Geochemical Characteristics Of Hydrocarbon Accumulation In The Baiyun Depression Of Pearl River Mouth Basin

The study indicates that Wenchang Formation and Enping Formation are the main source rock intervals of Baiyun Depression.Wenchang source rocks which are deposited in lacustrine environment are characteristic of abundant organic matter,good organic matter type and high hydrocarbon generating potential.Enping source rocks which are deposited in the shallow lacustrine-delta-swamp facies are feutred by abundant organic matter,mixed organic matter type and relatively high hydrocarbon generating potential.Zhuhai source rocks which are deposited in the marine-continental transitional facies with terrestrial organic matter input are stamped with medium organic matter abundance,mixed organic matter type and medium hydrocarbon generating potential.Crude oils of study area are identified in the mature or high-mature stage in light of analyses of the composition of light hydrocarbon,sterane and terpane maturity parameters.Vitrinite reflectance Ro values calculated by double-adamantane index are larger than 1.30%which distributes in the range of high mature oils.Different source rock intervals located in the different parts of depression have different undergone thermal evolution history,which caused different compositions and properties of hydrocarbon that would inevitable mix together in the process of migration and accumulation.As a result,there are some differences among the different maturity indicators corresponding to the size of the maturity.Based on the differences of biomarkers characteristics,Crude oils of study area can be divided into two major categories,which are eastern part and the western part respectively.In the western structure,the trend for relative abundance of the W-T-Ol compounds appears decreasing then to keep steady,which means that the bicadinanes(W and T)content are high and the abundance of the compound W is larger than that of the compound T,and the abundance of the oleanane is less than or the same as that of the compound T.In the eastern structure,the trend for relative abundance of the W-T-Ol compounds is that decrease firstly and then increase,which indicates that abundance of oleanane is higher but the abundance of resin compounds is relatively lower and the abundance of the compound W is larger than that of the compound T.Oil-source correlation shows that oils in the western structure have certain genetic relationship with Enping source rocks and both of them have abundant of resin compounds and low abundance of oleanane.The crude ois in the eastern structure are different from the Enping source rocks by study of oil-source correlation.These oils are rich of oleanane and resin compounds and could be derived from Enping source rocks while geological background and sedimentary strata are taken into consideration.In the Baiyun depression,the hydrocarbon gases generate in the thermal evolution of organic matter of the source rocks.Wenchang and Enping Formation source rocks in the main-lying areas are deeply buried and high maturity,which are the important source layer of nature gas.The characteristics of natural gas display a source-mixed gas,but generally biased in favor of oil-type gas.