Abstract:

The surface structure of the eastern Sichuan Basin consists mainly of the early and late Neocathaysian tectonic units.The early one,which was formed from the end of the Jurassic to the beginning of the Cretaceous,trends about 60°E.This unit is dominated by gentle folds with structure planes of compresso-torsional origin and is marked by two normal struc-tural belts,i.e.the Kaixian-Linshui and Fengjie-Fuling structural belts.The late Neocathaysian unit which was formed at the end of the Palaeo-gene,strikes about 20°E.This unit consists of some narrow,elongatedand high anticlines,compressive fractures of the same trend and relati-vely broad and gentle synclines,marked by two normal structural belts,i.e.the Huayingshan-Tongluoxia-Mingyuexia and Fangdoushan-Qiyue-shan structural belts.The compounding patterns of these two units are do-minated by transposition,but juxtaposition and super position patterns arealso present.The early and late Neocathaysian units as well as the hinterland middle-cathaysian unit(formed at the end of the middle Triassic)are the three principal units for oil accumlation within the area concerned.They allhave controlling effects on the Permian and Triassic oil and gas migration and accumulation.The late unit,however,is the more important one andthe anticlines formed in it constitute the main traps for oil and gas.The overlapping position of the normal structural belts of the three units is the most promising zone;the anticlines where the early and late units compoundare the most favourable structures,while the high-producing zone which controls the Permian and Triassic gas deposits is a fratured zone of tensionand tenso-torsional origin.