Fluid-Conduit Framework and Its Control on Petroleum Accumulation in the Junggar Basin

The Junggar basin is a huge, multiple-stage basin in Northwest China. Different structural units of this basin have quite different evolution histories, fluid-flow environments, fluid-conduit frameworks and petroleum accumulation histories. The northwest margin of the Junggar basin is in a normally pressured environment with a fault-unconformity conduit framework connecting source rocks and traps effectively. Due to the effectiveness of the fluid-conduit framework in this region, the major petroleum accumulation occurred almost at the same time as the main phase of oil generation. Very few faults developed in the center part of the Junggar basin, and faults in the shallow section could not connect with faults in the deeper section. As a result, petroleum generated in the Permian source rocks could not have migrated vertically into the overlying Jurassic reservoir rocks during the peak petroleum generation stage (Triassic to Jurassic) until the Cretaceous, when fracturing caused by overpressure occurred and the Jurassic reservoir rocks were connected with the Permian source rock by faults and hydraulic fractures. Because the fault-hydraulic fracturing and the resultant fault-fracture conduit framework developed a relatively long time after peak oil generation, the major petroleum accumulation came later than peak oil generation, and the accumulated oil had a relatively high maturity. This study confirms that in an overpressured environment the fluid-conduit framework is dynamic, and has a controlling effect on the petroleum migration pathway and accumulation intervals, as well as the injection histories.