| Due to the great differences in the development characteristics between marine source rocks and lacustrine source rocks,the research on the development characteristics,spatial distribution and development model of marine source rocks is particularly important in the process of changing from the domestic oil and gas exploration battlefield dominated by lacustrine source rocks to the overseas oil and gas exploration battlefield dominated by marine source rocks.Fine grained sediment is an important carrier of organic matter.It is widely developed in lakes,oceans,rivers and other environments.It contains rich sedimentological information and can effectively record the physical and chemical conditions for the development of source rocks.However,due to the limitation of analysis technology,this information has not been fully utilized.For a long time,due to the limitations of observation methods and experimental conditions,the research results of fine-grained rock petrology and sedimentology are relatively scarce,and the classification standards of lithology and lithofacies are not unified.Compared with the lithology and lithofacies analysis of classic sandy sediments,the lithofacies analysis of fine-grained sediments is weak,and the lithofacies division is too general,such as "middle mudstone deep lake","shelf mudstone" and "shallow sea mudstone",which can not meet the requirements of shale oil and gas reservoir prediction and fine evaluation of source rocks.Therefore,taking the exploration block of CNOOC lower Congo Basin as the target area,this paper discusses the rock structure,lithofacies,sedimentation diagenesis,typical minerals and source supply mode of marine fine-grained sediments,so as to provide a geological basis for the development characteristics,development mode and distribution law of marine source rocks;Through the combination of well and earthquake,the logging facies and seismic facies of source rocks are analyzed,the geological and geophysical characteristics of marine source rocks are comprehensively summarized,and the formation mechanism and development model of marine source rocks are discussed.On this basis,the spatial distribution of source rocks is predicted and evaluated.Deepening the study of petrology and sedimentology of fine-grained sediments not only has important theoretical significance,but also helps to better understand the enrichment mechanism of organic matter in fine-grained sediments and accurately evaluate and predict high-quality source rocks and shale reservoirs.In view of the fact that there are few first-hand drill core samples abroad,the heterogeneity of source rocks is strong,and the test,analysis and evaluation are not fine enough,and the organic geochemical test and analysis are greatly affected by oil-based mud drilling fluid and sample type,this paper puts forward the fine-grained sediment sedimentology Research Based on single rock debris particles,so as to fully tap the potential of rock debris samples,Improve the evaluation accuracy of source rocks.The arrangement mode of clastic particles,particle size distribution of clastic particles,clay mineral types and crystal morphology,microfossils,microbial minerals and mineral aggregates,alkaline metal sulfides,element geochemical characteristics Microbial Chemistry and organic matter distribution were analyzed to determine the source supply pattern,productivity and evaluation index of organic matter preservation of fine-grained sediments.Both the deep-sea drilling results of modern seabed sediments and the shale oil and gas exploration process at home and abroad show that the vertical heterogeneity of source rocks is very large,and only local intervals are effective source rocks in large sections of dark mudstone.Rock debris samples are abundant and can be sampled continuously,which can meet the requirements of source rock heterogeneity analysis.Intermittent suspended debris supply is the most effective way to supply nutrients required by algae and preserve organic matter.The mudstone in the interval rich in organic matter contains abundant foraminifera and radiolarian fossils and irregular phosphate nodules.Filamentous and fibrous silico calcareous mineral aggregates and biogenic cryptocrystalline cherts are developed in or around the foraminifera and radiolarian shells;Microfossils,biological silica,irregular phosphate nodules,alkaline metal sulfides,radial silica calcium aggregates related to microbial activities are common in shale rich in organic matter.These minerals are related to microbial chemical action and are important identification indicators of organic matter enrichment.Based on the analysis of fine-grained sediment lithofacies,logging facies and seismic facies,the whole process of fixed-point,zone and structure evaluation of source rock is realized through the combination of single rock debris particle fine-grained sediment lithofacies analysis(point),GRP cycle logging facies(line),seismic facies(surface)and seismic attribute analysis.The marine source rocks in the lower Congo Basin are evaluated by using single rock debris granular lithofacies,element geochemical analysis and geophysical parameters of high-quality source rocks,which overcomes the problem of source rock evaluation and prediction caused by the lack of organic geochemical analysis data in the study area,and defines the development model and spatial