Petroleum Accumulation System Formation And Occurance In The Kuqa Depression,Tarim Basin

The Kuqa Depression is a foreland basin with abundant oil and gas resources. As one of themost important gas bases of the East Gas Pipeline Project in China, it has been a hot researchfield of petroleum geology since the discovery of the Kela-2 gas field. The early work focusedon the study of natural gas, including gas resource evaluation, the genetic classification andenrichment patterns, while the study of crude oil is relatively weak. In this paper, crude oil isused as a clue for the hydrocarbon accumulation study by application of petroleum geochemicaltechniques. After fine anatomy of the oil biomarkers and oil & gas filling history in differenttectonic units of the Kuqa Depression, the process of hydrocarbon generation, migration,accumulation and late adjustment and transformation has been rebuilt; then the hydrocarbonoccurrence controlling factors has been discussed, which can provide the basis for effectiveprediction of hydrocarbon resources and their distribution.The distribution of hydrocarbon is quite different in the Kuqa petroleum system. In thecenter of the depression, natural gas is the main exploration target, and there are only two smalloilfields, i.e., Dawanqi and Yiqikelike. In the frontier uplift, however, most of the traps are filledwith liquid oil, including some condensate. This characteristic is closely related to thehydrocarbon generation and expulsion history of the source rocks, regional tectonic activity timeand so on. When the Triassic source rocks started to generate oil, the Qiulitage structural belt hasnot yet formed and the liquid hydrocarbons migrated along the unconformity or connectedsandstones from the centre to the south and north sides uplift; while the Jurassic source rocksstarted to generate natural gas, the Qiulitage structural belt became a barrier to prevent gasmigration southward, and a large number of thrust faults was developed in the center of thedepression, which was vertical pathway connecting the source and reservoir rocks, resulting innatural gas gathering in the shallow reservoirs of the center depression.Oil & gas occurrence in the Kuqa Depression is affected by the hydrocarbon charginghistory and tectonic setting. One of the obvious characteristics of the Dabei gas field is"deep gasand shallow oil", and oil and gas injecting in different phases at different tectonic settings led tothis differential distribution. The unusually high content of adamantine indicated a strong late gascharge in the Kela 2 gas field. The highly mature natural gas charged at late stage, dissolving alarge number of high-adamantane compounds volatile components of crude oil. There are twodifferent types of crude oil occurred in the frontier uplift. In the western part, the oil and gasmigrated laterally and distributed in different layers, while in the eastern part these two types ofoils mixed in some reservoirs due to the development of the vertical faultsIn the Kuqa petroleum system, oil and gas distribution is relatively regular. Paleogene andCretaceous are the main reservoirs, hydrocarbons are mainly concentrated in Kelasu Yiqikelikestructural belt, Qiulitage structural belt and frontier uplift, and the large scale of gas fields arelocated under the cap of gypsum. On the whole, it is obvious that the inner ring is rich in gaswhile the outer ring is rich in oil. Abundant organic matter, poor type and high maturity arecontrolling factors for the occurrence of natural gas; very late hydrocarbon charging period andthe role of capping are endurances of hydrocarbon effective maintenance; hydrocarbongeneration period and the corresponding migration pathway are the key factors for thedistribution of"oil in the outer ring gas in the inner one". As a result, it is very favorable forhydrocarbon generation, migration and maintenance, thus the trap is the final controlling factorsfor the exploration of the oil and gas accumulations.