Analysis On The Deformation, Rupture Of Thrust Belt And Preservation Conditions In Longmen Mountain

The overthrust nappe thrust in Longmen mountain refers to the area from the east part of Maowen-Wenchuan thrust in western Sichuan to Guankou-Zhangming thrust. Three gas fields(Zhongba, Qiongxi and Pingluoba gas fields) and many gas-bearing structures, such as Yazihe, Jinma, Dayi, Gaojiachang and Lianhuashan gas-bearing structures were discovered in the area of sub-thrust belt detachment structure and the front edge. Those areas with complicated structures in the west part of Guankou were of low level of exploration.The overthrust belt in Longmen mountain has been studied for many years and many books and articles about it were published, especially since the period of the tenth-five years, Sinopec has investigated prospecting funds over thousand millions for exploration and research. However, whether Sinopec or CNPC has not achieved great breakthrough. Why? The complicated structure conditions and less research work about preserve conditions are the direct reasons. For those faulting and crushing areas, preservation conditions are very important. Therefore, preservation conditions are key factors of gas accumulation for complicated structures in Longmen mountain. However, till now, there are less systematically and deeply research work about the preservation conditions.This article focused on the relationship between deformation and rupture of thrust belt and preservation conditions in Longmen mountain through integrated studying on field geological survey analysis, sampling and laboratory research (physical simulation, experiment and comprehensive study), adopting the combination methods of geology, geophysical and geochemical, macro and micro, qualitative and quantitative, utilizing outcrop, core, seismic, logging data etc, combining the preservation conditions of oil-gas-water and seals. It achieved some new understanding and new results, especially on the research of regional tectonic units, deformation features, characteristics of faults and sealing.The tectonic units of orogenic belt and the front edge in Longmen mountain were divided into five one-grade structure belts:â… :Songpan-Ganzhi folded belt;â…¡:Overthrust belt in rear Longmen mountain;â…¢:Thrust detachment belt in front Longmen mountain;â…£:Foreland basin in western Sichuan;â…¤:flat belt in middle Sichuan. The foreland basin in western Sichuan was further divided into six secondary structure belts:â…£1:front edge belt;â…£2:low and flat belt in the front of Longmen mountain;â…£3:flat belt in the north part of Longmen mountain;â…£4:low and flat belt near WE in the middle of Longmen mountain;â…£5:flat belt in the south part of Longmen mountain;â…£6:Longquan mountain-Xiongpo structure belt.From WN to ES, the level of structure deformation changed from strong toughness to brittleness, from high intensity to low intensity. All structural styles are included, such as vertical bend fold, fault related fold, imbricate fold, duplex structure, backthrust, nappe etc. Under the background of NE-trending structures, there were WE-trending folds and faults including vertical folds, flat folds and horizontal faults etc ., existed in the middle section of the front Longmen mountain.Based on the field research, it shows that the tectonic period and the stress level of central fault, front fault, front in-situ system, front klippen etc, became simpler and simpler from WN to ES. The central faults deformed nine times, such as (1) NW-SE ductile thrust; (2)NEE-SWW dextrorotation(enhanced semi-brittle deformation) (3)NW-SE thrust(cleavage) (4)SN levogyrational traction fracture cleavage; (5) levogyrational thrust(joint); (6)WE dextrorotation; (7)NW-SE stretch, traction fracture; (8)levogyration; (9)NW-SE thrust. The tectonic period of those faults in the front Longmen mountain became less, some of where experienced tectonic movement only one time. From the balanced cross section, it can be revealed that the tectonic style of front and foreland edge was mainly imbricate overthrust fault and related fold, with large reduction of fault and front expansion structure development. Physical simulation achieved reappearance of most tectonic style including front expansion ones.Based on the fission track study after sampling apatite in both hanging and foot walls in front Longmen mountain, it is proved that there existed different lift movement since Cretaceous period, the south one lifted apparently.The semi-quantitative relationship between deformation strength and preservation condition has been initially established. Only if one of the following conditions is achieved, the sealing can be preserved well, such as formation reduction amount(in terms of regional) is less than 7.3%,fold factor is 0.02-0.13,crack density is less than 5 per meter, rupture fractal dimension D is less than 1.3.Four methods to study fault sealing for Bailu Yazihe area have been adopted, such as pressure of fault plane, smear factor of mudstone, formation pressure and Allan profile. The study shows that Guankou fault sealed the foot wall, not hanging wall. Pengxian fault did not seal. Xiangshui fault in Anxian sealed when it entered certain depth. Dayi F1 fault sealed well.Through the above evaluation and studies, it can be concluded that part structures in the areas of front and foreland edge were of bad preservation condition; most low and flat structure belts and WE structures were of good preservation condition and proper strength; middle and flat structures were of good preservation condition and weak deformation.According to the above relationship between deformation strength and preservation condition, favorable gas accumulation area was evaluated in the research area. Different factors can be considered for the different layers. The south and north structures of front flat structure are the most favorable area in Xujiahe formation(â…£2),north, middle and WE structures of front flat structure belt in Leikoupo formation(â…£2),the north, middle structures of front flat structures(â…£2)in Jialinjiang formation.