NAPPE STRUCTURE CHARACTERISTICS AND EXPLORATION POTENTIAL FORECAST IN LONGMENSHAN, THE WEST OF SICHUAN PROVINCE

The Longmenshan nappe structure in the west of Sichuan Province can be divided into three parts in plane: the Indosinian fold in the north,the Yanshanian fold in the middle and the Himalayan fold in the south.There are 3 layers vertically: the Indosinian autochthon at the bottom,the Yan-shanian nappe structure in the middle and the Himalayan gliding nappe and superposition at the top.The Longmenshan structure has undergone multiple-epoch structural movements with strong deformation,thus its sealing conditions of preservation are the key factor to form reservoirs.In the north,faults are cut through and objective intervals are exposed,thus oil and gas reservoirs are destroyed.In the middle,objective intervals are exposed and sealed half-and-half and the preservation condition is general.In the south,the Middle and Lower Jurassic red bed,which is exposed to surface,develops continuous deformation resulting in seal to the fractured structure,thus the preservation condition has been improved. Hereby,it is concluded that the structural belt from Gaojiachang to Lianhuashan in the southern Longmenshan is the most favorable zone in which large and medium oil and gas fields may be discovered.The fault-fold zone in the middle is the favorable zone in which medium and small oil and gas fields may be found.Oil and gas reservoirs in the north have been destroyed in which small residual heavy oil and gas reservoirs may be discovered.In addition,Dayi,Yazihe,Jinma and Juyuan in the subtle area of the front of mountain should be paid more attention to.