distribution law of marine high-quality source rocks in the West African continental shelf basin,The analysis method is proved to be effective and feasible by exploration results.Through the comprehensive geological interpretation of heavy well earthquakes in the lower Congo Basin,this paper analyzes the seismic facies and sedimentary facies of madingo formation in the point interval,focuses on the test and analysis of drilling cuttings sampling and rock geochemistry,interpretation of high-resolution sequence stratigraphy and seismic attribute analysis of madingo formation in CNOOC 3D seismic work area,and obtains scanning electron microscope,energy spectrum analysis,surface scanning There are more than 2500 photos and analysis maps of micro major elements,laser particle size analysis,ordinary thin section test results,1200 seismic facies and forward seismic sections,30 seismic attribute analysis maps,3 sedimentary pattern and paleogeomorphic evolution maps,15 analysis maps of connecting wells and GRP cycles,and 1 map of hydrocarbon source rock prediction and evaluation.The main understandings are as follows:(1)Through the comprehensive analysis of mineral composition(clay mineral,calcareous and clastic particle content)and structure(clastic particle size,content and arrangement)of fine-grained sediments,it can effectively reveal the intensity and mode of terrigenous supply,and finely dissect the input state of terrigenous nutrient elements,which is an important supplement to the hydrodynamic analysis of coarse clasts.It provides a new idea for the sedimentological analysis of source rocks and fine-grained sediments,and it is also an important supplement to the hydrodynamic analysis of coarse clastic sediments.Since fine-grained sedimentary rocks are not composed of a single mineral and usually contain clay,felsic and carbonate minerals,the sedimentary structures of fine-grained sediments are divided into uniform mixed deposition,laminar mixed deposition and uneven mixed deposition structures,which correspond to suspended supply,intermittent terrigenous supply(seasonal precipitation)and biological disturbance or quasi syngenetic deformation respectively.Intensive sampling of mudstone cuttings from the upper Cretaceous madingo formation in the lower Congo Basin,quantitative analysis of clay minerals,microstructure analysis of mud single cuttings and laser particle size analysis reveal the transportation mode of fine sediments.The terrigenous clastic particles in the high TOC interval are fine(fine silt grade),mainly suspended transportation,and the content of clastic kaolinite increases intermittently,The suspension can be divided into three distinct stages,with debris particles distributed in a wide range of particle sizes,?=6-11,with the characteristics of multi-stage suspension supply and transportation,indicating that intermittent suspension debris supply is the most effective way to supply nutrients required by algae and preserve organic matter.(2)The surface scanning technology effectively reveals 100-200 mm amorphous phosphate particles and radial and filamentous carbon siliceous minerals widely distributed in high-quality source rocks.The mineral composition of fine-grained sediments is complex,mostly cryptocrystalline aggregates,coupled with the influence of microbial activities,the element composition of minerals varies widely,and the microstructure characteristics are not clear under ordinary optical microscope.It is necessary to judge the type and genesis of sedimentary diagenetic minerals by comprehensive analysis of X-ray diffraction,optical microscope,scanning electron microscope,backscatter and energy spectrum.The technical method of cryptocrystalline mineral aggregate analysis has important reference value for the analysis of sedimentary diagenetic minerals in source rocks.The mudstone in the interval rich in organic matter is rich in foraminifera,radiolarian fossils and irregular phosphate nodules.Filamentous and fibrous silico calcareous mineral aggregates are developed in or around the foraminifera and radiolarian shells,and biogenic cryptocrystalline cherts are developed in the foraminifera and radiolarian shells;Microfossils,Biosilica,irregular phosphate nodules,alkaline metal sulfides and radial silica calcium aggregates related to microbial activities are common in shale rich in organic matter.These minerals are related to microbial mineralization and are important indicators for the enrichment of organic matter.(3)This paper systematically summarizes and combs the fine analysis of marine source rocks based on single debris particles,and defines the sedimentological mechanism,control factors and identification marks of marine organic matter enrichment,which provides a new technical idea and analysis method for the evaluation and prediction of marine source rocks.The high-quality marine source rocks of the upper Cretaceous in the lower Congo Basin are mainly distributed in the shallow sea salt withdrawal trough within the influence range of suspended land source supply on the continental margin.The continental shelf is wide,weakly affected by gravity flow,high primary productivity,good organic matter preservation conditions,large-scale salt withdrawal trough in the continental shelf transition zone and great resource potential,It provides rich hydrocarbon sources for the structural stratigraphic