塔中隆起西北地区深部奥陶系天然气分布特征及运聚模式

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

综合油气产能、原油性质、天然气组分和同位素组成资料,结合研究区地质背景探讨了塔中隆起西北地区深部奥陶系天然气藏的分布特征及运聚模式,认为研究区目的层油气分布具有＂外气内油＂及＂羽状破碎带富气＂的特征,提出＂垂向输导运移-侧向差异聚集＂的天然气运聚成藏模式.研究结果表明：塔中隆起西北地区深部天然气通过塔中Ⅰ号断裂及左旋走滑断裂垂向运移至塔中隆起西北地区奥陶系储层后,一方面优先聚集于外部1~5号走滑断裂末端高孔渗羽状破碎带,导致在羽状破碎带油气产能、气油比、硫化氢质量分数、干燥系数、原油含蜡量异常高,分别可达199 442m^3/d,7 875 m^3/m^3,40 000×10-6,0.96,16.1%;原油密度异常低,低至0.77g/cm^3;天然气富集重碳同位素,最重值为-39.1‰;另一方面在不整合面、走滑断裂、高孔渗＂孔缝洞＂储层的侧向输导下,自北东向南西方向运移聚集,随着远离塔中Ⅰ号断裂带,气侵程度逐渐减弱,油气相态由凝析气藏向油藏过渡,气油比、天然气产能、干燥系数、硫化氢质量分数、原油含蜡量逐渐减小,分布范围分别为79~7 875m^3/m^3,1 198~199 442m^3/d,0.72~0.96,10×10-6~40 000×10-6,2.0%~16.1%,原油密度从0.77g/cm^3逐渐增大至0.89g/cm^3,甲烷碳同位素从-39.1‰逐渐变轻为-54.9‰.综合分析认为研究区深部或超深部天然气资源勘探潜力巨大,尤其是深大通源断裂附近的应力破碎带岩溶储层发育区,可作为深部天然气的优先勘探目标.

According to the production,properties and compositions of hydrocarbon,as well as the gas carbon isotopic data,combining with the geological background,the gas distributionand 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,H_2S content,dry coefficient,and wax content,which are199 442 m^3/d,7 875 m^3/m^3,40 000μg/g,0.96,and 16.1% respectively;the oil density is abnormally low,which is 0.77 g/cm^3;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,H2 S content,wax content decreases gradually,with the ranges from 79 to 7 875 m^3/m^3,1 198 to 199 442 m^3/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/cm^3;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.