Characteristics And Main Controlling Factors Of The Beach-facies Reservoirs In The Longwangmiao Formation In Gaoshiti-moxi Area

Recently,we have made a breakthrough in marine carbonate exploration and development,especially in discovering an important carbonate oilfield in the Sichuan basin.In 2011,we made great discovery in the Dengying Formation,specifically in Well GS-1 misering.In 2012,we acquired a gas production rate of about 190.68×104m3/d from well MX-8 in the Longwangmiao Formation;which boosted the exploration and development of the Longwangmiao Formation.Although scholars have done a lot of research in the Longwangmiao Formation in Gaoshiti-Moxi area,and because carbonate reservoirs are at great depth and with high heterogeneity,no attention was placed on the various controlling factors of petroleum entrapment,many problems were not been solved.Therefore,it is very essential to do the research about reservoir characteristics and main controlling factors of Longwangmiao Formation.Only in this way can we make a prediction of favorable reservoir distribution.Based on core observation and thin section identification,in addition to reservoir physical property data,our research on the characteristics and main controlling factors of the Longwangmiao Formation beach-facies reservoirs in Gaoshiti-Moxi area was facilitated.The main objectives are as follows:1.Based on the research of the basic characteristics and reservoir types of Longwangmiao Formation in Gaoshiti-moxi area,it was discovered that the reservoir rocks was grain dolostone and crystalline dolostone;The reservoir spaces mainly include intergranular dissolved pores,intercrystalline dissolved pores and bedding karst caves;These reservoirs mainly belong to low porosity;and low permeability reservoir,and partly to the low porosity;and high permeability reservoir category.The reservoir can be divided into five types: crystal grain dolostone reservoirs with dense bedding caves(Type ?-1),grain dolostone reservoirs with caves(Type ?-2),grain dolostone reservoirs with pinholes and separate caves(Type ?-1),grain dolostone reservoirs with pinholes(Type ?-2),“piebald” crystal grain dolostone reservoirs with dissolved pores and separate caves(Type ?).Type ?-1 reservoirs have high quality properties,while type reserv? oirs have low quality properties.2.From the perspective of lithofacies,high frequency sequence,diagenesis and tectonics,we analyze the main controlling factors of Longwangmiao Formation beach-facies reservoirs in the study area.Silt-fine crystalline dolostone and dolarenite have good quality reservoir properties.Longwangmiao Formation is a sequence,and can be divided into two fourth-order sequences: pss1 and pss2.The high quality reservoirs are at the top of deposit cycles in fourth-order sequence.Generally,the reservoirs in Moxi area have good qualities than those in the Gaoshiti area.In the Moxi area,the reservoirs are at the top of the pss1 and pss2 in the HST.In the Gaoshiti area,the reservoirs are in the middle and top of pss2.Karstification plays a key role in the development reservoir spaces,including dissolved pores and karst caves.Compaction and pressure solution,in addition to cemetation,are responsible for destroying the reservoir spaces.Fractures improve reservoir permeability.3.Research shows the diagenetic sequence of the Longwangmiao Formation reservoirs as follows: grain bank sedimentation,sea cementing,penecontemporaneous karstification,penecontemporaneous dolomitization,hypergene karstification,hydrothermal mineral filling,burial karstification,and fractures.Based on this diagenetic sequence,we divided the reservoirs into five stages: pore forming,hypergene karstification,hydrothermal mineral filling,burial karstification-asphaltic filling and fractures.Therefore,the evolutionary model of Longwangmiao Formation reservoir was made.Pore forming and hypergene karstification enhanced the porosity of the reservoirs.Hydrothermal mineral filling and burial karstification-asphaltic filling reduce the porosity of the reservoirs.Tectonics derived fractures,enhanced the permeability of the reservoirs.4.Based on the research on vertical sequence and horizontal correlation of the Longwangmiao Formation beach-facies reservoirs,and with the aid of the evolutionary model of the reservoirs,we made predictions about the horizontal distribution of the favorable reservoirs of the Longwangmiao Formation.In the Gaoshiti area,the pss1 reservoirs are general,and the pss2 reservoirs have good quality,towards the top of the pss2 and are the most favorable reservoirs.The reservoirs in the Moxi area have good quality than those of the Gaoshiti area.Type distributes in the intraplatform shoal and ?the edge of northwestern denudation area in the Moxi area,and transits to type ?-?reservoir eastward and southward.Compared to the pss1 reservoirs,the pss2 reservoirs have high quality,good connectivity and well-distributed in the study area.