Multiple Originanl Mechanisms Of The Ordovician Reservoir And Their Control On Hydrocarbon Charging In Shuntuoguole Area,Tarim Basin

The Shuntuoguole area is located in the northern slope of Tazhong Uplift in the Tarim Basin.It is located in the Saddle between the Manjiaer Depression and the Awati Sag,the Shaya Uplift and the Katake Uplift.It is commonly known as“Shuntuoguole lower uplift”or"A-Man ancient enemies."From north to the south,the tectonic units can be divided into the Shunbei gentle slope,the Shuntuoguole lower uplift and the Shunnan gentle slope.Among them,the Shunbei area is a new breakthrough in oil and gas exploration this year,with an exploration area of about 19,900 km~2,while the Shunnan area and the Shuntuoguole lower uplift are mainly gas.The exploration area of the gas is8000 km~2,currently in wells such as SN1 and ST1,obtained condensate and natural gas output.Based on the theories of oil and gas geology,reservoir geochemistry,sedimentology,petrology,and structural geology,this paper is based on previous studies of regional Ordovician reservoir characteristics and hydrocarbon charging,using imaging logging,casting thin sections and stress-sensitive experimental analysis,identify the reservoir space types of the Ordovician reservoirs in the Shuntuoguole region,and using the mercury intrusion method,scanning electron microscopy and laser confocal to restore the reservoir pore-throat structure.In particular,the algae pore structure developed by microbial rocks,the types and sources of diagenetic fluids from different periods are analyzed from fluid inclusions,carbon and oxygen isotopes and rare earth elements,and reservoir formation,seismic data and diagenesis are discussed to the controling factors of reservoir formation and the control effect of karst fluid and fault activity on reservoir formation,and analyze the dominant factors of microbial reservoir formation.On this basis,oil and gas geochemical analysis,fluid inclusion system analysis and fracture activity analysis are used to determine hydrocarbon charge,further discussion on the control of karst fluid and cracking on hydrocarbon filling in different blocks and different fault zones in the Shunbei area are also analysised.On the basis of macroscopic and microscopic observation of reservoir space,it is identified that the type of Ordovician carbonate reservoir in the study area is a fault-karst reservoirs,and the reservoir space of different regions and different strata is slightly different.The main reservoir space of Yijianfang formation in Shunnan area is high-angle crack,followed by dissolved pores and micro-cracks,and pores are mainly dissolved intra-granular pores and algae pores.The main reservoir space in the upper part of Yingshan Formation is crack,dissolved pores and quartz intercrystalline pores;the lower part of the Yingshan Formation is dominated by cracks,a small number of pores,and dolomite intercrystalline pores appear locally.The reservoir space of the Ordovician carbonate reservoirs in the Shunbei area is dominated by tectonic fractures,accompanied by the development of dissolved pores.The space of reservoirs in the main faults are high-steep fault-related caves,high-angle structural fractures and associated dissolved pore aggregates.The pores of reservoirs in the secondary fault zone are low-angle structural fractures,high-angle structural fractures and dissolved pores.In addition,both the main fracture and the secondary fracture develop dissolved intragranular pores,dissolved intercrystalline pores,micro-cracks and algae pores.The pore-throat structure characteristics of different lithological developing pores were restored by mercury intrusion method,laser confocal microscopy and scanning electron microscopy.It was found that the limestone pores and throat volume with higher degree of silicification were larger,while the microbial pores developed by microbial rocks,although the pore volume was larger,but the interconnectivity between the pores is poor.The microbial reservoirs developed in the Ordovician area of the study area show better reservoir properties than other lithologies in the porosity-permeability map.Based on the study of the Ordovician reservoir formation mechanism and the observation of microbial reservoir space,combined with the analysis of logging data,the dominant factors of the development of microbial reservoir space are summarized:(1)The formation of primary dolomite can promote calcite dissolution;(2)Rich organic matter content can not only cause organic acid dissolving in the burial diagenesis stage,but also reduce the ultimate strength of microbial rocks and easily break and form cracks under the action of structure.On the basis of the observation of the lamina,it was identified that the Ordovician reservoirs in the study area experienced dissolution,cracking,dolomitization,cementation,compaction and pressure dissolution,mud crystallization and silicidation.Wherein the improved properties of the reservoir main constructive diagenesis is the dissolution,the effect of cracks and the dolomitization;destructive diagenesis which can reduce the reservoir property,including pressolution and cementation.It also includes that can improve reservoir performance in some cases,and in some cases the opposite of dual effects,such as silicidation.Cathode light observation showed that there were mainly four generations of calcite cements,which were dark,dark brown,orange and yellow.The early calcite cement is mainly filled in the dissolution hole,the mold hole and the recrystallized calcite affected by the stress.The color of the cathode light is mainly dark and dark brown.The crack filling calcite is mainly orange yellow cathode light,and the local area is cut through.In the early caves,due to changes in fluid properties,some crack-filled calcites have ring zones that develop yellow cathode light.The developed compression-dissolving suture cuts the crack of the orange-cathed calcite cement and has late cracks passing through the compression-dissolving suture and the dissolution hole.According to the cathode of the light color and cutting relationship,the determined diagenetic ordinal:dark?dark brown?orange?yellow,unfilled fractures most advanced.Karst and cracking are important formation mechanisms of the Ordovician reservoirs in the study area.The karstification of the early diagenesis superimposed the deep karstification,which greatly improved the reservoir physical properties.The