Sequence Stratigraphic Geochemical Investigation On The Upper Cretaceous Yogou Formation Of The Termit Basin,West Africa Rift System

The Termit Basin is one of the hydrocarbon prolific basins in the West and Central Africa Rift System?WCARS?.The marine mudstones from the Upper Cretaceous Yogou Formation have been proven to be the main effective source rocks in the Termit Basin.However,the occurrence and formation mechanism of these potential source beds are yet poorly understood,which directly restricts the prediction of effective source rocks and the estimation of hydrocarbon resources.Based on 47 well-loggings and 24 2D seismic sections,as well as geochemical data of 229 mudstone samples,this thesis attempts to study the spatial distribution,geochemical characteristics and the accumulation process of organic matter within the sequence stratigraphic framework of the Yogou Formation.The occurrence and formation mechanism of the Yogou marine source rocks have also been discussed in this thesis.Main results obtained are showed as follows.A total of three third-order sequences?YSQ1,YSQ2 and YSQ3,from bottom to top?can be identified in the Yogou Formation of the Termit Basin.The stratigraphic sequence of the Yogou Formation consists of a series of upward coarsening depositional cycles.Three types of sedimentary facies,i.e.offshore,shoreface and delta,can be distinguished among the Yogou Formation in the area of tectonic slope and depression.As a result,five types of source rocks,i.e.lower offshore mudstone,upper offshore mudstone,deltaic mudstone,shoreface mudstone and coal/carbonaceous mudstone,can be divided on the basis of sedimentary facies and lithology.The lower offshore mudstones are the dominated source rocks in the YSQ1 sequence which distributed thoughout the basinal area,whereas they restrictly distributed around the depression center and gradually transformed into upper offshore mudstones in the tectonic slope area during the period of YSQ2.However,all of these five types of source rock can be found in the YSQ3 sequence in the tectonic slope and depression area.The upper offshore mudstones mainly occurred in the lowstand system tract?LST?,which gradually transformed into lower offshore mudstones in the transgressive system tract?TST?.While the shoreface mudstones,deltaic mudstones and coal/carbonaceous mudstones were mainly distributed in the highstand system tract?HST?of YSQ3.However,the deltaic mudstones and coal/carbonaceous mudstones only restrictly occurred in the basinal edge area,characterized by thin thickness,narrow distribution area and lower quantity.The lower offshore mudstones and upper offshore mudstones show with similar geochemical characteristics,such as medium total organic carbon content?TOC?ranging from 0.59%to 2.76%?with an average value of 1.10%?and dominanted type II₂ to type III kerogen.Besides,they also represent with the unimodal n-alkane distributions with maxima at n-C₁₉ to n-C₂₁,lower Pr/Ph ratios?Pr/Ph<2.0?,higher gammacerane index(Ga/C₃₀H)ranging from 20%to 52%.In contrast,the shoreface mudstones are characterized by relatively higher TOC in the range of0.97%¹2.90%?with an average value of 4.60%?as well as dominanted type II to type III kerogen.Moreover,they show with the bimodal n-alkane distributions with a predominance of nC₁₈ and nC₂₉,relatively higher Pr/Ph ratios?Pr/Ph>2.0?and lower gammacerane index(Ga/C₃₀H<20%).In this study,the palaeodepositional conditions for the organic matter accumulation,as well as the accumulation models,have been reconstructed on the basis of the organic geochemical characteristics of soure rock samples from different sequences of the Yogou Formation.Results indicate that the global sea level changes and the influx of terrigeous freshwater and sediments are the main controlling factors leading to different seawater circulation patterns between the internal water column and external ocean,which are also the critical factors to the palaeodepositional conditions,such as the nutrient conditions of the internal water column,paleo-water-depth,paleosalinity and redox condition.Besides,the kerogen types,quantity and preservation of organic matter are also influenced by all of these geological factors.During the period of marine invasion in the Termit Basin,the critical factor leading to the occurrence of source rock with relatively higher amount of organic matter is the supply of terrigeous organic matter and nutrient materials from the basinal peripheral area.