Abstract:

Dachiganjing structural zone is one that is quite a typical high-steep anticlinal fault folded zone.Its subsurface structural pattern has varied obviously,and its concealed faults and traps are developed well.Hence it has become a favourable Carboniferous gas reser-voir place with fracture and fracture-porous preserving spaces.The structural zone underwent following evolution process:in Himalayan period-1,a wide symmetrical anticline formed;in Himalayan period-2,recoil faults formed on the anticline,and a secondary concealed anticline derived from the footwall of the steep limb;in Himalayan period-3,the steep fault-depressed belt varied and two subrecoil faults derived from the secondary concealed anticline;in Himalayan period-4,a overthrust belt developed on the culmination of the main structure and it made the structure disintergrate into monofault traps or fault-block traps.The migration,acculmulation and distribution of hydrocarbon in the structural zone are controlled by following two types of strctures.Type-1,the recoil fault assemblage (compressional faults),formed a kind of fault-culmination trap,which make the filling degree of Carboniferous gas pools equal or greater than 1.0;type-2,shear-compressionalfaults,usually reformed the primary gas pools,as the result,the gas pool filling degree of Carboniferous reservoirs in Diaozhongba and Longtou area decreased to less than 1.0.The hydrocarbon pools in the study area are primarily anticline-fault type,and then fault-block and fracture types.The later ones are generally not controlled by anticlines or culminations.