Effect of Mantle-Derived CO2 on Hydrocarbon Accumulation in Central African Rift System:A Case Study of Palogue Oilfield
The discovery of Palogue oilfield in Melut Basin has opened a new exploration direction of the Central African Rift Paleogene and confirmed that the northern sag of Melut Basin is rich in oil and gas resources.The Lower Cretaceous source rock is the major source rock of Melut basin,the Palaeogene is its main seal-reservoir assemblage.The formation mechanism and model of oil and gas accumulated were proposed based on the specific tectonic-stratigraphic assemblages and evolution history of the Melut Basin.Oil and gas accumulations were influenced of mantle CO2.Relationship between CO2 and hydrocarbon accumulation was investigated using a suite of hydrocarbon generation and expulsion history,tectonic evolution history,fluid inclusions,CO2 and crude by taking Palogue oil reservoir as an example.The results show that the Late Paleogene-Neogene,the occurrence of the mantle source CO2 migration tectonic thermal event have an important influence on oil and gas accumulation in Palogue oilfield.Under the action of tectonic-thermal events,the thermal evolution of source rocks rapidly enters the mature stage,and intense hydrocarbon generation and expulsion occurs,CO2 in supercritical state quickly blends into crude oil.The crude oil saturated with supercritical CO2 quickly migrated through the reservoir and developed a large number of high-temperature mixed hydrocarbon inclusions,which showed the rapid accumulation event under the abnormal high heat event.After the crude oil full of supercritical CO2 entered the Palogue trap,CO2 gas began to separate out from the crude oil layer by layer due to the change of formation temperature and pressure conditions,CO2 gas carries light components into the upper reservoir for mixing.This results in a series of changes in vertical physical properties.
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