苏丹Muglad盆地Fula坳陷油气地球化学特征与成藏意义

Geochemical characteristics of crude oil and their implications for hydrocarbon accumulation in the Fula Sub-basin,Muglad Basin,Sudan

Fula坳陷位于Muglad盆地东北部,面积约5 000km2,FN油田是该坳陷最大的亿吨级油田。油气储层主要为白垩系Abu Gabra、Bentiu和Aradeiba组。应用油藏地球化学的方法对FN油田的成藏期次进行了探讨。FN油田Abu Gabra、Bentiu和Aradeiba组原油特征相似,来源于同一套烃源岩,原油既表现出正构烷烃、藿烷、甾烷系列分布完整等正常油特征,又表现出色谱基线抬升、高峰度UCM、25-降藿烷出现等生物降解油特征,这些特征表明FN油田经历了至少两期成藏过程,早期充注的原油遭受生物降解后又接受后期成熟度较高的原油充注。根据构造演化、埋藏史和生排烃史可以研究生烃期次及油气成藏特征。Abu Gabra组烃源岩发生了两次生烃作用,第一次生烃作用发生在晚白垩世,Ro达到0.6%以上,进入生排烃作用阶段,晚白垩世末构造抬升作用使这次生烃产物遭受较为严重的生物降解;第二次生烃作用发生在古近纪,Abu Gabra组烃源岩进入生油高峰阶段,是本区最重要的一次生油作用和成藏过程,与该次成藏事件相关的构造圈闭是Fula坳陷油气勘探的重点。

The Fula Sub-basin, located in the northeast Muglad Basin, has an area of 5000 km2. The FN oilfield is the biggest oilfield discovered in this sub-basin with 100 million tons of oil reserves. Its main hydrocarbon reservoirs included the Aradeiba, Bentiu and Abu Gabra Formations of the Cretaceous. Based on a systematic geochemical analysis of crude oil, the reservoir-forming stages of the FN oilfield are discussed. The geochemical characteristics of the crude oil from the three formations are very similar, which indicates that all hydrocarbons originated from the same source rocks, i.e., the Abu Gabra source rocks. The crude oil has both normal oil characteristics, such as full series distribution of n-alkanes, hopanes and steranes, as well as biodegraded oil characteristics, such as raised chromatogram baseline and appearance of sharp UCM and 25-norhopanes. It revealed, therefore, that the FN oilfield had undergone two oil infilling stages, i.e., the earlier infilling oils had relatively low maturity and was strongly biodegraded, and the later ones had relatively high maturity. The tectonic, burial and thermal evolution histories were used to analyze the hydrocarbon generation and charging and accumulation stages of the FN oilfield. It was shown that the Abu Gabra source rocks experienced two periods of hydrocarbon generation. The first one took place during the Late Cretaceous, with the maturity （Ro） hydrocarbon generation window; then the crude of Abu Gabra source rocks reaching 0. 6% within the oil suffered relatively serious biodegradation due to tectonic reversion and uplift by the end of the Cretaceous. The second one happened during the late Paleogene as the Abu Gabra source rocks entered the hydrocarbon generation peak stage. This was the most important process for hydrocarbon generation and oilfield formation in the region. The structural traps related to the second period are the main targets for hydrocarbon exploration in the Fula Sub-basin.