Paleo-fluids And Hydrocarbon Accumulation Studies In The Ordovician Carbonate Reservoirs Of The Shunnan And Guchengxu Areas,Tarim Basin

The Tarim basin is located in the Xinjiang Uygur Autonomous Region,NW China,and is a large petroliferous basin that built on the Tarim block.The Shunnan and Gulong areas are located in the footwall of the Fault Tazhong 1 in the Central Tarim.The Shunnan area is the second-order tectonic unit named ‘Shunnan Gentle Slope',which belong to a first-order tectonic unit named ‘Guchengxu Uplift'.The Gulong uplift is the west part of the second-order tectonic unit ‘Guchengbitu',which also belong to the Guchengxu Uplift.Based on the diagenesis,fluid inclusion and isotopic geochemistry studies,we try to identify the activities of paleo-fluids in the Shunnan and Gulong areas,aiming to discuss how and when the reservoirs formed and when the gas charged.Several diagenetic events are identified in the Middle Ordovician of the Shunnan and Gulong areas,including intergranular calcite cement,dissolution,stylolites,coarse calcite cement(CC1 – 6 and CC2-gl),replacement quartz(Qz1),quartz cement(Qz2 – 4)and saddle dolomite(SD).The dissolution crosscut intergranular calcite cement and grains in the limestone.In the Shunnan area,from the wall to the center,internal sediment,bladed calcite,radiaxial fibrous calcite(RFC),medium crystalline dolomite,CC1 and saddle dolomite fill in the vugs.CC1 is mainly found in vugs in the Shunnan and Gulong areas,crosscut by stylolites.CC2 is mainly found in the fractures developed in limestone and slightly silicified limestone,crosscutting stylolites and postdating Qz1.CC3 fills in the intercrystalline pores of Qz1,postdating Qz1.CC4 fills the fractures in the strongly silicified limestone,crosscutting stylolites and Qz1.In some samples,CC3 filling the pores is transitional to CC4 in the facture.CC5 fills the residual pores of Qz2 – 4.Qz1 is the replacement quartz found in limestone,slightly silicified limestone and strongly silicified limestone.Qz2 – 4 are quartz cements only found in strongly silicified limestone.Qz2 is the equant quartz,and Qz3 and Qz4 are the core and the rim of the bladed quartz,respectively.CC2-gl is mainly found in the fractures developed in limestone in the Gulong area,crosscutting stylolites.In the Gulong area,saddle dolomite fills the fractures in the lower member of the Yingshan Formation,crosscutting the stylolites.CC6 fills the residual pores of saddle dolomite in the Gulong area.The carbon and oxygen isotopes are analyzed on micrite,CC1,CC2,CC2-gl,CC4,CC6 and SD.The ?13C value of micrite is 0.19 ‰ and the ?18O value is-6.58 ‰.The ?13C values of CC1 in the Shunnan area are-2.22 – 0.48 ‰ and the ?18O values are-8.31 –-4.60 ‰.The ?13C values of CC2 are-2.48 – 0.24 ‰ and the ?18O values are-13.26 –-7.76 ‰.The ?13C values of CC4 are-1.98 –-1.82 ‰ and the ?18O values are-11.09 –-6.99 ‰.CC2-gl has ?13C values of-0.55 – 0.35 ‰,and ?18O values of-11.44 –-7.56 ‰.CC6-gl has ?13C values of-2.17 –-1.92 ‰,and ?18O values of-12.19 –-11.97 ‰.The strontium isotope is analyzed on micrite,CC2,CC2-gl and CC4.The 87Sr/86 Sr value of micrite is 0.70875.T he 87Sr/86 Sr values of CC2 is 0.708898 – 0.709167.CC4 has 87Sr/86 Sr values of 0.709489 – 0.709721.CC2-gl has 87Sr/86 Sr values of 0.709715 – 0.709909.The silicon and oxygen isotopes are analyzed on Qz1 and Qz2 – 4.Qz1 has ?30SiV-NBS28 values of 1.7 – 2.1 ‰.Qz2 has ?30SiV-NBS28 values of 2.3 – 2.4 ‰,and ?18OSMOW valuesof 19.2 – 20.6 ‰.Fluid inclusions in CC1 – 5 mainly occur in random populations and mainly are liquid-dominated biphase aqueous fluid inclusions(L-D),which are interpreted to be of unknown origin.Some L-D inclusions,vapor-dominated biphase inclusions(V-D)and vapor-only monophase inclusions(V-O)coexisted in long trails that crosscut crystal boundary of CC4,and the inclusions are interpreted to be of secondary origin.Fluid inclusions in Qz2 and Qz3 occur in clusters and growth zones,which are interpreted to be of primary origin.L-D,V-D and V-O inclusions coexist in some growth zones in Qz2 and Qz3.Fluid inclusions in Qz4 occur in clusters,and are L-D inclusions,which are interpreted to be of unknown origin.Fluid inclusions in CC2-gl occur in long trails and are L-D inclusions,which are interpreted to be of secondary origin.Qz2 – 4 contain some L-D,V-D and V-O inclusions coexisted in long trails that crosscut crystal boundary of quartz,and those inclusions are interpreted to be of secondary origin.L-D,V-D and V-O inclusions coexist in long trails in CC6,and they are interpreted to be of secondary origin.Primary inclusions in Qz2,Qz3,CC3 and CC4 have valid homogenization temperatures(Th)of 143 – 166 °C,and ice-melting temperatures(Tm-ice)of-22.3 –-10.7 °C.Unknown inclusions in Qz4 have valid average Th values of 125 – 132 °C,and average Tm-ice values of-32.1 –-25.4 °C.Unknown inclusions in CC1 have valid Th values of 103 – 113 °C,and Tm-ice values of-6.8 –-5.0 °C.Unknown inclusions in CC5 have valid Th values of 86 – 101 °C,and Tm-ice values of-29.6 –-27.7 °C.Secondary inclusions in Qz2 – 4 and CC4 have valid average Th values of 172 – 186 °C,and Tm-ice values of-6.5 –-0.9 °C.The Raman spectra analysis on the V-D and V-O inclusions shows that the gas in the inclusions is methane.Based on the petrography,fluid inclusion and isotopic geochemistry studies,sea water(and slightly modified sea water),meteoric water,formation water and deep fluids are identified in the Middle Ordovician carbonate reservoirs,in the Shunnan and Gulong areas.Bladed calcite and CC1 are interpreted to be formed form the sea water and slightly modified sea water,and are the most important cements that occlude early pores in the reservoirs.The dissolved vugs filled by bladed calcite and CC1 are interpreted to be formed from the meteoric water.The formation water is considered to form CC2 and SD,and may be derived from the Cambrian strata.CC2-gl,CC3,CC4 and Qz1 – 4 are interpreted to be formed from the deep fluid,and the deep fluid is interpreted to be derived from the Sinian strata and the base.The pattern of fluid inclusion Th evolution with paragenetic sequence,i.e.,a decrease from Qz2,Qz3 and CC4 to Qz4 and CC5,suggesting that the deep fluid is of hydrothermal nature.Furthermore,the relatively low temperatures recorded by CC5(at most 134 °C after pressure correction)imply that the hydrothermal event possibly took place in the Late Ordovician – the Carboniferous time.In combination with previously published studies about sequence stratigraphy,petrography and reservoir porosity-permeability data,the Yijianfang Formation reservoirs in the Shunnan and Gulong areas are interpreted to be controlled by the meteoric water during the LST period of the third-order sequence.However,the upper member of the Yingshan Formation reservoirs are mainly formed by the hydrothermal silicified fluids,instead of the meteoric water.