Genesis Of Hydrocarbons And The Petroleum System Of The Termit Basin,West And Central Africa Rift System

The Termit Basin in Niger is the second largest petroliferous basin in the West and Central Africa Rift System（WCARS）,and also an important overseas oil and gas interest block for the China National Petroleum Corporation（CNPC）.Previous work has yielded rich achievements on the tectonics,sedimentary reservoirs,and geochemical characteristics of crude oil in this basin.However,there is still lack of systematic research on the sedimentary basin background of source rock development,genesis and origin of crude oil,and hydrocarbon accumulation system.A total of ten third-order sequences were identified in the Upper Cretaceous–Paleogene in the Termit Basin based on geological data such as logging and seismic profiles.Two third-order sequences were identified（DSQ1 and DSQ2,from bottom to top）in the Upper Cretaceous Donga Formation.The Yogou and Madama Formations were divided into three（YSQ1,YSQ2,YSQ3,from bottom to top）and one（MSQ1）third-order sequences,respectively.Paleogene were divided into four（SSQ1,SSQ2,SSQ3 and SSQ4,from bottom to top）third-order sequences.By using inorganic and organic geochemical data and the paleogeographic evolution history of the Late Cretaceous of the WCARS,this thesis determined the depositional paleoenvironment and evolution of the Upper Cretaceous source rocks in the Termit Basin.During the deposition of the DSQ1–YSQ2,the paleoclimate is warm and wet,both the relative higher salinity and reducing–weakly reducing bottom water environment all indicated the occurrence of marine transgression during Late Cretaceous in the study area,which belongs to the eastern Trans-Saharan Seaway.The first large-scale transgression occurred during the Late Cenomanian period,and reached maximum extent during the Santonian,and then gradually regressed.The seawater withdrew from the Termit basin during the Late Campanian.This thesis clarifies the geochemical characteristics of source rocks under the stratigraphic framework established on the geochemical experimental analysis data of 693source rock samples.The DSQ1 and DSQ2 source rocks are characterized by relatively low total organic carbon contents（TOC）,typeⅢkerogen,and at the thermal mature–high mature stage.The YSQ1 and YSQ2 sequences as well YSQ3 lowstand–transgressive system tracts source rocks are at the mature stage,with relatively moderate TOC contents,type II₂–III kerogen.The YSQ3 highstand system tract source rocks are at low-mature stage,with good hydrocarbon generation potential and type I–II kerogen.The TOC contents and kerogen type of source rocks from the SSQ1 and SSQ2 are similar with those of YSQ3 source rocks.In contrast,the SSQ1 and SSQ2 source rocks are mainly at low maturity stage.Only source rocks occurred in east part of the Dinga step-fault zone are in the oil generation window.Based on the assemblage of biomarker parameters of 56 crude oil samples,oils from the Termit Basin can be devided into two families,i.e.Family-ⅠandⅡ.The widely distributed Family-Ⅰoils are characterized by relatively low values of Pristine/Phytane（Pr/Ph）,terrigenous/aquatic ratio（TAR）,C₁₉₊₂₀ tricyclic terpanes（TT）/C₂₃TT ratios,but high n C₂₁-/n C₂₂+and Gammacerane（Ga）/C₃₀Hopane ratios.The Family-Ⅱoils,which only occur in east part of the Dinga step-fault zone,have high values Pr/Ph,TAR,C₁₉₊₂₀TT/C₂₃TT,and low n C₂₁-/n C₂₂+,Ga/C₃₀H ratios.The result of oil-source correlation indicated that Family-Ⅰoils are mainly sourced from the YSQ1–YSQ2.Family-Ⅱoils are derived form SSQ1–SSQ2 source rocks,probably with some contribution of YSQ3 source rocks.Moreover,Family-Ⅰoils have generally suffered biodegradation with varying degrees which ranging from non-degraded to severely degraded（PM 6–7）.The burial depth of oil reservoir,post-charging reservoir temperature,the development characteristics of faults and the thickness of cap rocks are the geological factors controlling the degrees of biodegradation of Family-Ⅰoils and the burial depth of reservoir is the primary factor.According to the mentioned factors,it is predicted that the Dinga step-fault zone in the Termit Basin is the main production area for light oil,while the northeastern of the Moul Depression,the Dibeilla and Oyou wells area in the Araga Graben are heavy oil production areas.Two petroleum systems,i.e.northern Dinga petroleum system and southern Moul petroleum system,have been subdevided in the Termit Basin,in terms of the geological elements,geological processes and the occurrence of discovered oil reservoirs.In Dinga petroleum system,the primary source rocks are YSQ1 and YSQ2 sequences offshore mudstone,the lacustrine mudstone of SSQ1–SSQ2 in the east part of Dinga step-fault zone and the shoreface mudstone of YSQ3 have a minor contribution for the discovered oil.The primary reservoir rocks are SSQ1–SSQ2 sequences sandstone.Mudstone in SSQ3 sequence as the effective caprocks overlaying structural traps within SSQ1–SSQ2sequences.The oil vertically migrated by faults and carrier system from the underlying source rocks in the Dinga depression and accumulated in good permeability sandstones in the Dinga step-fault zone and the Araga Graben.In Moul petroleum system,the primary source rocks are YSQ1 and YSQ2 sequences offshore mudstone.The primary reservoir rocks are SSQ1–SSQ2 sequences sandstone,sandstone of YSQ3 sequence are minor reservoirs.Mudstone of SSQ3 sequence is the mainly effective caprocks.Trap types are mainly structural and lithologic traps.The oil vertically migrated by faults and carrier system from the underlying source rocks in the Moul depression and accumulated in sandstone reservoirs in the Yogou Slope and the Fana Uplift.