traps around the depression.The high-quality marine source rocks in the lower Congo Basin are mainly developed in shallow sea depressions.Small canyons are developed in the depressions near the direction of land-based supply.They are mainly distributed within the scope of suspended land-based nutrient supply,and the primary productivity is high;At the same time,the circulation of water is limited and the preservation conditions of organic matter are good.The continental margin of this model provides a macro Geographical Framework for the prediction of marine source rocks.The hydrocarbon reservoirs in the continental margin of West Africa South America from the upper Cretaceous marine source rocks are mainly distributed around shallow sea depressions(salt withdrawal depressions),while the hydrocarbon reservoirs in the undeveloped area of shallow sea depressions are mainly from the lower Cretaceous lacustrine source rocks.The shallow sea depression(salt withdrawal depression)in the continental margin transition zone has the largest scale,large thickness of marine source rocks and good preservation conditions of organic matter.If it is located in the coverage area of suspended land source supply,it has high primary productivity and is a natural place for the development of high-quality marine source rocks.(4)Analyze the seismic facies and logging facies of different genetic source rocks,extract the sensitive geological and geophysical parameters of various source rocks,and obtain the prediction model and evaluation technology with operability and popularization;Then,the development model and spatial distribution evaluation technical system of marine source rocks are established to clarify the distribution range and development scale of source rocks in the target interval of key sea areas,so as to provide technical support for zone optimization.Based on the lithofacies analysis of fine-grained sediments and the "four facies" analysis of sedimentary facies,logging facies,seismic facies and organic facies,this paper summarizes the identification indicators of terrestrial supply,paleoproductivity,preservation conditions,micro diagenetic characteristics and pore types of marine source rocks of different lithofacies types,and organically combines the micro sedimentological indicators with the macro indicators of "four facies",Realize the rapid prediction and evaluation of source rocks under the background of different exploration degrees.The nearshore clastic sedimentary body is characterized by low frequency,strong amplitude and continuous lenticular reflection.It is a sand mud interbedded sedimentary combination under the condition of high energy oversupply.It is dominated by medium high frequency medium amplitude GRP cycle,and the terrigenous clastic particles are relatively coarse;The shallow water shelf is a sedimentary combination of shallow sea mudstone with thin marl,which is characterized by low frequency,strong amplitude,continuous and parallel reflection.It is dominated by medium frequency medium amplitude GRP cycle,which is a shallow sea deposit under the condition of weak supply.The deep-water shelf is characterized by intermittent medium strong reflection under the background of weak reflection,corresponding to the sedimentary combination of mudstone mixed with thin-layer siltstone.It is the product of intermittent suspension supply under the background of deep-water shelf or shallow sea depression.The stratum thickness is relatively thick and is mainly distributed in salt withdrawal depression.(5)Based on the work of logging facies and seismic facies,the geological and geophysical technical methods and work flow for the evaluation and prediction of marine source rocks are established,which provides a technical reference for the prediction and evaluation of marine source rocks under the background of different exploration degrees,and overcomes the difficulties in the prediction and evaluation of source rocks due to the lack of organic geochemical data in the study area.The high-quality source rock of madingo formation in the shallow water trough of lower Congo Basin fluctuates with high gamma amplitude,and has the logging curve characteristics of high gamma value,high resistivity,high acoustic time difference and low density.The seismic reflection shows intermittent medium strong reflection under the background of weak amplitude,surrounded by low-frequency continuous strong amplitude reflection on the shallow sea shelf,and the sedimentary facies belt is continuously and completely distributed.Through the combination of fine-grained sediment lithofacies analysis(point),GRP cycle logging facies(line),seismic facies and sedimentary facies(surface),the whole process of fixed-point,zone and structure of source rock evaluation is realized.Firstly,accurately calibrate the development horizon of high-quality source rocks,secondly determine the development facies belt and paleogeographic background of source rocks,and finally realize the construction of 3D TOC data volume through quantitative analysis of logging parameters and seismic attributes under the constraint of four facies.The above evaluation methods are used to evaluate the marine source rocks in the lower Congo Basin,and the analysis method is proved to be effective and feasible by the exploration results. |
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