Structural superimposition and conjunction and its effects on hydrocarbon accumulation in the Western Sichuan Depression

Records of unconformities and lithologic facies and analysis of paleo-structure show that at least eleven tectonic boundaries exist in the Meso-Cenozoic continental strata in the Western Sichuan Depression. Affected by the Longmenshan thrust belt on the west side, Micangshan structural belt on the north side and Chuandian structural belt on the southwest side, NE-trending, sub-EW-trending and sub-NS-trending structures were developed correspondingly in the Western Sichuan Depression. The NE-trending structures were developed in varying degrees in most tectonic movements, while the sub-EW-trending and sub-NS-trending structures were formed in the Yanshanian and Himalayan, respectively. The three groups of structures were superimposed by one another in multi-stage tectonic movements, producing mainly five types of structural complexes, i.e. coaxial superimposition, orthogonal superimposition, oblique superimposition, multi-direction superimposition, and structural conjunction. Structural controls on hydrocarbon accumulation in the Western Sichuan Depression can be expressed as the following three aspects, i.e. timing, location and modification. Hydrocarbon distribution and the formation of primary gas pools were mostly controlled by early structures (in the Indosinian and Yanshanian), whereas late structures (mainly in the Himalayan) only had modification effects on early gas pools, resulting in structural displacement, reservoir improvement, the destruction of primary gas pools and the formation of secondary oil pools. Accurate reconstruction of the Indosinian-Yanshanian paleo-structures and fracture behaviors during the multi-stage and multi-direction structural superimposition are two important aspects that need to be paid more attention to in the future hydrocarbon exploration in this region.