controlled cracking by the strike-slip fault activity provides a seepage channel for multi-stage dissolution,and finally forms a“fault-karst”reservoir.Based on the diagenesis type and diagenetic sequence observation,combined with the analysis of fluid inclusions,carbon and oxygen isotopes and rare earth elements,it is concluded that the fluids of the Ordovician reservoirs in the Shunnan area mainly have three endmembers,including exogenous synbiotic brines,ascending thermal fluids and mixed fluids of these two hydrothermal fluids with synbiotic seawater;and there are mainly four fluid endmembers in the Shunbei area,namely atmospheric freshwater,exogenous synbiotic brine,ascending thermal fluid and syngenetic seawater mixed with two hot fluid.The dissolution of several fluid endmembers on the reservoir is carried out under three water-rock reaction regimes:(1)atmospheric freshwater-synchronous seawater gravity flow evolution system;(2)fault control atmospheric paleo-water seepage system;(3)The igneous rock activity drives the evolution of the upwelling hydrothermal fluid and the syngenetic water.The early active fluids of the Ordovician reservoirs in the Shunnan area and the fluid activities of the first and second scenes in the Shunbei area are the dissolution of syngenetic seawater and atmospheric freshwater;the fault control of atmospheric paleo-water seepage mainly plays an important role in the Shunbei area.The oxygen isotope of the calcite filled with orange-yellow cathode light in well SHB7 is obviously negatively biased,and the hydrothermal properties are not shown in the relationship between oxygen isotope and fluid inclusion homogenization temperature.Therefore,the second curtain fluid has the effect of atmospheric fresh water.In the igneous rock activity driving the remote hydrothermal fluid and the syngenetic water mixed upwelling evolution system,due to different hydrothermal sources,it can be divided into igneous rock activity to drive acid hydrothermal fluid and deep source rock thermal brine upwelling thermal alteration.The development of hydrothermal minerals in the Shunnan area represents the process of thermal alteration;the third act of fluid activity in the Shunbei area is silicon-rich hydrothermal fluid,forming cryptocrystalline silicon.In the thermal alteration of the deep source brine upwelling,the rare earth elements in the Shunnan area show that the calcite vein formed by the hydrothermal fluid and the quartz vein formed by the magmatic hydrothermal fluid have different sources,which confirms the mechanism of action.The fourth act of fluid activity in the Shunbei area is exogenous hot brine,and the fluid property in the Himalayan period is changed to acidity,which is converted into alkaline fluid during the process of dissolving calcite veins.It is this process of acid-base alternation that causes the formation of calcite cement and the occultation of cryptocrystalline silicon in some cracks in well SHBP3 and SHB7.The cracking is mainly controlled by the fault activity.The Shunnan area mainly occurred in the fourth episode of the mid-Caledonian,the Late Caledonian,the Late Hercynian and the Himalayan period.The Shunbei area also occurred in the four stages of the Middle Caledonian and Late Caledonian-early Hercynian,Late Hercynian and Yanshanian-Himalayan periods,while the No.7 fault zone was less affected by the tectonic movement of the Himalayan period.The exploration results show that Shunnan area are mainly on sulphide oil and natural gas and light oil in Shunbei area.The geochemical data of crude oil and natural gas indicate that the Ordovician Yijianfang and Yingshan Formation crude oils in Shuntuoguole area are originated from the source rocks of the Cambrian Yuertusi Formation.The crude oil and natural gas maturity of the No.1,No.5 and No.7 fault zone are gradually decreased.Based on the study of reservoir formation mechanism and oil and gas geochemical characteristics,fluid inclusions and microscopic fluorescence methods were used to determine the hydrocarbon filling period,and the fault activity was determined by seismic data.The results showed that karstification and cracking controlled the oil and gas filling and the difference of the degree of cracking caused by the strength of the fracture activity,the tectonic stress state of the geese.The Shunnan area and the Shunbei No.5 and No.7 fault zones are weakly filled in the late Caledonian period,and the Shunbei No.1 fault zone is filled more strongly.Therefore,the dissolution pores formed by the early diagenesis controlled the oil and gas filling in the Shunbei No.1 fault zone.The tectonic fault activity in the Shunnan area is relatively strong,which causes the hydrothermal activities in the late Hercynian-Indosinian and Himalian superimposed the karstification of the early diagenesis,and accompanied by oil and gas charging.The No.1 fault zone in Shunbei area has strong tectonic activities and in the pull-out environment during the tectonic movement,which makes the cracking degree higher and easy to open,resulting the freshwater and synbiotic mixed fluids of the late Caledonian and the late Hercynian magmatic hydrothermal fluid,the deep hydrothermal fluid of the middle and late Yanshanian and the deep karst fluid of the Himalayan formation water control the hydrocarbon charging;Although the fracture activity intensity of the No.5 fault zone is higher than that of the No.1 fault zone,however,the No.5 fault zone is in the right-hand left-stage in the late Caledonian period,so the degree of chemicalization is not high,and the current stress direction is inconsistent with the fracture direction resulting the internal crack is not easy to open,which causes the control on the hydrocarbonate charging of early diagenetic karst fluid in the Late Caledon is weak.Although the No.7 fault zone has four stages of cracking,but the early tectonic activity is weak,and the fault zone is in a compression environment as a whole.Not only the cracking degree is relatively weak,but the developed crack is not easy to open inside,so only a period of oil and gas charging occurred under the action in the late Hercynian hydrothermal fluid.The research results in this paper have obtained the advantages of the development of microbial reservoirs and the multi-causes mechanisms of faults controlling on formation of hydrocarbon charging.This indicates the direction for the next oil and gas exploration in the Shuntuoguole area.