尼日尔Termit盆地上白垩统成藏条件分析与勘探策略

尼日尔Termit盆地纵向上主要发育白垩系和古近系两套大的成藏组合，其中上组合古近系Sokor组为主力成藏组合，下组合白垩系为潜在成藏组合。盆地整体从下到上发育了下白垩统陆相-上白垩统海相-古近系陆相沉积旋回，发育了特殊的海侵层序。随着Termit盆地古近系主力成藏组合的勘探程度越来越高，亟待开辟新的勘探领域。基于研究和勘探实践，对Termit盆地下组合(白垩系)勘探新层系进行了探索。下组合白垩系又可以细分为K1、Donga组和Yogou组上段(YS3段)3个成藏组合，通过构造建模、沉积体系研究和油气成藏条件分析，认为YS3段具有较好的成藏条件，是下步勘探的重点领域。YS3段形成于海平面高频震荡下行时期，总体为海陆过渡相，三角洲体系发育，以砂泥互层沉积为主；其中发育的泥岩为该盆地主力烃源岩且大面积成熟，生烃潜力大，同时又是非常好的局部盖层；三角洲相石英砂岩结构和成分成熟度高，可做良好储层，且与烃源岩交互发育，埋藏适中，因此YS3段具有良好的“近源”油气成藏条件。结合构造研究和钻井揭示特征进行Yogou组时期沉积体系研究，建立时间与空间演化序列特征，评价物源体系和储层发育特征，进而优选有利勘探区带。通过盆地东、西和南缘沉积期古地形的对比研究，认为Yogou组时期盆地在不同盆地边缘位置具有不同的地形和沉积体系发育特征，西部Dinga断阶带为“陡坡带”特征，隆起区通过几组深大断裂快速进入深凹区，连接隆起和深凹区的斜坡区较窄；南部Yogou斜坡区和东部Fana低凸起地区地形较缓，斜坡区域面积大，有利区带广，据此建立了“陡坡带”和“缓坡带”沉积模式。东部隆起带在晚白垩世为Termit盆地的主力物源区，西部和南部为次要物源区，因此在海侵层序后期在东部广泛发育大规模三角洲前缘砂与泥岩互层的层序，南部次之，西部规模最小。烃源岩研究认为YS3段烃源岩为陆源有机质，与物源规模有关。因此沉积体系研究认为盆地西南部和东南部“缓坡带”储层和烃源岩都更加发育，成藏条件和可勘探面积都优于盆地西部“陡坡带”。下组合白垩系Yogou组YS3段与上组合古近系Sokor组比缺乏区域盖层，上组合反向断块勘探策略不适合YS3段层系，应重点针对背斜圈闭、小断距反向断块、顺向断块和岩性圈闭进行勘探。

Analysis of hydrocarbon accumulation condition and exploration tactics of the Upper Cretaceous strata in the Termit Basin,Niger

There are two plays developed in the Termit Basin vertically, namely the upper Paleogene and lower Cretaceous formations. The upper Sokor and lower Cretaceous formations were the dominant and potential plays, respectively. In addition, a continental marine continental facies cycle, from the Early Cretaceous to Palaeogene, developed an unique marine transgressive sequence in the basin. With increasing degree of exploration on the Paleogene main strata in the Termit Basin, it is desirable to look for a new exploration stratum in the basin. Here, we studied the hydrocarbon accumulation characteristics of a newly explored upper Cretaceous strata in the Termit Basin. We showed that the Lower Cretaceous can be further subdivided into K1, the Donga Formation and Upper Yogou Formation (YS3). Based on the researches of structural modeling, depositional system evolution, and oil and gas accumulation condition, we considered YS3 to be the new prospecting layer. Our study showed that the YS3 group was formed in the fall stage of a high frequency sea level fluctuation period. It had transitional facies, and developed the interbedded delta front sand and shale system. The YS3 mudstones were the main source rocks of the Termit Basin, extensively matured with good hydrocarbon generation potential. The delta quartz sandstone was a dominant reservoir of high compositional and texture maturity. It was mostly located in the favorable hydrocarbon vertical migration domain, near the source rocks of proper burial depth. Altogether, these features of YS3 afforded it favorable hydrocarbon accumulation conditions. Based on tectonic study and well data, we carried out investigation on the depositional system of the Yogou Formation: we built time and spatial evolution sequences, evaluated provenance system and reservoir features, and selected the optimum exploration zone. In addition, through a paleotopographic comparative study of the depositional period in the eastern, western and southern margins, we discovered that different margins had different topography and depositional features in the Yogou period: the Dinga step fault in the western margin assumed a steep slope with several deep fractures between uplift and depression, and the belt is narrow; the Yogou slope in the southern margin and Fana low uplift area are gentle slopes with wide open areas, and steep and gentle slope models were built based on the Yogou slope. Our investigation further reveal that in the Late Cretaceous, the eastern uplift belt was the dominant provenance zone, whereas the western and southern uplifts were minor; as a result, large scale delta front sand, interbedded with mudstone, developed widely in the east of the basin in the later period of transgressive sequence, while the south and west came second and third in development scales, respectively. The source rock study showed that the YS3 mudstone source rock had terrigenous origin and its development was controlled by the amount of sediment supply. The depositional system analysis showed that the gentle slope belts in the southwest and east of the basin are more mature than the western steep slope zone. Compared with the upper Paleogene Sokor layer, the lower layer Cretaceous Yogou Formation YS3 layer lacks substantial cap rock, therefore, in future exploration, we should adopt a different strategy, aiming at the anticline trap, small throw reverse normal fault, consequent normal fault and depositional trap.