to the Bogeda Mountain, and now, the basin presents the characteristics of the foreland basin [15][16][17][18][19][20][21]. The study area, Taibei Sag, mainly lies in the Turpan Depression of the basin.…”

“…The characteristics of the geothermal field and thermal evolution pattern are basically clarified. It is indicated that the Turpan-Hami Basin has been cooling down since the Carboniferous Period, which is vital for restoring the thermal history of the basin [17,41,49,54,55]. Table 1 summarizes the values of heat flow (densities) in different sags of the Turpan Depression of the Turpan-Hami Basin during different periods.…”