塔里木盆地轮南低凸起奥陶系深层天然气地球化学特征及成藏模式

Geochemical characteristics and accumulation model of Ordovician deep gas in Lunnan low uplift of the Tarim Basin

塔里木盆地轮南低凸起奥陶系油气资源是深层油气勘探的重点领域之一,但目前对其天然气地球化学特征、成因及来源、成藏过程及成藏模式等缺乏系统研究。为此,基于轮南低凸起奥陶系深层天然气组分及碳同位素特征,结合该区构造演化特征、烃源岩生排烃史及油气运移输导条件,探讨了研究区奥陶系深层天然气成因及来源、分布规律及成藏模式。研究结果表明:①奥陶系深层天然气为有机成因的油型气,主要为寒武系-下奥陶统海相腐泥型烃源岩在高成熟-过成熟阶段形成的原油裂解气和干酪根降解气,但西部斜坡带及中部斜坡带西侧天然气为奥陶系早期捕获的寒武系-下奥陶统烃源岩在成熟-高成熟阶段的原油伴生气;②奥陶系油气藏具有“三期成藏、两期调整改造”和“早期聚油、晚期聚气、多期充注”的成藏特征,油气藏分布受区内不整合面、缝洞体系及断裂构造等因素联合控制,整体呈“东气西油”的分布特征;③晚期天然气主要沿轮古东走滑断裂垂向运移进入奥陶系,沿不整合面及岩溶储层自东向西侧向运移、聚集成藏,气侵强度具有自东向西逐渐减弱的特征。结论认为,奥陶系次生凝析气藏是喜马拉雅中-晚期寒武系-下奥陶统深层/超深层来源的高成熟-过成熟天然气气侵作用的结果,油气相态多样,指示研究区奥陶系及寒武系深层/超深层具有较大的油气勘探潜力。

Ordovician oil and gas resource in Lunnan low uplift of the Tarim Basin is one of the important focuses of deep oil and gas exploration,but the geochemical characteristics,origin,source,accumulation process and accumulation model of its natural gas have not been studied systematically.In this paper,the origin and source,distribution laws and accumulation model of the Ordovician deep gas in Lunnan low uplift are discussed based on its compositions and carbon isotope characteristics,combined with the tectonic evolution characteristics of this area,the hydrocarbon generation and expulsion history of the source rocks,and the hydrocarbon migration and transport conditions.And the following research results are obtained.First,Ordovician deep gas is oil-type gas of organic origin,which is mainly crude oil cracking gas and kerogen degradation gas formed by the Cambrian-Lower Ordovician marine sapropel source rocks in the high mature-over mature stage.However,natural gas in the west slope zone and the west side of the central slope zone is oil-associated gas which is formed by the Cambrian-Lower Ordovician source rocks in the mature-high mature stage and captured in the early Ordovician period.Second,the Ordovician reservoir has the hydrocarbon accumulation characteristics of"three-stage hydrocarbon accumulation,two-stage adjustment and reworking"and"early oil accumulation,late gas accumulation and multi-stage charging",and its distribution is under the joint control of unconformity,fracture-cave systems and fault structures in the area,presenting an overall distribution pattern of"east gas and west oil".Third,the late natural gas mainly migrates vertically into the Ordovician along the Lungudong strike-slip fault,and then laterally along the unconformity and karst reservoir from east to west to accumulate,and the gas invasion intensity gets weaker gradually from east to west.In conclusion,the Ordovician secondary condensate gas reservoir in Lunnan low uplift is the result of the invasion of high mature-over mature natural gas sourced from the Cambrian-Lower Ordovician deep/ultra-deep reservoirs in the middle-late Himalayan period,and its phase state of oil and gas is diverse.The Ordovician and Cambrian deep/ultra-deep reservoirs in the study area have greater potential of oil and gas exploration.