A Research Of Characteristics And Main Controlling Factors Of Longwangmiao Reservoir In Central Sichuan Basin

The exploration of lower Cambrian Longwangmiao Formation in Sichuan basin has begun in 1960s,the early stage exploration principle of Caledonian paleouplift is in favor of oil-gas accumulation is not succeed.There are weak gas content in structure of Longnüsi,Weiyuan and Gaoshiti,the development of well is not good.The exploration and development prologue of Longwangmiao Formation gas reservoir has been opened by PetroChina since 2012,when the production of Moxi8well is 190.68×10~4m~3/d of Longwangmiao reservoirs in Central Sichuan Basin.Longwangmiao Formationin Central Sichuan Basin is a deep carbonate reservoir in which experienced multi-stage tectonic superposition and multi-stage accumulation of oil and gas.The diagenetic evolution is intricate,reservoir heterogeneity is strong and high quality reservoir formation and preservation mechanism is complex.Based on geological,seismic,logging,geochemical and test data,combining with the early Cambrian mianyang-changning intracratonic sag and the evolution of palaeouplift in Central Sichuan Basin,oil and gas accumulation and sedimentary-diagenetic evolution in Longwangmiao Formation,characteristics and main control factors of high quality reservoir in Longwangmiao Formation have been studied.The sedimentary of Longwangmiao Formation is under the control of early Cambrian mianyang-changning intracratonic sag.Intracratonic sag is in dying stage of the Longwangmiao period and inheritance deposition of paleo-geomorphology.Gaoshiti-Moxi area in a high position of paleo-geomorphology,center of intracratonic sag is relatively low.Central Sichuan Basinis mainly of carbonate gentle slope sedimentary,development of grain dolomite reservoir is in high part area of the secondary sedimentation paleo-geomorphology.Moxi structure is located in the high position of paleo-geomorphology,development of grain beach is the best.Gaoshiti structureis located in the high position in paleo-geomorphology slope,development of grain beach is better.The center position of intracratonic sag is low,development of grain beach is poor.Affected by deposition in palep-geomorphology and high frequency sea-level change,vertically developed 2~3 grain beach sedimentary cycles in the study area,the upper part is more development than the lower part.Longwangmiao Formation reservoir heterogeneity is strong and differences between different parts of the reservoir is big.Longwangmiao Formation high quality reservoir lithology are mainly of dolarenite dolomite and crystalline dolomite,second mainly of oolitic dolomite and a small amount of gravel dolomite.Reservoir space types are mainly of intergranular dissolved pore,intragranular dissolved pore,intergranular(dissolved)hole.There is also a small amount of moldic pores,local development solution pores,caves and fractures.Porosity is decrease with increasing well depth,and porosity of higher porosity values(more than 2%)of finely crystalline dolareniteis more than 2%.The difference of matrix and pore in Longwangmiao Formation is not big,high quality reservoirs are mainly of development section in solution pores.Micro-throat are mainly of retractile throat and tubular throat,retractile throat is mainly development in granular dolomite,tubular throatis major development in the crystalline dolomite.In horizontal direction,the reservoirs of Moxi area is superior to that of Gaoshiti,the poor intracratonic sag center development.In the longitudinal direction,upper part is more development than the lower partin Longwangmiao Formation reservoir.Longwangmiao Formation reservoir is a deep carbonate reservoir,diagenetic evolution process is complex which controlled by the intracratonic sag and paleouplift evolution.Diagenesis is include micritization,dolomitization,cementation,dissolution,compaction-pressure solution,recrystallization,filling,fracture and hydrocarbon(including organic acid)filling.Key diagenesis are Seawater dolomitization,penecontemporaneous fresh-water dissolution,and the hydrocarbons(including organic acid)filling.(1)Seawater dolomitization occurred before burial role,grain dolomite is more resistant to compaction and pressolution than grain limestone,which is critical diagenesis condition for the early pore preserved.(2)Affected by sedimentary paleo-geomorphology,penecontemporaneous fresh water dissolution in Moxi area is the most development,which formed a large number of early fabric selective secondary pore intergranular dissolved pore,intragranular dissolved pore and moldic pores and so on;Gaoshiti area is the second development,intergranular dissolved pore and intragranular dissolved pore are development in the upper of Longwangmiao reservoirs.Intergranular dissolved pore and intragranular dissolved pore are only development in upper intracratonic sag,have a small contribution to the reservoir.(3)Hydrocarbons(including organic acid)dissolution are mainly of second phase of the late Permian-Middle Triassic,the bitumen by ancient reservoir cracking is widely development,which filling in the solution pores,intergranular dissolved pore,intragranular dissolved pore,intergranular(dissolved)in the holes and fractures.In the longitudinal direction,upper part is more development than the lower partin Longwangmiao Formation reservoir.In horizontal direction,Moxi structure is located in the high part of paleouplift during this period,dissolution is the most development.Gaoshiti structure is located in the slope area of paleouplift during this period,dissolution is less than the Moxi area.The center position of intracratonic sag is relatively small,hydrocarbon(including organic acid)dissolution is the smallest.Combining with the early Cambrian mianyang-changning intracratonic sag and the evolution of paleouplift in Central Sichuan Basin,oil and gas accumulation and sedimentary-diagenetic evolution in Longwangmiao Formation,main control factors of high quality reservoir in Longwangmiao Formation is grain beach,water dolomitization,penecontemporaneous fresh water dissolution and hydrocarbon(including organic acid)filling effect.The development of grain beach is controlled by palaeogeomorphology and high frequent sea-level change,mainly developed in the secondary structure high of palaeouplift,which is the important material base for high quality reservoir formed.seawater water dolomitization and penecontemporaneous fresh water dissolution are the key to early high quality reservoir formed.The former is used for early diagenesis retention,the latter is used for early constructive diagenesis.Hydrocarbons(including organic acid)filling effect is used for important constructive and retention diagenesis in advanced stage high quality Longwangmiao Formation.High quality pore-cave reservoirs are formed by multi-stage karstification superposition in Longwangmiao Formation,and keep well in later period overpressure gas reservoirs.Overall,high quality grain dolomite reservoirsis is the best in Moxi area,that of Gaoshiti is the better,the center of intracratonic sag is poor.In the longitudinal direction,upper part is more development than the lower part in Longwangmiao Formation reservoir.