P-T And Phase Evolution Of Hydrocarbon In Reservoirs Charging Periods In The Transitional Zone Between The Awati And Manjiaer Depressions,Tarim Basin

The study of temperature and pressure field and the phase evolution of hydrocarbon has become one of the key issues with the further development of deep-ultra-deep oil and gas exploration.In this paper,the transitional zone between the Awati and Manjiaer depressions enriched with deep oil and gas in the Tarim Basin is taken as the research object.By various analysis,tests and thermodynamic simulation of fluid inclusions in Ordovician reservoirs,the charging periods as well as the temperature and pressure(P-T)characteristics were declared,the relationship between P-T field and distribution of hydrocarbon phase were restored,the phase evolution model and influencing factors were summarized,which provide a theoretical basis for establishing the distribution law of deep oil and gas.Research shows that there are three hydrocarbon accumulation periods of Ordovician reservoirs in the study area,including late Caledonian,late Hercynian and Himalayan.Due to multi-stage tectonic movements in the Tarim Basin,there were certain differences in depositional history,tectonic activity periods and fluid activity in the study area,resulting in some differences in both hydrocarbon accumulation stages and P-T characteristics.In the Tabei-Shunbei area,there were three hydrocarbon accumulation periods,including late Caledonian,Hercynian and Himalayan,during which the reservoirs were characterizezed by low temperature and normal pressure.In the Shuntuo area,the early hydrocarbon accumulation stage was in the background of low temperature and normal pressure,and the late Hercynian and Himalayan periods were in the background of high temperature and overpressure.In the Shunnan-Gucheng area,there were two hydrocarbon accumulation periods,including late Caledonian and Himalayan,both of which were in high temperature and overpressure conditions.There were also differences in evolution characteristics of hydrocarbon phase in key accumulation periods and secondary structural units.In late Caledonian,except Shunbei 1 Fault zone,the whole area was filled with crude oil in a large scale,forming medium-light oil reservoirs,and the hydrocarbon in the reservoirs was in single oil phase.In late Hercynian,hydrocarbon was still in oil phase,forming light-volatile reservoirs.During Himalayan period,because source rock was in high-overmature and reservoir temperature and pressure rised,gas invasion and crude oil cracking occured in different degrees.As a result,the hydrocarbon phase gradually transformed from oil phase into condensate gas phase and single gas phase from north to south in the study area,that is,volatile oil reservoir formed in the Tabei-Shunbei area,condensate gas reservoir formed in the Shuntuo area due to strong gas invasion,and dry gas reservoir formed in the Shunnan area due to the crude oil cracking.Based on the above analysis,three evolution models of hydrocarbon phase are established,which are “early low-mature oil phase—temperature and pressure adjustment—late highmature oil phase”,“early oil phase—gas invasion modification—late gas phase” and “early oil phase—oil cracking—late gas phase”.The hydrocarbon phase was influenced by the initial phases of the hydrocarbon originated from source rock during the main hydrocarbon accumulation periods,the changing process of temperature and pressure in reservoirs,and the gas invasion after the main hydrocarbon accumulation periods.