Organic Matter Enrichment Mechanism For The Source Rock Of The Enping Formation And Its Contribution To Hydrocarbon Accumulation In The Baiyun Sag,Pearl River Mouth Basin

The Paleogene Enping Formation provides the principal source rocks for hydrocarbon accumulation in the Baiyun Sag,Pearl River Mouth Basin,South China Sea.However,the sedimentary environment of the Enping Formation is still controversial because of the low exploration degree in the deep-water area of the northern South China Sea.Less work has been done on the organic matter enrichment mechanism of the Enping Formation source rocks,as well as their contribution to hydrocarbon accumulation in the study area.In this study,the sedimentary environment and organic matter accumulation of the Enping Formation source rocks were investigated using biomarkers,sulfur isotopes,and elemental compositions.Organic facies of the Enping Formation were classified on the basis of deposition environment and organic matter characteristics.Moreover,the contribution of source rocks of differernt organic facies to oil accumulation in the Baiyun Sag was also identified.The marine transgression occurred during the deposition of the SQ3 of the Enping Formation（EPSQ3）rather than in the early deposition of the Zhuhai Formation,which is supported by the increasing of paleo-salinity,regular changes of pyrite sulfur isotopic composition,triaromatic dinosteroid concentration in the EPSQ3 mudstones.Four different types of source rocks can be distinguished on the basis of sedimentary facies,including shallow lacustrine mudstone,fan delta mudstone from the SQ1-2 of the Enping Formation（EPSQ1-2）,and braided delta mudstone,shallow marine mudstone from the EPSQ3.Source rocks from different sedimentary facies are characterized by predominance of terrigenous organic matter,and medium to high abundance of organic matter,with TOC content mainly ranging from 0.8%to 2.0%.The proportions of terrigenous organic matter are probably higher than 80%in shallow lacustrine,delta and shallow marine source rocks from the Main Subsag.By contrast,shallow marine source rocks from the East Subsag contain lower content of terrigenous organic matter（65%～80%）.The provenance of the Enping Formation source rocks is primarily composed of felsic igneous rocks.Proxies of chemical alteration index（CIA）,P/Ti,V/Cr and Zr/Al ratios in the Enping Formation source rocks indicate a warm and humid paleoclimate and an oxidizing environment,with low paleo-productivity,high terrigenous clastic input.Although the transgression resulted in the increase of water depth,the salinity as well as the sedimentary area,the factors affecting the accumulation of organic matter in the EPSQ3 and EPSQ1-2 did not significantly change.High input of terrigenous organic matter is crucial for the accumulation of organic matter in the study area.Moreover,the multi-stage deltas may provide access to terrigenous organic matter into the Baiyun Sag.While,low paleoproductivity,oxidizing environment and strong dilution condition are not conducive to the enrichment of organic matter.The accumulation of organic matter therefore may be affected by the synergistic effect of paleoclimate,dilution conditions and terrigenous organic matter input,rather than one of a single factor.This is consistent with the weak correlations between TOC and proxies for these depositional conditions.A number of higher plant-derived biomarkers,including bicadinanes（V,W,T,T1,and R）,oleanane（OL）,taraxastane,des-A-oleanane,rearranged oleananeⅠ,ⅡandⅢ,were detected and identified in oils and source rocks of the study area.The relative abundances of these higher plant-derived biomarkers in oils are primarily controlled by angiosperm inputs and sedimentary conditions of their source rocks.On the basis of multiple biomarker parameters,oils from the study area can be devided into two families,i.e.Family-ⅠandⅡ.Family-Ⅰoils,including oils from the West Subsag,northern Main Subsag and western part of the East Subsag,are characterized by relatively high values of Pristine/Phytane（Pr/Ph）,bicadinane-T/C₃₀ hopane(T/C₃₀H),（Ⅰ+Ⅱ+Ⅲ）/OL,low values of OL/C₃₀H.By contrast,Family-Ⅱoils,which occur in eastern part of the East Subsag,the East Low Uplift and eastern Main Subsag,have lower values of Pr/Ph,T/C₃₀H,（Ⅰ+Ⅱ+Ⅲ）/OL,but higher values of OL/C₃₀H.The line-fitting formula between vitrinite reflectance（Ro）and Methyl-Phenanthrene Distribution Fraction（F₁）of source rocks（Ro=1.6182×F₁+0.0427）was evidenced to be an effective formula for calculating the maturity of oils（Rc）.Calculations suggest that oils from the study area are products of source rock in the stage of mature,with Rc values in the range of 0.79%～1.13%.This study defined five organic facies in the study area,including the Fan Delta Terrestrial Organic Facies A,Shore-shallow lake Terrestrial Organic Facies B,Braided Delta Terrestrial Organic Facies C,Littoral-Neritic Sea Mixed Organic Facies D and Littoral-Neritic Sea Terrestrial Organic Facies E.Organic facies B,D and E source rocks have better hydrocarbon generation capacity than that of organic facies A and C source rocks.The EPSQ1-2 source rocks are composed of organic facies A,B and C source rocks.While,organic facies C,D and E source rocks are mainly located in the EPSQ3.According to maturity evolution history,source rocks of these five organic facies are within mature stage at key moment for hydrocarbon accumulation（16 Ma）,implying relatively good oil generation properties.Systematic oil-source correlation indicated that Family-Ⅰoils are mainly sourced from organic facies C source rocks.Practically,oils from the West Subsag and northern Main Subsag are sourced from the organic facies C source rocks,possibly with minor contribution of the organic facies B source rocks.Oils distributed in western part of the East Subsag are derived from organic facies C and D source rocks,of which C facies source rocks contribute more.While,Family-Ⅱoils are derived from organic facies A,E and D source rocks.Oils from eastern part of the East Subsag are mainly generated by organic facies D source rocks,with the contribution of the organic facies C source rocks.The East Low Uplift oils are dominantly derived from organic facies A and E source rocks,possibly with minor contribution of organic facies D source rocks.By contrast,oils located in eastern Main Subsag are mainly sourced from organic facies E and A source rocks.