关键词: 尼日尔, Termit盆地, 油气勘探, Yogou组

Abstract: 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.

Key words:  Niger, Termit Basin, oil and gas exploration, Yogou Formation