Natural Gas Geochemistry in the Tarim Basin, China and Its Indication to Gas Filling History*
Tongwei Zhang1, Quanyou Liu2, Jinxing Dai2, and Yongchun Tang1
Search and Discovery Article #10131 (2007)
Posted August 20, 2007
*Adapted from oral presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1California Institute of Technology, Covina, CA ([email protected])
2Research 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 as probably later than 5mybp.
For more than 20 years petroleum and natural gas exploration, Chinese geologists have discovered two family groups of natural gas in the Tarim basin, China
Gas filling history is an important issue for natural gas exploration, and quantitative gas generation model is an useful tool to address the following issues:
Geological Application of GOR-Isotope Model in Tarim Basin
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