Study On The Different Phase Hydrocarbon Components And Its Accumulation Characteristics In The Lower Plaeozoic Reservoir,Tazhong Area

Turning to the deep carbonate strata in marine basins is one of the most prominent trends of hydrocarbon exploration in China.Deep carbonates have been widely developed in the Tarim basin,which have great potential for exploitation.The Tazhong area is the key area of hydrocarbons exploration and development in marine carbonate strata.The current exploration achievements show abundant Lower Palaeozoic hydrocarbons with complex distribution of hydrocarbon phase states.The origin and formation mechanism of hydrocarbons in different phases have been unknown,and the accumulation process has been controversial,and hydrocarbon accumulation mode needs to be summarized.In this paper,based on the latest exploration achievements,comprehensive utilization of geological analysis,geochemical and geophysical methods,following the comparison analysis and accumulation characteristics of hydrocarbon in different phases,systematically dissect the control factors of hydrocarbons phase.Try to analyze the origin,distribution characteristics and accumulation process of hydrocarbon in different phases,and establish the hydrocarbon accumulation mode.The research obtains understandings as follows:?1?Multiphase hydrocarbons have been widely developed in the Lower Palaeozoic strata of the Tazhong area,including condensate gas,volatile oil and normal oil.From south to north,hydrocarbons in different phases show the characteristics of“gas-riched in fault zone,while oil-riched in platform area”.From west to east,hydrocarbons present“gas-riched in eastern,while oil-riched in the western”.Besides,in different stratas,hydrocarbons show the characteristics of“gas-riched in deep strata,while oil-riched in shallow strata”;in the same strata,hydrocarbons show the characteristics of“gas-riched in high position,while oil-riched in low position”.?2?Compared with the western Tazhong,most of the gases in the eastern Tazhong are characterized by higher gas dryness coefficients and ¹³C-enriched methane.Compared with platform area,most of the gases in the fault zones are characterized by higher gas dryness coefficients and ¹³C-enriched methane.The differential distribution of gas geochemical characteristics is related to the mixture of gases from different sources.The gas of the eastern No.1 and No.10 fault zones belongs to the high maturity oil cracking gas from the Cambrian-Lower Ordovician source rock,mixed with a small amount of oil associated gas.The low maturity oil associated gas from the Middle-Upper Ordovician source rock and high maturity oil cracking gas from the Cambrian-Lower Ordovician source rock are both developed in the western No.1 fault zones and the north platform area.The carbon dioxide in the study area with low content are mainly from inorganic origin.The nitrogen with high contents mainly generated from the thermal evolution function in carbonate hydrocarbon source rock.Hydrogen sulfide mainly comes from the early period of TSR.Compared with the eastern Tazhong,the active sulfate concentrations in the western Tazhong are higher which can be more favorable to accelerate TSR and produce H₂S.?3?Compared with the north platform area,most of the oil in the No.1 and No.10fault zones is characterized by the low density,high wax content and heavy carbon isotope.Compared with the Lianglitage Formation,the oil in the Yingshan Formation is characterized by the low density,high wax content and heavy carbon isotope.The differential distribution of oil geochemical characteristics is related to the mixture of oil from different sources.On the whole,the oil is from the Middle-Upper Ordovician and Cambrian-Lower Ordovician source rocks,and Middle-Upper Ordovician source rock has done much more contributions than Cambrian-Lower Ordovician source rock.In particular,compared with the Lianglitage Formation,the Yingshan Formation are mainly from the Cambrian–Lower Ordovician source rock.With the increase of buried depth,the contribution of Cambrian-Lower Ordovician source rock continues to increase.?4?The phase states of Lower Palaeozoic hydrocarbons in the Tazhong area are influenced by the synergetic action of various factors.The phase of hydrocarbons in the tectonically stable zone mainly controls by the source rock type,its thermal evolution and multiple hydrocarbon charging.The north platform area have experienced multistage hydrocarbon generation and expulsion.In Caledonian and late Hercynian periods,the oil mixed filling to generate volatile oil and normal oil reservoirs.The phase of hydrocarbons in the tectonically active fault zone mainly controls by the gas invasion.The early reservoirs in the No.1 and No.10 fault zones were transformed by the strong gas invasion,generating large areas of condensate gas reservoirs.Thermal cracking makes the main control effect to the hydrocarbon phase state in the deep strata.The Cambrian paleo-reservoir crack to generate a lot of oil cracking gas.Besides,TSR function reduces oil thermal stability and critical temperature of oil cracking,and further promote oil cracking to generate gas.?5?Lower Palaeozoic hydrocarbons in the Tazhong area which from the Cambrian–Lower Ordovician and Middle-Upper ordovician source rocks vertical migrated through fractures to the Ordovician reservoirs in Caledonian and late Hercynian periods,and lateral migrated through the fault,unconformity surface and conducting layer.It priority accumulated in the high-quality reservoirs in traps which located in the structure high position covered with the cap-rocks.The reservoirs were transformed by the strong gas invasion in Himalayan period,and formed multiphase hydrocarbon finally.It is divided into two main types of accumulation modes of multiphase hydrocarbon in the Tazhong area:gas-invasion reformed type condensate gas reservoir and filling mixed type oil reservoir.The former is mainly distributed in the tectonically active fault zones,and the latter is mainly distributed in the structure stable platform area.According to the characteristics of gaseous components,the gas-invasion reformed type condensate gas reservoir can be further divided into H₂S-riched type condensate gas reservoir in western Tazhong and the H₂S-depleted type condensate gas reservoir in the eastern.