Study On Front Structural Features In The Middle Segment Of Longmen Mountains And The Conditions Of Hydrocarbon Preservation

The anterior margin in the middle of Long menshan locates in the zebraic region of producing much hydrocarbon, which is close to the hydrocarbon kitchen. Whatmore, the reservoir rocks in this region are much better, which belong to the terrestrial and paralic facies. This field is a good one to form oil and gas accumulation because there are a lot of folds which are affected by Guankou fault. However, because the Longmen Shan thrust nappe with complex, multi-phase characteristic and secondary fault develops quite well, so the outcome of exploration activity is lessly satisfied in Jinma-Yazihe tectonic belt.So in order to breakthrough oil and gas exploration in the Longmen Mountains front, it is necessary to study the preservation conditions in the area depthly.This research has discriminated buried faults in the middle of Long menshan with many methods, such as the explanation of remote sensing geology, the measure of gas radon in nuclear physics, the magnetotelluric method, the magnetotelluric deep sounding and the field investigation. We verify that these buried faults are the important channels to basin, which leap the up and down Guankou fault. We also find that the remote sensing techniques and the geophysicaland geochemical methods are very feasible, serviceable and profitable.The main buried faults in the region are much active. They not only cause the disclosure of gas reservoir below ground, but also are the canals to transport gas. So researching the activity of buried faults is significative. To demonstrate the activity of major buried fault in this area and the effect of changes in radon concentration as earthquake happen, we apply the radon gas measurement carried out on regular monitoring of aftershocks,and find that the aftershock is much relevant to the concentration of gas radon, which demonstrates that gas radon not only is used to discriminate buried faults, but also keeps watch on the activity.Longmenshan was tensional before the middle period of Norian, which means to be the centripetal fault. After it, the centripetal fault was reversed and Longmenshan moved upward. In this period, the thrust faulting of Longmenshan is very strong and the distance of disposition is much far. The fracture belt of Longmenshan is of sinistral strike slip before Late Pleistocene, and after that, it became to be of right lateral movement. All the drainage systems in this region are opposite"S".We can also analyse Guankou fault and its derivative architecture from mechanics and geological geomorphology. These buried faults are mainly northwest, east north east and latitudinal direction.The northwest faults are developed well, which cutout deeply and leap the upper cap rocks of Lianhuakou formation. We further know that the northwest faults are of sinistral strike slip and the east north east and latitudinal faults are of right lateral movement. The neotectonic movement is strong and the tenso shear is much apparent. The construction of those buried faults is"ç±³"type.As the boundary line in the allegiance of basin and mountain, Guankou fault is thought to be the one of resisting water, and covers well the trap of Xujiahe formation. But in the assessment of the luted feature in this fault, we only take value of the leaped mudstone and geostatic stress except secondary fault. According to the geochemistry of hydrology in oil gas and the regular pattern of groundwater, we can conclude that the secondary fault makes Guankou fault to be the one of diversing water. The type of groundwater is NaHCO3 in shallow layer, and CaCl2 in deep zone. This indicates that shallow layer is affected seriously by surface water, and the feature of sealing gland in deep zone is good, which belongs to sedimentogenic water. So there are hydrocarbon reservoirs of hydrodynamic closure in the region, which is far away from Guankou fault, and is affected weakly by superficial water. In the belt of anticlinal structure in Juanyuan-Yazihe, there may be fracture type reservoirs if buried faults and splits are growed well. The surficial water is much in the Yunxi-Xujiachang nose upfold, and the roof of the upfold is damaged easily. So the shallow gas pools are difficult to be generated, but the deep ones can be created. We also notice that gas accumulations may be found in downfolds in Zhongxing-Yutang of Dujiangyan region.