Lateral Sealing of Faults and Gas Reservoir Formation in Volcanic Rocks in Xujiaweizi Fault Depression

Deep layers of the Xujiaweizi fault depression refer to the layers below 1st and 2nd Member of the Lower Cretaceous Quantou Formation (K1q1+2),in which there are 3 sets of volcanic rock reservoir: the Huoshiling Formation (K1hs),1st and 3rd Member of the Yingcheng Formation (K1yc1 and K1yc3). At present larger scale gas reservoirs have been found in the 2 sets of volcanic rocks in K1yc. Fault systems in the Xujiaweizi fault depression are complex and obviously control gas reservoir formation. Reservoir-controlling faults controlled volcanic fissure eruption in the early stage and experienced strong activity in late K1q—early K1qn. Faults which sealed in the top in regional cap rocks were volcanic conduit when they were formed. There are no obvious fault planes. Fault breccia zone was formed by fault activity in late K1q—early Qingshankou Formation(K1qn). Gas migrated vertically along faults on a large scale and then accumulated in volcanic rocks of K1yc. Because of the poor lateral seal ability of fault breccia zone,gas was occluded laterally by conglomerates which had poor physical properties in K1yc4. For normal faults,gas mainly accumulated in the bottom wall,while for reverse faults mainly accumulated in the hanging wall. Xuzhong strike-slip fault is main reservoir-controlling fault. Gas that accumulated in two walls is characteristic of one increases as another decreases because of the changes of dip and property of faults. This accumulation regularity of gas has important directive significance for exploration deployment,gas reservoir borders determination and reserves submission.