Accumulation Mode Of Hydrocarbon In The Ordovician Carbonate Reservoir In The Tazhong Area,Tarim Basin

Much hydrocarbon exists in the Ordovician carbonate reservoir of the Tazhong-Gucheng area.In this study,the latest seismic,logging,geology and geochemistry data are applied to research the distribution characteristics,origin,source,and secondary alteration of multiphase hydrocarbons in the Ordovician in the Tazhong-Gucheng area.On the basis of the main controlling factors of hydrocarbon accumulation,the accumulation process of hydrocarbon,the origin and source of hydrocarbons and the control factors of multiphase hydrocarbons,the accumulation modes of hydrocarbons in the Tazhong and Gucheng areas are summarized.(1)Multiphase hydrocarbons have been discovered in the Ordovician,Tazhong-Gucheng area,including dry gas in the Gucheng area and condensate gas,volatile oil and normal oil in the Tazhong area.In the Tazhong area,oil and gas are enriched in the west and east,respectively.In the west,condensate gas is enriched in the TZ1 and TZ10 fault zones and strike-slip faults through the source rocks;volatile oil and normal oil are predominantly distributed in areas far from these faults.In the east,condensate oil is located in the TZ1 fault zone;volatile and normal oils are distributed in the inner zone.Vertically,in different formations,natural gas and oil are enriched in deep and shallow locations,respectively.In the same formation,oil and gas are enriched in lower and higher locations of the structure,respectively.Oil and gas are distributed along unconformities.In the Gucheng area,natural gas is enriched in the fault zone in the structural high and slope parts;vertically,gas is distributed in the beach reservoir in the Ying-3 member.(2)In the Tazhong-Gucheng area,natural gas is the cracking gas of oil from Cambrian source rock.The drying coefficient,maturity and gas-oil ratio(GOR)of gas in the eastern Tazhong are greater than those in the western Tazhong;compared with gas in the platform area,the gas in the fault zones has a higher drying coefficient.This may be caused by the genetic difference of the natural gas: the overmature dry gas in the Gucheng area migrates to the east of the Tazhong area along the TZ1 fault and mixes with gas there,which causes the higher drying coefficient and maturity of gas in the east compared to the west.Low mature source rocks are distributed in the north platform area of the Tazhong area,and these rocks generate kerogen-cracking gas coexisting with oil cracking gas there.H2 S is generated from thermochemical sulfate reduction(TSR)driven by CIP ions.Compared with the eastern Tazhong,higher concentrations of Mg2+ and total mineralization in the western Tazhong are prone to TSR reaction,which leads to higher contents of H2 S.CO2 and N2 in the Tazhong and Gucheng areas are generated from the thermal degradation of organic matter.(3)In the Tazhong area,based on thermal simulation data of gold tube experiments,effective geochemical parameters for the oil source are redefined.Aryl isoprenoids,carbon and sulfur isotopes are applied to analyze the oil source,and the oil is primarily generated from the Cambrian source rock.Compared with the western Tazhong,the density and viscosity of the oil are greater,and the maturity and carbon isotope of the oil are lower in the eastern Tazhong.This phenomenon may be related to the maturity differences of the source rocks.The Cambrian source rock in the western Tazhong,which is buried more deeply than that in the eastern Tazhong at approximately 1000 m,has a higher maturity and generates oil with a higher maturity.The geochemical characteristics of oil in the fault zones are related to the higher maturity of the source rock and gas invasion,which leads to reductions in the density and viscosity of the oil.(4)In the Tazhong area,multiphase hydrocarbons are related to the maturity of the source rock,secondary alteration and multistage fillings of hydrocarbon.The maturity of the source rock and multistage fillings of hydrocarbon affect the phase state of hydrocarbon in the plateau region;biological degradation of oil occurs in the high part of the top of the Ordovician.TSR can reduce the thermal degradation temperature of oil and accelerate oil cracking.Oil in the Ordovician cracks slightly due to the lower temperature of the reservoir,whereas oil in the Cambrian cracks intensely.Gas invasion is important in the Tazhong area.Vertical and lateral gas invasion mainly occurs in the west and east of Tazhong,respectively.Natural gas in the Gucheng area is the cracked gas of oil filled during the late Caledonian.(5)In the late Caledonian,oil from the Cambrian source rock migrated to the Tazhong-Gucheng area.In the early Hercynian,paleo-reservoirs in the Tazhong area were destroyed by tectonic movement,and the paleo-reservoirs in the Gucheng area were hardly destroyed.In the late Hercynian,source rocks were buried deeply again to generate hydrocarbons,which migrated vertically to the Ordovician through faults and laterally to high quality reservoirs;in the Gucheng area,source rocks generated a small amount of hydrocarbons.In the Himalayan period,oil cracked gas in the Tazhong area migrated to the Ordovician and mixed with early oil reservoirs,forming condensate gas;oil cracked gas in the Gucheng area partially migrated to the eastern Tazhong along the TZ1 fault.Dry gas was accumulated in the Cambrian or Ordovician reservoirs in the Gucheng area.The accumulation mode of oil can be further divided into oil(volatile oil and normal oil)and gas invasion type condensate gas.The former is distributed in the platform area in the western Tazhong,which is far from the gas source fault in the middle and inner zone in the eastern Tazhong;the latter is distributed in the gas source fault zone,which is further divided into vertical gas invasion in the western Tazhong and lateral gas invasion in the eastern Tazhong according to different directions of gas invasion.