Basin Modeling of Carbon and Hydrogen Isotopes to Quantify Gas Maturity and Formation Age - Unique Diagnostic Tool for Basin Modeler

Yunyan Ni¹, Jinxing Dai¹, Feiyu Wang², Qisheng Ma³, and Yongchun Tang³
¹PetroChina Research Institute of Petroleum Exploration and Development, Beijing, China
²China University of Petroleum, Changping, Beijing China
³Power, Environmental, and Energy Research Center, California Institute of Technology, Covina, CA, USA

Carbon (¹³C) and hydrogen (D) isotope measurements are important analytic tools for geochemists. Particularly, the Kinetic Isotope Effect (KIE) of natural gas systems provides key genetic information on gas maturity, thermal history and formation age. Using KIE we can constrain our basin modeling uncertainties and give a new dimension for petroleum system evaluation. We present here a case study from Tarim Basin China where maturity of natural gas and yields were determined based on both conventional basin modeling and isotope modeling from field natural gas isotope fractionation

In the Kuqa depression which is located in the northern Tarim Basin, Nothwest China (Figure 1), the most likely gas source rocks are the Middle-Lower Jurassic coal measures containing mainly of type III organic matters. Most of the source rocks are mature or overmature with the vitrinite reflectance values (Ro,%) ranging from 0.6% to 2.5%. Based on the geothermal gradient of the Kuqa depression, both cumulative and instantaneous modes have been applied for ¹³C/D kinetic isotopic models. It is demonstrated that the gas charging model is more toward instantaneous pattern when we cross plotted the gas isotope from theoretical modeling versus field data. Good agreements were achieved for gas maturity (i.e., vitrinite reflectance value) between the ¹³C/D kinetic isotopic models. Similar to the basin modeling results for the Ro values of the modern Jurassic bottom organic matters, our calculated gas maturity based on the KIE also demonstrates that the gas was mature or overmature in the central depression with Ro values up to 1.8% in the Kela2 gas field, but the gas maturity was much lower in the southern Front Uplift with Ro values as low as 1.2% (Figure 2). On the other hand, the calculated gas maturity in the Dawanqi oil-gas field is relatively lower than those of the source rocks predicted by the basin modeling. Our calculated gas maturity in the southern Front Uplift is systematically higher than that of the local source rocks based on the basin modeling. This probably implies that natural gases in the depression were formed at the deeper depth; and the expelling and migration from north to south might result in the accumulation in the southern Front Uplift. Meanwhile, for the hydrocarbon generation depression, gases migrated vertically along faults to some traps and formed giant gas fields such as Kela2 gas field.

Our calculated gas formation age based on KIE is also consistent with previous studies on the burial history according to basin modeling. Both models imply the gas formation age is within five million years, mostly around 2 Myr for the gas from the Kela2 gas field. These results indicate the gas generation and entrapment in the Kuqa depression were mainly occurring within last five million years.

References:
Liang D., Chen J., Zhang B., Zhang S., Wang F. and Zhao M. (2004) Terrestrial oil and gas generation in the Kuqa depression, Tarim Basin. Oil Industry Press, Beijing (In Chinese).
Qin S., Dai J. and Liu X. (2007) The controlling factors of oil and gas generation from coal in the Kuqa Depression of Tarim Basin, China. Int. J. Coal Geol. 70, 255-263.

Figure 1. Location map of the Kuqa depression (After Qin et al., 2007).