Main Controlling Factors And Hydrocarbon Accumulation Regularities Of The Paleogene Sokor2 Formation In Termit Basin, Niger

CNPC achieved its scaled hydrocarbon discoveries in the Paleogene Sokor1 Formation in Termit basin,Niger in the recent years.Despite the substantial potential is revealed by the latest exploration activities in the Paleogene Sokor2 Formation,the(potential)new hydrocarbon exploration play,studies and literatures on hydrocarbon accumulation of the Sokor2 Formation are rarely reported.Focusing on the Sokor2 Formation hydrocarbon play and utilizing the frontier geophysical technologies of reservoir rock prediction,this study integrates the source rock evaluation and oil-source correlation,carried out the sedimentology analysis and hydrocarbon accumulation regularities research of the Sokor2 Formation.The Sokor2 Formation reservoir rock distribution and the main controlling factors of hydrocarbon accumulation are defined;the hydrocarbon accumulation models are built.The exploration activities in Termit basin are well orientated with the outcome and geological understandings of this study and will further benefit from this study.Source rock evaluation and oil-source correlation analysis shows that two different source rocks are developed in Termit basin: marine source rocks and terrestrial source rocks.Marine source rocks are mainly shales developed in the 3rd sequence of Yogou Formation of Upper Cretaceous,when the basin entered the first post-rift depression stage.With the widespread marine transgression,the main source rocks of the basin deposited and are widely mature at present.Terrestrial source rocks are mainly shales developed in the Paleogene Sokor1 Formation when the basin entered the second rifting stage with the continental lacustrine deposits.The terrestrial source rocks are only mature locally surrounding the depocenter.It was found that the organic matter in the Late Cretaceous in the Termit Basin was dominated by the input of higher-level land-sourced plants.The high-abundance source rocks were controlled during the low sea level,and the delta environment controlled the accumulation of organic matter and the distribution of favorable hydrocarbon source rocks.Based on the characteristics of the development of marine source rocks on the passive continental margin,the “A restricted in-land sea enriched with terrigenous organic material” was established.The oil-source correlation analysis suggests,in the different zones,the oil in the Sokor2 Formation could be charged by marine or terrestrial source rocks.For the first time,the bi-source Upper Sokor charge model thought is proposed.In this study,based on the sequence correlation of wells,sedimentology analysis and seismic data interpretation,the Sokor2 Formation is subdivided into three sand units,i.e.the S21,S22 and S23 sand units.The characteristics of the sedimentary system of the Sokor2 Formation in the Paleogene are well defined.The sedimentary system of the shoreline shallow lake dominates the development of the underwater distributary channels,estuary dams,and far sand dams in the delta front around thelake.It is pointed out that the S23 sand group can be an effective reservoir.It can be a new exploration play in the Termit basin.During the study,it was further raised that the southern Dinga Terrace,southern Yogou Terrace and southern Moul Trough are favorable zones for the new exploration play of S23 sand unit,which guided the recent exploration activities with new successes.The Sokor2 Formation hydrocarbon accumulation model is built: bi-source charging from marine or terrestrial source rocks,vertical migrating through faults pathway,trapping controlled by sand units' development.The accumulation model could be depicted by the main controlling factors as below: Extensive hydrocarbon generation of marine source rocks of Cretaceous Yogou Formation;Locally mature and near source charging of terrestrial source rocks of the Paleogene Sokor Formation;Vertical migration of hydrocarbon controlled by both strength scale of Paleogene faulting and the thickness of Low Velocity Shale above the Sokor1 Formation;Quality of S23 sand unit reservoir rock controlled by the overlapped Paleogene rifting scale.The study enriched “the petroleum geological theories of the Central-Western African passive rift basins”;the new understanding raises the “regional cap rocks” as a new hydrocarbon exploration play,which filled the research gap of the region.Through this study,the new hydrocarbon exploration play of Sokor2 Formation is defined to widen the exploration domain,which will support the sustainable and substantial discovery rate of the Termit basin hydrocarbon exploration.