Genetic Origin of CO2-Rich Oil Reservoirs and Pool-Forming Period in Melut Basin, Southern Sudan
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摘要: Palogue油田的发现打开了Melut盆地古近系勘探新方向，证实了北部凹陷为富油气凹陷.Palogue油田具有以下白垩统为主力烃源岩，古近系为主力成藏组合的跨时代运聚风格，幔源CO2气体对油气的聚集和改造具有不可忽略的作用.通过对CO2气体、原油特征、生标特征、包裹体特征进行研究，结合地层埋藏史，分析油气特征及成藏期次.结果表明：Palogue油田具有两期成藏的特征，古近纪中期发生第1期油气成藏事件，随后原油被降解；古近纪晚期-新近纪以来，伴随幔源CO2气体油气发生第2次运移充注事件，且CO2气体对油藏进行了强烈的气洗作用.Abstract: The oil discovery in the Paleogene of Palogue oilfield of the Melut basin is a milestone in the potentiality of new exploration play direction, it confirmed that the northern sag is hydrocarbon⁃rich sags. The Lower Cretaceous source rock is the major source rock of Melut basin, the Paleogene 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 accumulation were influenced by mantle CO2. Analyzed characteristics of oil and hydrocarbon accumulation were investigated using a suite of burial history, fluid inclusions, CO2 and crude biomarker characteristics by taking Palogue oil reservoir as an example. The results show that the Palogue oil reservoir has experienced two episodes of hydrocarbon accumulations, the first episodes of the hydrocarbon accumulation happened in the Middle Paleogene, but the crude oil was strongly biodegraded.Since the Late Paleogene to Neogene, the second episodes of hydrocarbon accumulations accompany by mantle CO2.The oil and gas enter traps to form the anticlina reservoirs, CO2 under the gravity differentiation has carried on the intense gas washing effect to reservoir.
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Key words:
- Melut basin /
- Palogue oilfield /
- accumulation process /
- evaporative fractionation /
- Southern Sudan /
- petroleum geology.
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图 1 Melut盆地构造单元划分及地层综合柱状图
Fig. 1. Classification of structural units and strata column of Melut basin
图 2 Melut盆地Palogue油田剖面（剖面位置见图 1）
Fig. 2. Oil reservoir cross section of the Palogue structural belt, Melut basin
图 3 Palogue油田原油地球化学成因判别图
Fig. 3. Crude oil geochemical genesis discriminant diagrams in Palogue oilfield
图 4 Palogue油气田储层流体组分组成
Fig. 4. Ternary diagram showing fluid components of reservoirs in Palogue oilfield
图 5 Palogue油气田原油次生变化识别
Fig. 5. Dentification of secondary changes in crude oil in Palogue oilfield
图 6 Palogue油田原油Pr/Ph、ααα-20RC27/C29与含蜡量的关系
Fig. 6. Diagram of Pr/Ph, ααα-20RC27/C29 and wax content of crude oil in Palogue oilfield
图 7 原油中正构烷烃和25-降藿烷分布
Fig. 7. The distribution of alkanes and 25-norhopane in crude oil
图 9 Palogue油田砂岩显微荧光照片
a.颗粒边缘发棕褐色光，粒间发蓝白色光；b.粒间孔隙受烃类浸染，发蓝白色光，发光强度中
Fig. 9. Micrograghs of fluid inclusion in sandstone of Palogue oilfield
图 10 Palogue油田埋藏热演化史及油气充注期次
Fig. 10. Burial-thermal history and oil accumulation stages of Palogue oilfield
表 1 F-2井Yabus组砂岩样品烃类包裹体测试
Table 1. Hydrocarbon inclusion test of sandstone sample in Yabus Group of Well F-2
类型 气液比（%） 分布特征 气相颜色 液相颜色 均一温度（℃） 次生 6 石英加大边群体分布 浅褐色 无色 77.8 6 石英中群体分布 浅褐色 无色 71.8 8 石英中群体分布 无色 无色 235 10 石英边部零星分布 浅褐色 无色 73.6 6 石英愈合缝中 浅褐色 无色 89.7 8 石英中群体分布 无色 无色 220 
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