Development Characteristics And Reservoir Evaluation Of Shiniulan Formation Reefs In Southern Chongqing And Northern Guizhou

A large number of early Silurian carbonate deposits and changes of marine environment in southern Chongqing-northern Guizhou area resulted in a large number of oxygen-loving benthic organisms,which formed a large range of reefs in the late Shiniulan Formation.In this paper,the reefs and reservoirs of Shiniulan Formation in the study area are comprehensively studied by means of structural,logging interpretation,outcrop data,core analysis,thin-section microscopic identification,element analysis and reservoir physical property analysis,and the following understandings are obtained.The Shiniulan Formation is divided into four sections from bottom to top,mainly sedimentary nodular limestone,bioclastic limestone,biohermal limestone,sandstone,mudstone and dolomite.The Shiniulan Formation is dominated by sea-land transitional facies and marine facies and is characterized by shelf-carbonate gentle slope sedimentary environment.The reefs of Shiniulan Formation are developed in the gentle slope zone of epicontinental sea.Reefs are distributed in the upper and middle strata and Reef-building organisms are mainly tabulate corals,rugose corals and less stromatoporoids.Reefs are small-scale patch reefs and biostrome which the uplift height is 10-30 meters,and the extension generally does not exceed 10 kilometers.According to the longitudinal development characteristics of reefs,the growth and development of reefs can be divided into four stages: stabilisation,colonization,diversification and domination.The main factors of reef growth and extinction are discussed by thin section identification,analysis of isotope and REE,including the agitation of terrigenous debris,fluctuation of sea level,temperature of seawater and tectonic movement.It is clear that reefs are mainly distributed mainly in warm or torrid tropical and subtropical climate zones and a marine sedimentary environment with shallow water in thedeposition period.Based on the comparison of global reefs,the reefs of Shiniulan Formation in the same period were distributed on the Laurent,Baltic,Siberian and Kazakhstan blocks.The similarities and differences of the Menier Formation and Shiniulan Formation reefs in the same period were discussed with an example.With the help of core data and microscopic thin section identification,reservoir space types of Shiniulan Formation are mainly fracture and pore,among which fracture is mainly structural fracture and dissolution fracture,and pore includes primary and secondary pore.Diagenesis includes compaction-pressure solution,silicification,cementation,filling,recrystallization and dissolution.According to physical property data and mercury injection test analysis,the porosity and permeability of the Shiniulan Formation are low.Oil and gas are mainly stored inbiohermal limestone,bioclastic limestone,argillaceous limestone and sandy limestone,mainly in low-porosity and ultra-low-permeability reservoirs,compact reservoirs,and a few low-porosity and ultra-low-permeability reservoirs and ultra-low-porosity and low-permeability reservoirs.Based on reservoir physical properties and sedimentary facies distribution pattern,the distribution range of reef reservoirs of the Shiniulan Formation in the study area is divided into type II reservoir,type III reservoir and type IV reservoir.It is clear that the exploration and development of reef oil and gas in Shiniulan Formation are related to well distribution,fracture development,dissolution and dolomitization.