| The Baiyun Sag is located in the Zhu II depression of the Pearl River Mouth Basin in the northern part of the South China Sea.It is the main area for natural gas exploration in the deep water area of the basin.In recent years,the discovery of a series of oil reservoirs has broken the traditional concept that the Baiyun sag is chiefly favorable for natural gas exploration.Although a large number of studies have been carried out on the Baiyun Sag,understanding of the petroleum system of the sag is still limited.In this study,the research mainly foucus on two aspects of the Baiyun Sag.Firstly,due to the lack of samples,the organic matter abundance of source rocks cannot be accurately and effectively evaluated during the deepwater petroleum exploration.An evaluation model for the total organic matter content(TOC)was established based on effective medium theory.The model is composed of the nonconductive rock skeleton,conductive shale,organic components(kerogen and oil and gas)and pore water.The average relative error between the measured TOC and the predicted value is 14%,which proved the effectiveness of the method.Compared with the classical ?log R method and neural network method,the primary advantage of the new model is that the consideration of lithological variation on the logging response of the rock.Therefore,the TOC evaluation model established in this work is not only applicable to the large-scale mudstone section with stable lithology.It is also suitable for tight sandstone sections with large lithological changes.Secondly,systematical investigations of the geochemical characteristics of oils and source rocks of the Baiyun Sag have been performed to figure out the source origin of crude oil and the hydrocarbon generation mechanism.In addition,the hydrocarbon accumulation characteristics of typical oil accumulations in the sag was discussed with a perspective of geochemistry.According to the sedimentary evolution process of the source rocks of the Baiyun Sag,the source rocks can be divided into five types,including the lacustrine mudstones of the Wenchang Formation,the lacustrine mudstone and delta/coal mudstones of the Enping Formation,and the shed mudstones and marine delta mudstones of the Zhuhai Formation.Source rocks in the Wenchang and Enping formations searve as the chief origin of hydrocarbon in the Baiyun Sag.Three oil families has been identified in the Baiyun Sag.The Family I oils are mainly distributed in the northern tectonic belts of the sag,and they are more likely to be derived from the coalrelated source rocks deposited in the Enping Formation.The Family II oils are mainly distributed in the eastern to northeastern part of the sag.Source rocks for this group are probably the shallow lacustrine/delta mudstone of Wenchang/Enping Formation.In this study,a new lacustrine oil group was discovered in the southwest of the Baiyun Sag.This discovery confirms the exploration potential of lacustrine oil in baiyun sag.The study on organic hydrocarbon generation mechanism shows that the geothermal background and organic matter type are the main controlling factors of the organic hydrocarbon generation in the Baiyun Sag.The Main Sub-sag of the Baiyun Sag is mainly controlled by the high geothermal gradient background,which leads to the overall compression of the oil window.The West and East sub-sag are controlled by the low geothermal background,and the corresponding oil generation period is stretched.There exists two accumulation models in the eastern part of the Baiyun Sag.The hydrocarbon accumulation of “upper gas and lower oil” in the W3-2 structure to the H29 A tectonic belt is mainly controlled by evaporation fractionation,while the W3-1 structure is high mature crude oil migration through high angle faults.Moreover,the Zhuhai Formation in the southwest of the Baiyun is the favorable exploration area for the new discovered lacustrine oil,and the interior of the Main Sub-sag is more prone to the accumulation of condensate gas. |
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