Introduction 

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

• Coal-type gas: derived from Jurassic coal measures of type-III kerogen, d¹³C₂>-28‰;

• Oil-type gas: derived from Ordovician ,Cambrian marine sources of type-II and type-I kerogens; or oil secondary cracking. d¹³C₂<-28‰.

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:

• Gas generation and expulsion from sources

• Gas thermal maturity

• Gas filling history and gas charging time

• Gas reserves evaluation

 

Selected Figures 

	Three sets of potential source rocks in the Tarim basin, China (modified from Chen et al., 2000). Triassic- Jurassic continental source rocks--Type III kerogen (humic) with an average TOC of 1.8% to 67%. Carboniferous –Permian source rocks--Type II and III kerogen with TOC = 0.47% - 5%. Sinian-Ordovician marine source rocks--Type I kerogen (sapropelite) with TOC = 0.2-3.4%.
	Tectonic elements and gas fields in the Tarim basin,China, with location of cross section A-A’.
	Structural cross section (A-A') of the Tarim basin. Sinian-Lower Paleozoic Unit (highly mature to post-mature marine carbonate sediments with thickness >9500m). Upper Paleozoic Unit (mature to highly mature clastic deposits with a maximum thickness of 4500m). Mesozoic-Cenozoic Unit (terrestrial clastic deposits up to thickness of 11,000m in thickness in the sedimentary center). (According to Chen et al., 2000.)
	Carbon isotopic compositions are effective indicators for gas origin identification. Coal-type gas possesses heavier carbon isotopes of ethane and propane compared with oil-type gas. A positive relationship of δ13C2 and δ13C1 and δ13C3 suggests that the thermal genetic gases from organic matter cracking under high temperature and pressure are a dominant source in the Tarim basin. GOR-isotope quantitative model can apply to the understanding of natural gas-formation and gas-filling history.

 

Geological Application of GOR-Isotope Model in Tarim Basin 

• Timing of gas formation and expulsion.

• Gas filling history.

• Gas thermal maturity.

• Gas reserves estimation.

• Gas recharging time.

 

Additional Selected Figures 

	Burial thermal history of Jurassic source rock in the Tarim basin: Burial history curve (left) and time-temperature curve of the base of Jurassic source (right) (according to Liang et al., 2003).
	Distribution of gas fields in Kuqa depression (upper) and Ro (%) contour of Jurassic coal as source in Kuqa depression (lower) (according to Qin et al., 2006).

  

Conclusions 

• Oil-type gas and coal-type gas can be clearly differentiated by the carbon isotopic compositions of C₁, C₂, and C₃ in natural gases from the Tarim basin. Coal-type gas possesses heavier carbon isotope of ethane and propane compared with that of oil-type gas.

• Thermal genetic gases from organic matter cracking under high temperature and pressure are a dominant source in the Tarim basin.

• Quantitative kinetics model provides a useful tool for dynamically understanding gas recharging history, determining gas maturity, predicting gas reserves and recharging time in an effective trap.

• Expelled gas from Jurassic coal occurred about 27mybp once the produced gas is abundant enough to meet coal absorption. The recharging of Kela 2 large-sized gas field probably occurred about 2my before present time.

• Expelled gas retained in the Jurassic coal until faults as gas migration pathway became available at about 2mybp.

 

Selected References 

Chen, J., Yanchou Lu, and Guoyu Ding, 1998, Quaternary tectonic deformation in Jiuxi Basin, West Qilianshan Mountain, Northwestern China: Journal of Earthquake Prediction Research, v. 7, p. 510-522.

Chen, J., D. Burbank, K. Scharer, J. Yin, C.M. Rubin, and R. Zhao, 2000, Timing and shortening rates of hinterland-vergent growth folding and active faulting along the SW margin of the Chinese Tian Shan. Abstracts for AGU 2000 Fall Meeting.

Chen, J., J. Yin, G. Qu, and K. Zhang, 2000, Timing, lower boundary, genesis, and deformation of Xiyu Formation around the western margins of the Tarim Basin: Seismology and Geology, v. 22(suppl.), p. 104-116.

Chen J., Y. Qin, B.G. Huffe, D. Wang, D. Han, and D. Huang, 2001, Geochemical evidence for mudstone as the possible major oil source rock in the Jurassic Turpan Basin, Northwest China: Organic Geochemistry, v. 32, p. 1103–1125.

Chen, J., D.W. Burbank, K.M. Scharer, E. Sobel, Yin Jinhui, C. Rubin, and Zhao Ruibin, 2002, Magnetochronology of the Upper Cenozoic strata in the Southwestern Chinese Tian Shan: Rates of Pleistocene folding and thrusting. Earth Planetary Sci. Lett., v. 195, nos. 1-2, p. 113-130.

Li, M., J. Bao, R. Lin, L.D. Stasiuk, and M. Yuan, 2001, Revised models for hydrocarbon generation, migration and accumulation in Jurassic coal measures of the Turpan Basin, NW China: Organic Geochemistry, v. 32, p. 1127-1151.

Liang, D., S. Zhang, J. Chen, F. Wang, and P. Wang, 2003, Organic geochemistry of oil and gas in the Kuqa depression, Tarim Basin, NW China: Organic Geochemistry, v. 34, p. 873-888.

Qin, Shengfei, Jinxing Dai, and Xianwei Liu, 2007, The controlling factors of oil and gas generation from coal in the Kuqa Depression of Tarim Basin, China: International Journal of Coal Geology, v. 70, issues 1-3, p. 255-263.

Zhao, W., S. Zhang, F. Wang, B. Cramer, J. Chen, Y. Sun, B. Zhang, and M. Zhao, 2005, Gas systems in the Kuche Depression of the Tarim Basin: Source rock distributions, generation kinetics and gas accumulation history. Organic Geochemistry, v. 36, 1583-1601.

Zou, Yan-Rong, Changyi Zhao, Yunpeng Wang, Wenzhi Zhao, Ping’an Peng, and Yanhua Shuai, 2006, Characteristics and origin of natural gases in the Kuqa Depression of Tarim Basin, NW China: Organic Geochemistry, v. 37, p. 280–290.