ZHA, MING, ZHANG, WEIHAI, and QU, JIANGXIU
University of Petroleum, Shandong, China

Abstract: Overpressured Compartments in Junggar Basin, Northwest of China: Mechanism and Hydrocarbon Distribution

There are different types of overpressured hydrocarbon compartments in the Junggar basin, Northwest of China. In the center of the basin, overpressure occurs within Permian, Triassic and the lower Jurassic sequences. The top boundaries of compartments are impermeable mudrocks with higher contents in carbonate and silicate, and depths are at 3900-4400m which cut across structural and stratigraphic boundaries. The mechanism of overpressuring is mudstone diagenesis, sedimentation and hydrocarbon generation of Jurassic coal sequences. In the south portion, however, anomalously high pressure exists within the lower Tertiary soft mudrock and forms "pressure sealing", which is related to disequilibrium compaction of thick mudstone sequences that were rapidly buried. The mudstone has been identified as an important sealing for valuable gas accumulation in this area.

The study shows that overpressuring controls on hydrocarbon evolution, entrapment and distribution in the Junggar basin. In the center, there is more liquid hydrocarbon within overpressured compartments with higher temperature than in normal pressure rocks. Because overpressuring restrained compaction and diagenesis and resulted in microfractures, rocks around the compartments show higher porosity and permeability. In south of the basin, "pressure sealing" of mudstone improves the capability to seal industrial gas. Under the control of overpressuring, most of liquid hydrocarbon occurs within high-pressure zones, and gas is entraped above the compartments.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas