苏丹Melut盆地Palogue油田高酸值原油成因

Origin of high acidity oils in the Palogue Oilfield,Melut Basin,Sudan

苏丹Melut盆地Palogue油田原油主要为中质油(20<API°<34),其次为重质油(API°<20),但普遍高含酸,酸值最高可达10 mg/g。高酸值原油主要产于古近系砂岩储集层,上白垩统产层原油酸值相对较低,一般低于1 mg/g。地球化学分析认为:下白垩统Alrenk组烃源岩于晚白垩世末期开始生烃,并在上白垩统和古近系储集层中聚集成藏;由于渐新统末期构造运动造成地层抬升和剥蚀,烃源岩生、排烃停止,地表淡水注入古近系,形成重碳酸钠型地层水,使古近系油藏遭受生物降解,形成高酸值重油,深大断裂活动造成地壳深部CO2气体侵入,加速了高酸值油藏的形成;新近纪盆地发生拗陷作用,使烃源岩继续深埋,重新开始生、排烃,新生成的正常原油不断充注到古近系油藏,形成早期降解油与后期正常油混合的混合油藏,由此形成了酸值不等的高酸值油藏。

Oils produced from the Palogue Oilfield,Melut Basin,Sudan are medium oils（20API°34）,followed by heavy oils（API°20）,having high total acid numbers（TAN）.High TAN oils are mainly produced from Paleogene sandstone reservoirs,their TANs can be up to 10 mg/g.Low TAN oils are produced from Upper Cretaceous reservoirs,lower than 1 mg/g.Detailed geochemical analyses indicated that the Lower Cretaceous Alrenk Formation source rocks began to generate and expel hydrocarbons at the late of Upper Cretaceous,which accumulated in the Upper Cretaceous and Paleogene reservoirs,but stopped at the late of Oligocene due to uplifting and erosion.NaHCO3 formation water was formed due to the charge of fresh water into the Paleocene strata,and high TAN oils were formed due to biodegradation by NaHCO3 formation water,and faulting-caused CO2 intrusion from deep crust accelerated the forming of high acidity oils.During the Neogene,the Alrenk source rocks were buried deeply due to the basin depression and restarted to generate and expel hydrocarbons.The normal oils generated lately charged into the Paleogene reservoirs to form mixed oils with biodegraded oils,and finally,form high acidity oils with various TANs.