Abstract:

The main controlling factors of shale gas enrichment in the Ordovician Wufeng FormationSilurian Longmaxi Formation in the Sichuan Basin and its adjacent areas include: (1) High quality shale formed in favorable palaeoenvironment is the base for shale gas enrichment. Sichuan Basin and its adjacent areas experienced intracontinental depression in a weak compressional setting during the deposition of Wufeng and Longmaxi Fms. A spacious deep water shelf was formed during this period due to rapid global sealevel rising after the glacial epoch, providing a favorable setting for massive breeding of simple organisms. In the mean time, the obliterative bay provided a favorable geochemical environment for the preservation of organic matter, resulting in the high organic content in the strata. A large number of organic silicon, generated by special types of biology, formed rock framework with high compress strength in the early stage of diagenesis, which provided favorable space for crude oil generated in the early phase, as well as a material base for the formation and preservation of abundant organic pores in the stage of oiltogas. (2) Organic pores and special fractures can guarantee the enrichment of shale gas. High quality shale with high organic carbon content contains more organic pores and larger gas content, in which the proportion of free gas is higher. Special fractures include beddingparallel fractures and small scale fractures. Hydrocarbonforming organisms accumulating on bedding surface, such as graptolites and algae, promoted the development of beddingparallel fractures. Small scale fractures enlarged reservoir space and improved filtration capacity of shale, which was in favor of shale gas accumulation. (3) Effective preservation under moderate uplifting is the key factor for shale gas enrichment. However, if deformation and denudation of strata were too strong, the closed preservation system would be destroyed, both conventional and unconventional petroleum system might become invalid totally or partially. It would be the best if the microfractures caused by uplifting did not form large crossingbed fractures. The subdued strata with certain burial depth and without tectonic faults, especially opening faults, have favorable conditions for preservation of shale gas. The difference in preservation conditions between Sichuan Basin and its adjacent areas was mainly due to different tectonic deformation, sedimentation, uplifting and denudation, controlled by the middle Yanshanian tectonic movement. Shale gas play fairways of the Ordovician Wufeng FmSilurian Longmaxi Fm in Sichuan Basin and its adjacent areas are located in the composite region where thick black shale enriched in organic matter were deposited in the central area of deepwater shelf, the shale layers with prosperous organic pores, microfractures and beddingparallel fractures were well developed and the timing of uplifting in the Yanshanian was late and the amplitude of uplifting in the Himalayanian was relatively small.

Key words: shale gas, main controlling factors of enrichment, Ordovician, Silurian, Sichuan Basin and its adjacent areas