Facies Analysis And TOC Prediction Using 3-D Seismic And Well Log Data For Characterization Of The Source Rock Potential In Lower Congo Basin,West Africa

In the past few decades,the ongoing exploration of fine grained rocks in offshore areas has gained much importances,this dissertation is based on the study of hydrocarbon potential evaluation and characterization of Lower Congo Basin,west Africa.Preliminary objective of the study is to carry out interpretation of 3D seismic data on 200 sq km area of Block A and S3 Lower Congo basin.The main reflectors are marked in 3D volume named as;Paloukou(Oligocene-Holocene),Madingo(Turonian-Oligocene),Likouala(Cenomanian-Turonian),Sendji(Albian)and salt top(Aptian).Faults associated to them are also marked.Source rock information and calibration points are previously not well understood and scarce due to the lack of well data,a considerable contribution aimed in this reaserach by applying a systematic approach using integrated geophysical,geological,geochemical data.Moreover,the emphasize is to depict source potential qualitatively and qauntitaviley.Based on initially interpreted 3-D seismic data and then the sufaces,thickness and structural maps were generated for each horizon,the horizon of interest is studied in detail.The main hydrocarbon bearing strata,the Madingo Formation is divided as: Upper and Lower Section Formation.The palaeogeographic research and the development of marine source rock can effectively predict the distribution of high-quality source rocks under limited wells.Based on the geographical background of ancient and modern organic-rich sedimentary development pattern,channel pathways,depositional style of Madingo Formation,it is interpreted that the area is suitable for accumulation and preservation of the Organic matter.The integrated approach of facies analysis was intended to analyze the quality of the source rock using geological and geophysical responses and background knowledge.The physical and chemical properties of organic matter make the well logging response of source rock and nonsource rock distinctly different,which aids in source rock evaluation both qualitatively and qauntitaively.The list of available logs is named as;GR(gamma Ray),RHOB(density),DT(sonic),NPHI(porosity)and AT90(resitivity).Basically,the GR logs response helps in understanding the depostional style,facies and the biological development of higher TOC content.The maximum GR often reflected in the deepest period of water corresponds to the Condensed Section and higher TOC values e.g Plankton has a strong adsorption of uranium ions widely present in the marine environment,so the marine source rock with generally a high gamma values then the terrestrial source rock.Depositional pattern,vertical facies,lateral correlation of genetically related facies the sequence stratigraphic framework of the main well logs are studied.The systematic and cyclic strata stacking pattern of the lithology coupled with the gamma-ray log patterns led to identification of three Para-sequence types: upward-increasing Gamma Ray Para-sequence(GRP),upward-decreasing GRP and intervals of constant GRP.The fall in GRP shows a regressive process,the GRP increasing upward displays the transgression process and the steady change in the GRP shows the water stable stage.1)Likouala Formation,the GRP have medium amplitude dentated trend corresponding to stable sea-level rise and small-scale condense section development.2)Madingo Formation,GRP displays high-amplitude fluctuation corresponding to three organic matter enriched condensed sections identified as the maximum flooding surfaces.3)Paloukou Formation,GRP have low-amplitude GRP cycles corresponding to stable rise and fall in the sea level.The GR cuve demonstrating the changes in the water body due to fall and rise in sea level.The development of the flood surface and(MFS)for relatively long period of time show that the,mudstone deposited with high organic matter content displays large GR values.Then the seismic facies studied using reflection features showing source rocks seismic reflection characteristics are significantly different.Reflections characteritics such as frequency,continuity and amplitude there thickness,internal and external geometries which is very easy to identify are studied and helps to identify the source rock nature e.g the low-frequency,high-continuity,and strong-amplitude reflect a dense reflection section,which is generally a continental sediment with rich organic matter,parallel-sub-parallel reflection structure is mainly a lacustrine stratum with horizontal deposition mainly in deep water environments.The reflection structures,displaying the continuous deposition,continuous subsidence and no signifcant unconfirmties or tectonic movements so the thickness,distribution range,and sufficient burial depth will support that the mudstone enterd oil generation threshold and whether it can be an effective source rock.The application of seismic structural interpretation and attributes analysis in combination with well logs,has not only improved strategies for seismic facies development but also improved the method for development of seismic facies planar distribution with marine lithological settings.The development of source rocks is inseparable from the spatial distribution of sedimentary facies,combination of seismic and well data aids to understand the depositional setting at each well and seismic properties like frequency,reflection pattern and amplitude along with paleogeomorphological data helps in the identification planar distribution of sedimentary facies.The conventional well logs are used to predict TOC using an integrated work flow by several methodologies,mainly the empirical and Graphical method is applied to resolve this task,the adopted method is the cheapest and fast.Firstly,the log parameters correlation with TOC was analyzed.Several methodologies are used named as;multivariate analysis,ΔLog R and schomeker methods results are calculated and interpreted and implemented on the study area then it is found that the results of multiple regression are good.All these predictions are not considerd to be fine enough,a model is considerd to be premium if the correlation is 0.9.Then,ANN(artificial neural network)back propagation technique was designed for more accurate prediction of the TOC.(ANN)data-based approach comprised of three layers;input,hidden and output layer,using different training alograthim the logs are input,and the output is TOC,predicted TOC with 0.95% accuracy.The generated model reduced the relative error and estimated results are compared with with the real data to affirm its reliabilty.Furthermore,the geochemical parameters obtained by pyrolysis of source rocks were calibrated with the results obtained from logging data.The Upper Cretaceous rock succession has shown very good source rock properties with kerogen type that Type II kerogen.The absence of core data and other geochemical data sets,logging data is the cheapest and best way to check the oil and gas potential of any hydrocarbon source rock.The results of these studies are based on the seismic structural interpreted data,paleogeomorphological maps,well logs data and rock pyrolysis analysis,petrophysical analysis(some thin sections)which comprehensively resolved key problem for prediction of economic viability of Madingo Formation,Lower Congo Basin and this technique is readily applicable to areas of same geological settings.