Structural Features In The Northern Segment Of Longmen Mountains And The Discussion On Its Hydrocarbon Prospects

The northern segment of Longmen intracontinent composite orogen refers to the extension of the northeast part, bounded to Beichuan,An'xian, with the main features exposing Jiaoziding complex rock and Tangwangzhai syncline. The overall characteristics are complex thrust-nappe structure. Through detailed field survey on the surface structure, survey on the distribution of the surface bitumen, re-interpretation of the regional seismic profiles, etc, the present paper has conducted a detail study on the surface structural characteristics, the underground structural characteristics, the process of formation and evolution, distribution characteristics of the surface bitumen and prospects for oil and gas exploration in the area. The northern segment of Longmen Mountains has three main faults:Qingchuan-Yangpingguan fault, Liangshui-Lin'ansi fault and Majiaoba-Zhuyuanba fault. The three faults have multiple phases of deformation at different times with different movement characters. The first two are mainly ductile deformation in early stage and brittle deformation in late time, and the Majiaoba-Zhuyuanba fault is mainly dominated brittle deformation. Based on the three main faults, the northern segment of Longmen Mountains and it's near area are divide into four structure units. They are Bikou micro-block, back-Longmen intracontinent orogen, pre-Longmen intracontinent orogen and the northern sichuan composite foreland basin. Tectonic deformation of the every tectonic unit is significantly different. From northwest to southeast tand from the orogen to the basin, the deformation-metamorphism reduced significantly.The northern segment of Longmen Mountains can be divided into upper, middle and lower layer-structure in vertical direction according to the interpretation of seismic profiles. The upper structural layer is composed of imbricate thrust sheet by Silurian, Ordovician and Cambrian. The middle structural layer is composed of relatively flat structural layer by Permian, Devonian, Silurian, Ordovician and Cambrian. The under-structural layer is made up of Sinian and basement.The northern segment of Longmen Mountains developed a wide range structural styles, who mainly includes imbricate thrust structure, duplex, fault-related fold, and klippen etc.The basic shape of the northern segment of Longmen Mountains has been deformed in Indo-Chinese epoch, experienced Yanshan and Himalayan movements. Regression; general uplift and weak folding are in early Indo-Chinese epoch.Strong folding and thrusting happened in late Indo-Chinese epoch. Uneven uplifting took place and alluvial fan developed in Yanshan movement. The fault-folding enhanced and strong thrusting happened in the Himalayan movement.Beacause of the strong structure movements in the northern segment of Longmen Mountains, the early ancient reservoir has be destroyed and a large number of bitumen exposed on surface. The ancient reservoir of Dengying series in Ningqiang Shanxi province is subjected to uplifting and denudating, and bitumen dispersely distributed in the Dengying reservoir. The Kuangshanliang area in Guanyuan the bitumen is vein and mainly along the fracture distribution. It showed the ancient reservoir under the area has been destroyed on some extent because of the fault devlopment, and even maybe completely destroyed. The gravel-like bitumen of Lower Cambrian in Qingchuan area indicated that there was oil and gas generation and gathering at late Sinian. The three different bitumen have the same source rock of the upper Sinian Doushantuo formation.The northern segment of Longmen Mountains has original favorable conditions of oil and gas accumulation, but later the trap's storage conditions deteriorated because of strong structure movements. The hydrocarbon accumulation of the northern segment Longmen Mountains was mainly controlled by the late tectonics. And the hydrocarbon accumulation of the northern Sichuan foreland basin mainly was controlled by the later structural trap and fracture system. Some small and medium-sized oil and gas field can be found in the front range of the northern segment of Longmen Mountains, and it's front area has a good exploration prospects especially the western ramp region of Zitong depression because of adequate oil and gas source and late weak deformation.