Analysis On The Reservoir Development Controlling Factors Of The Lower And Middle Ordovician In The Shuntuoguole Area Of The Tarim Basin

Shuntuogole area in the Tarim Basin is a super large oil and gas region with overall oil and gas accumulation and local enrichment.The existing exploration results show that the Middle and Lower Ordovician oil-gas reservoirs are controlled by reservoir development.However,the buried depth of the reservoirs is more than 6000m,which has experienced multiple structural fractures and the transformation of buried fluid,and part of reservoirs are controlled by specific lithology.The exploration in the study area has been restricted by the uncertainty of the main controlling factors of reservoir formation.In order to find out the key period and the main controlling factors of reservoir formation in the study area,seismic attributes,logging,drill-logging response,and core properties of pore and permeability are used to analyze reservoir type and distribution characteristic in this paper.Dunham classification is used to describe the lithology of the core.The controlling effect of different lithologies on the distribution of pores and fractures in the core is analyzed.The structure fluid environment evolution history of the Middle and Lower Ordovician in Shuntuoguole is reconstructed by comprehensive analysis of the active stages of strike-slip faults,the formation mode and distribution characteristics of fractures,and the morphology and geochemical characteristics of cements and fillings in pores and fractures.Based on the statistical results of formation water composition analysis and the evolution history of structure fluid environment,the effects of formation temperature and pressure,salinity and carbon dioxide content of formation water,and special ions,such as sulfate radical,ferrous ion,manganese ion and magnesium ion,on the activity of calcite dissolved in formation water,a Ca CO_3(aq),are quantitatively analyzed by using the thermodynamic software SUPCRT96 and ENRTL-RK model of chemical software Aspen Plus V9.When a Ca CO_3(aq)is lower than the solubility product of calcite at the same temperature,the Middle and Lower Ordovician limestone would be dissolved.Then,the main controlling factors of reservoir formation can be evaluated by using the difference of a Ca CO_3(aq)and calcite solubility product under different conditions.The statistics of fracture density show that within 4km from the strike slip fault,there are favorable areas for fracture development.The fracture density reaches the highest within the range of 0.5km to 1.5km.The simulation results show that the increase in temperature and pressure caused by burial and the great volcanic province in the Shunbei area is not conducive to the dissolution of limestone.However,atmospheric fresh water seeped along strike-slip fractures and SO₄^2--rich buried fluid associated with the Middle Cambrian gypsum-salt rock upwelling under compression in the Late Caledonian and Early Hercynian periods could lead to significant dissolution of limestone.The mixing of SO₄^2--rich buried fluids,which was driven by the large volcanic province and was related to the Middle Cambrian gypsum-salt rock,in the Late Hercynian period could also cause dissolution of limestone at low salinity.Combined with the distribution characteristics of the Middle and Lower Ordovician reservoirs,it was shown that the upwelling of SO₄^2--rich fluids in the Late Caledonian and Late Hercynian periods are more important to the formation of reservoirs than the infiltration of atmospheric fresh water in the Late Caledonian and Early Hercynian periods.Algal clastic limestones related to Proaulopora and Girvanella in the Upper Section of the Yijianfang Formation in Shuntuo area and the Lower Section of the Yijianfang Formation in Shunnan area are the basis for the formation of high-quality reservoirs.The particle size difference between the Proaulopora and surrounding limestone provides lithologic basis for the selective dissolution after deep burial.The mixing of the Cambrian formation water,which included the compaction fluid related to the pressure dissolution of the Cambrian ankerite and the SO₄^2--rich fluid related to the gypsum-salt rock,is the key factor of reservoir dissolution and preservation.There were organic matter-related fluids leading to limestone dissolution in the early stage of diagenesis in Shuntuo area,and hydrothermal fluids related magmatism leading to the precipitation of cryptomicrocrystalline quartz and limestone maralitization in the Late Hercynian period.The formation water from the detrital rocks of the Sugetbulak Formation of the Sinian system flowed through the gypsum-salt rock of the Middle Cambrian,and flowed into the Yingshan Formation along the strike-slip faults,forming cavity-fractured type reservoirs related to silica rocks in the Shunnan area in the Yanshanian period.The formation water related to the Cambrian ankerite compression dissolution in the Late Hercynian and the Himalayan periods was not only the diagenetic fluid that formed the interlayer dolomite reservoir in the lower part of the Yingshan Formation in Shunnan area,but also the key diagenetic fluid that causes the pores of the Yijianfang Formation and Yingshan Formation to be preserved after deep burial.Compared with lithology,the Middle and Lower Ordovician reservoirs in Shuntuoguole area were mainly controlled by the structure-fluid environment.The atmospheric fresh water infiltration and upwelling of SO₄^2--rich fluid in the Late Caledonian and Early Hercynian were the most favorable structure-fluid environments for reservoir formation.The mixing of SO₄^2--rich fluid driven by the great volcanic province and related to the gypsum-salt rock in the Late Hercynian period was the most important for formation of secondary pores of reservoir.The compaction fluid related to the pressure dissolution of the Cambrian ankerite in the Late Hercynian and the Himalayan periods was beneficial to the preservation of reservoir pores.The silica-rich fluid in the Late Hercynian and the Yanshanian period transformed and filled the early pores of reservoir,but had little contribution to the increase of porosity.