Geochemical characteristics and migration-accumulation mode of the deep Ordovician gas in the northwestern Tazhong uplift, Tarim basin

WANG Yangyang; CHEN Jianfa; PANG Xiongqi; SHEN Weibing; CHEN Zeya; ZHANG Ke; WANG Yifan; LUO Guangping

Abstract：According to the production,properties and compositions of hydrocarbon,as well as the gas carbon isotopic data,combining with the geological background,the gas distribution and the migration-accumulation characteristics of the deep marine carbonate reservoir in the northwestern Tazhong uplift,Tarim basin was discussed.The hydrocarbon distribution demonstrates the characteristics of "oil inside and gas outside" and "gas enriched in feather-like sub-fracture zones",and a gas accumulation mode of "vertical hydrocarbon migration and lateral differential accumulation" in the study area was put forward.Results show that after the deep gas vertically migrates to the Ordovician reservoir through the No.1 fracture zone and the sinistral strike-slip faults,the gas primarily gathers around the feather-like sub-fracture zones in the ends of No.1 to No.5 strike-slip faults on one hand,and results in abnormally high natural gas production,gas oil ratio,H2S content,dry coefficient,and wax content,which are 199 442 m3/d,7 875 m3/m3,40 000 μg/g,0.96,and 16.1％ respectively;the oil density is abnormally low,which is 0.77 g/cm3;methane trends to enrich with the heavy carbon isotope,and the heaviest value is －39.1‰.On the other hand,the natural gas invades the early reservoir by the lateral migration system which consists of unconformity surface,strike-slip faults system,and pore-cavity-fracture reservoir,leading to differential accumulations of hydrocarbon in different regions from northeast to southwest;when far away from the Tazhong No.1 fault,the degree of gas washing weakens,and the hydrocarbon phase transits from the condensate gas to the normal oil;the gas oil ratio,natural gas production,dry coefficient,H2S content,wax content decreases gradually,with the ranges from 79 to 7 875 m3/m3,1 198 to 199 442 m3/d,0.72 to 0.96,10 to 40 000μg/g and 2.0％ to 16.1％;the density of crude oil increases from 0.77 to 0.89 g/cm3;the methane carbon isotope becomes lighter,ranging from －39.1‰to －54.9‰.The comprehensive analysis indicates the tremendous resource potential of the deep and ultra-deep strata gas.Further explorations in the deep Ordovician of the Tazhong uplift should be conducted in the structural fracture zones with karst reservoirs around the deep faults.