Terrestrial Hydrocarbon Distribution in Kuqa Foreland Basin--Illustrating Impacts of Migration Pathway Transformation on the Petroleum System

The Kuqa Foreland Basin is located in the South Tianshan, stretching from west to east with 400 kilometers long and 30-120 kilometers wide. It has experienced two major tectonic evolvements, i.e., the extension subsidence in the Mesozoic and foreland thrusting in the Neozoic. Two types of terrestrial source rocks occurred in the Triassic and Jurassic, i.e., lacustrine shale and paludal or coaly shale. The former is apt to generate oil while the latter tends to generate gas.

According to hydrocarbon generation history, oils were generated during Jidike Stage (N1j, about 23~12Ma) and Kangcun Stage (N1-2K, about 12~5Ma). At that time, the liquid hydrocarbon migrated along the relatively stable ramp through unconformities underneath the Kapushaliang Group (K1kp) and the Tertiary, and the connected sandstone of the Bashijiqike Formation (K1bs), and finally accumulated in the traps of the north and south structures. The tight shale & siltstone of the K1kp and the salt rock of the Tetiary, two sets of widely occurred regional seal, assured the migration of the oil in long distance. There were also some liquid hydrocarbons in the center of the basin, where the early formed small scale normal faults were major migration pathways.

Natural gases were generated during Kuqa Stage (N2k, about 5~2Ma) and early Quaternary (2~0.5Ma) when the basin subsided quickly and faults thrust intensively. The horizontal migration routes were no longer effective due to the obstacle of the faults, and then the vertical migration pathways were developed quite well owing to the formation of a series of thrusting faults from north to south. It was these newly-developed vertical faults that functioned as key migration pathways and made it possible for the formation of the huge gas pools in the shallow traps. The abundant source rocks could have generated huge amount of natural gases, thus nearly all of the traps, no matter deep or shallow, could become gas pools. As far as the natural gas pools in the Kuqa Foreland Basin were concerned, where there was a trap, there was a gas reservoir. But there were only a few associated gases in both north and south oil pools, for the invalidation of the horizontal migration pathways.

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