Main origins of high quality reservoir of Lower Cambrian Longwangmiao Formation in the giant Anyue gas field, Sichuan Basin, SW China

The reservoir lithology of Longwangmiao Formation of Lower Cambrian in Anyue giant gas field mainly includes residual granule dolomite and crystalline dolomite. The major reservoir space is dissolved vug, followed by residual intergranular pore and intercrystalline pore. The types of reservoir belong to fractured-vugs and fractured-pores with the average porosity of 4.24% and thickness about 36 m. The vertical and horizontal distribution of the reservoir is controlled by grain shoal facies in inner-ramp and the material basis of which is grain carbonate rock. The constructive factors during the reservoir evolution include: (1) The intergranular pore of grain carbonate rocks lay a foundation of the reservoir space generation; (2) Penecontemporaneous atmospheric water eluviation results in the continuous distribution of large-area massive vugs; (3) Penecontemporaneous dolomitization was favorable for the preservation of earlier porosity; (4) Supergene karstification in Caledonian stage improved the reservoir performance; (5) Fracture cracking and burial dissolution improved the reservoir performance further. The destructive effects of the reservoir suffered are mainly cementation and compaction, and then the filling of hydrothermal minerals and bitumen. After sedimentation and diagenesis comprehensive analysis, the following recognitions are summarized. The necessary condition for the formation of high quality reservoir is grain shoal facies; Penecontemporaneous atmospheric water eluviation is the precondition for the formation of vuggy reservoir; Undergoing penecontemporaneous dolomitization is a prerequisite for the preservation of earlier porosity; Supergene karstification superposition is necessary for the formation of best reservoir. So the main controlling factors of high quality reservoir generation are “Advantage Facies+Penecontemporaneous Atmospheric Water Eluviation+Penecontemporaneous Dolomitization+ Supergene Karstification”. © 2015, Science Press. All right reserved.