Natural Gas Geochemistry in the Tarim Basin, China and Its Indication to Gas Filling History

Zhang, Tongwei¹, Quanyou Liu², Jinxing Dai², Yongchun Tang¹ (1) California Institute of Technology, Covina, CA (2) Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China

Natural gases in the Tarim basin, China are primarily composed of methane, whose content is more than 70%, with variable contents of ethane (<1% ~18%) and propane (<0.5% ~9%). According to the carbon isotope composition (C₁, C₂ and C₃) of 261 natural gas samples collected from the entire basin, two genetic groups, coal-type gas (from Triassic and  Jurassic humic matter) and oil-type gas (from Paleozoic marine sapropelics) can be clearly differentiated. Coal-type gas enriches ¹³C isotope for methane, ethane and propane, especially for ethane and propane derived from the humic matter are much heavier than those of natural gas from the Paleozoic marine source rocks.

A positive relationship among d¹³C₁, d ¹³C₂, d ¹³C₃ suggests that the thermal genetic gases from organic matter cracking under high temperature and pressure are a dominant source for natural gases in the Tarim basin. Being taken coal-type gas as an example, dynamically understanding gas recharging history to an effective trap is demonstrated by using quantitative gas isotope kinetic modeling. The preliminary isotope modeling results show that the natural gases derived from the Triassic-Jurassic humic matter are not following the cumulative pattern, which indicates that earlier generated gases were gone before the reservoir formed. Accumulated gas in reservoirs mainly represents the late-stage generated gases from the Triassic - Jurassic source. Based on the gas isotope pattern, we can reconstruct the gas filling history. Our gas isotope modeling gives the recharging time of Kela 2 large-sized gas field in the Tarim basin probably happened later than 5mybp.