Sedimentary Facies Of The Longwangmiao Formation In The Northern Sichuan Basin And Its Relationship With Reservoirs

Longwangmiao formation is one of the significant oil-bearing strata in Sichuan basin and Anyue gas field has become the largest single marine integrated gas reservoir profiting from the Longwangmiao formation,which is achieved the great exploration breakthrough in the middle area of Sichuan.As the strata of Longwangmiao formation widely developed in the north of Sichuan,it has become an important area for oil-gas exploration potentially.With the guide of carbonate sedimentary,petroleum geology and carbonate reservoir geology and applying the materials of drilling,logging,field profiles and analyzed laboratory data comprehensively,this paper investigated the Cambrian strata in the north of Sichuan and studies the Longwangmiao formation,sedimentary facies,reservoir and the relationship between them combined with regional geological surveys and previous research achievements.Stratigraphic studies indicate that the bottom and top boundaries of Longwangmiao formation in the north of Sichuan are distinct,which is conformity with the underlying Canglangpu Formation and overlying Douposi Formation.The rock type of Changlangpu formation is main consist of purple,gray-green mudstone,silty mudstone.And argillaceous-arenaceous dolomite in Douposi formation with purple mudstone in local region.The rock type of Longwangmiao formation is diverse,including grain dolomite dominated by carbonate,crystalline-grain dolomite,calcareous dolomite,micrite,siltstone constituted by carbonate and clastic rock,pelitic siltstone and a small mount of sand-bearing grain dolomite etc.In south of Sichuan basin,the distribution of Longwangmiao formation is stable.Its thickness is 70 to 150m.Form northwest to southeast,the trend of thickness is gradually increase.Longwangmiao formation in the north Sichuan has three depositional systems including tidal flat,restricted platform and open platform.From the west to the east,the sedimentary facies are tidal flat,restricted platform,open platform and slope-basin in the plain,which further divided into subfacies as mixed tidal flat,the lagoon,platform beach etc.and microfacies.Each sedimentary facies has certain sedimentary characteristics.When it deposited in south of Sichuan basin,Longwangmiao formation is deposited shallow in west,deep in northeast and southeast.The most favorable reservoir rock in Longwangmiao formation of the north Sichuan is grain dolomite,followed by crystalline-grain dolomite and the reservoir thickness is generally thin.The reservoir spaces include intergranular pores,intergranular dissolved pores,intercrystalline pores,intercrystalline dissolution pores and moldic pores etc.Additionally,the fractures for seepage are not developed in this area.According to the physical properties of statistics,the average porosity of Longwangmiao formation is 3.19%and the average permeability is 0.05mD,which illustrated the type of reservoir is porosity reservoirs.Based on the criteria established for reservoir classification,most reservoir layers of Longwangmiao formation is classified in the third class.In some areas,such as the top of Longwangmiao formation in Ba Mountain,the average of porosity is up to 4.58%,which can be classified in the second class.In general,the reservoir performance of Longwangmiao formation is poor.The paper delivers the conclusion that the most beneficial sedimentary facies for reservoir developing is the platform beach,which is the subfacies of restricted platform,followed by the dolomitic lagoon,which is the microfacies of lagoon.Moreover,it is concluded that the open platform is not conductive for reservoir development and mixed tidal plat is the worst.