Kela-2 Gas Field — Is It an Instantaneously Accumulated Gas Pool?

The Kela-2 Gas Field, located in the middle of the Kuqa Forland Basin, is one of the biggest gas-pools in China. Due to its huge scale and some particular properties, a series of studies have been carried on this gas field since its discovery. So far, most of the studies have been focused on the formation of natural gas, but seldom research on the associated oil has been reported.

Many researchers pointed out that the Kela-2 gas pool was formed in two episodes characterized as “oil accumulated in early stage, followed by dry gas”. When it comes to the origin of the natural gas, people tend to believe that it is the result of instantaneous accumulation, i.e., the Kela-2 trap was formed fairly late and it could only entrap some late-generated gases during the high to over maturity. The basis of this deduction is stable carbon isotope of the natural gas. The δ13C1 value of Kela-2 gases ranges from -27.07‰ to -27.8‰ and δ13C2 from -17.87‰ to -19.40‰, while it is about -31‰ and -21‰ in gases of the Dabei Gas Pool and -34‰ and -23‰ in gases of the Dina Gas Pool.

But this hypothesis can hardly explain why there are still some liquid oils in this gas pool which should have been generated even earlier than gases. Here, if we take the geochemical properties of oil and gas in the Kela-2 Gas Field as a whole, we can have an impression that these abnormal features of both oil and gas may have been caused by similar effect.

The oils in Kela-2 Gas Field contain much more diamondoid hydrocarbons than those in other structures. The concentration of 3-methy-diamantane + 4-methy-diamantane, being considered as one of the most important indexes of oil cracking and oil maturity, is as high as 2000ppm in oils of Kela-2 Gas Field, while it is only 200ppm in oils of other structures. The content of the polycyclic aromatic hydrocarbons (PAH) in oils of the Kela-2 Gas Field, such as chrysene, pyrene and fluoranthene, is also much higher.

High temperature could have played a key role on both oil and gas. As known to all, relatively high geothermal field can increase hydrocarbon maturity, resulting in high concentration of diamondoid hydrocarbons and PAH in oil, and heavier carbon isotope of natural gases as well. The abnormally high formation temperature has been confirmed by some fluid inclusions whose uniform temperature is more than 300°C in the Kela-2 reservoirs.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California