Enrichment Mechanism Of Organic Matter In The Lacustrine Mudstone Of The Eocene Wenchang Formation,eastern Yangjiang Sag,Pearl River Mouth Basin

As a complete early-middle Eocene sedimentary record,the lacustrine sediments of Wenchang Formation in the eastern Yangjiang sag of the Pearl River Mouth Basin(PRMB)provide a unique perspective for understanding the early-middle Eocene paleoclimate and paleoenvironment evolution in southern China.Meanwhile,the organic-rich mudstone of Wenchang Formation is an important source rock in the PRMB and even South China,and identifying its organic matter enrichment mechanism will provide useful enlightenment for oil and gas exploration in the PRMB and similar basins.According to the high-resolution three-dimensional seismic data and typical drilling data,on the basis of seismic sedimentology research in the study area,this paper analyses the magmatic and hydrothermal activities,paleoclimate and paleo-sedimentary environment evolution characteristics of Wenchang Formation by carried out systematic mineralogy,organic petrology,organic geochemistry,element geochemistry,isotope geochemistry analysis,the enrichment mechanism of organic matter was discussed.Based on drilling and logging analysis and three-dimensional seismic interpretation,through the fine analysis of single well sedimentary facies and seismic facies,four types of sedimentary facies including fan delta,braided river delta,lake and gravity flow are identified in the Wenchang Formation of study area.The development scale of steep slope fan delta and gentle slope braided river delta was limited during the deposition of Wenchang Formation in the study area,indicating that the deposition period was dominated by the input of small and medium provenance from the low uplift area between sags.The boundary faults were violently active during this period,and the high tectonic subsidence promoted the extensive development of lacustrine deposits.Especially in Enping 20/21 subsags,the semi-deep lake and deep lake facies are the most stable in vertical superposition and horizontal distribution,which lays a good material foundation for the development of organic-rich lacustrine mudstone.According to the analysis of organic petrology,mineralogy and geochemistry,the Wenchang Formation mudstone macerals of eastern Yangjiang sag contains abundant liptinite components(lamalginite,telalginite,sporinite and liptodetrinite)and a small amount of vitrinite and inertinite components,and the mineral composition are mainly in quartz and clay minerals.The lacustrine mudstone is characterized by high organic matter abundance,with total organic carbon(TOC)content of 1.58-3.56%,hydrocarbon generation potential(S1+S2)can reach up to 20.82mg/g,and hydrogen index(HI)can reach up to 602.0mg/g TOC.Tmax,CPI,OEP and Ts/(Ts+Tm)indicate that the organic matter in a low mature stage of Wenchang Formation.The results of major element analysis show that there are enriched in Fe2O3 and MnO elements,Co,Zn and Sr elements in trace elements are obviously deficient,while Tm and light rare earth elements are obviously enriched in rare earth elements.Following the collapse of the Yanshanian orogenic belt,mantle magma uplift,crustal stretching and thinning,and multiperiod strong magmatic hydrothermal activities occurred during the Eocene in South China.Based on the detailed explanation of the external morphology,internal reflection of magmatic rocks,and the deformation and contact relationship of sedimentary surrounding rocks,two types of magmatic diapir,concealed diapir and pierced diapir,were identified in the early-middle Eocene of eastern Yangjiang sag,and they were divided into Wen-3,Wen-2 and Wen-1 stages.Magmatic diapir is mainly developed in Enping 20/21 subsags in the eastern of study area,which probably related to its location in the Yangjing-Yitong’ansha Fault Zone,and the magmatic emplacement is easy to occur at the special structural position where the transition zone and the tension fracture zone overlap.Ratios of major elements((Fe+Mn)/Ti),rare earth elements(δEu*and δCe*),δ18O-δ13Ccarb isotope value intersection diagram and hydrothermal indicator discrimination diagrams(Zn-Ni-Co and(Cu+Ni+Co)×10-Fe-Mn triangular plot,etc.)jointly supported three episodes of hydrothermal activity during early-middle Eocene Wenchang Formation deposition,which indicates that the multi-stage magmatic diapir in the early-middle Eocene of eastern Yangjiang sag induced multiple hydrothermal activities in the same sedimentary period.The element ratios(Sr/Cu,Rb/Sr and SiO2/Al2O3)are used to reconstruct the paleoclimate evolution process during the deposition of Wenchang Formation in eastern Yangjiang sag.They jointly indicate a warm paleoclimate during the deposition of Wenchang Formation,and the weathering indexes(CIA and PIA)further suggest the strong weathering conditions during the deposition period,while in the hydrothermal sedimentary unit,a warm-cool semihumid paleoclimate and the weakening of land weathering are shown.Negative excursions of organic carbon isotopes(δ13Corg)in the study area,indicating the prosperity of microalgae and plankton in the lake,and further confirmed the warm paleoclimate during the deposition of Wenchang Formation.In addition,inorganic carbon and oxygen isotopes(δ13Ccarb and δ18O)also indicates a warm and humid paleoclimate background,but the multiple shifts of isotope curves suggest that there were some dry wet alternate climate intervals during the sedimentary period,which is consistent with the results of element geochemistry.Redox proxy indicators,including element ratios(V/(V+Ni),Ni/Co)and element enrichment factors(UEF,MoEF)indicate that a suboxic-anoxic water environment was dominated in the deposition of Wenchang Formation,while the weak positive deviation of 15N value also suggests the advantage of incomplete denitrification.In addition,the trifluorene compounds further characterized the anoxic water column conditions during the hydrothermal deposition,which probably related to the reducing gas released by the hydrothermal effusion.Paleosalinity proxy indicators,including 100*Mg/Al,Ca/Mg,GI,ETR and C35H/C34H jointly revealed the high input of fresh water during the deposition of Wenchang Formation,while the deep hydrothermal fluids input led to the salinity fluctuation of brackish water.The organic matter types are mainly in Ⅱ1 and Ⅱ2,and the study of organic macerals and biomarkers jointly suggests the prosperity of aquatic organisms(algae and bacteria)and terrestrial vascular plants,resulting in high bioproductivity during the deposition of Wenchang Formation.The moderate Zr/K and Ti/Al ratios of mudstone samples reflect the moderate terrigenous clastic input during the deposition of Wenchang Formation,which mostly comes from the weathering of Mesozoic granite in the uplift area around the depression.The research of tectonic evolution shows that the transformation and extension of Wenchang Formation during the sedimentary period brought high tectonic subsidence to eastern Yangjiang sag,and abundant rainfall in warm-humid paleoclimate led to extensive deep-water environment.At the same time,the warm-humid paleoclimate promotes the development of surface runoff and the increase of terrestrial nutrient input,which leads to the common prosperity of lake algae and terrestrial vascular plants,and improves the primary productivity of the lake.In addition,high lake level is easy to form anoxic environment of bottom water,which provides excellent preservation conditions for organic matter enrichment.The warm and humid paleoclimate is the main controlling factor for the enrichment of organic matter during the deposition of Wenchang Formation.The extensive magmatic diapir during the deposition of Wenchang Formation often promoted the rapid detachment and hanging wall rotation of the detachment fault system,and further promoted the development of the under compensated lake environment in the study area.Synsedimentary magmatic diapir may have induced the occurrence of multi-stage lake bottom hydrothermal activities,hydrothermal exhalation promotes the increase of nutrient elements(Cu,P,Ba,etc.)required for the growth of algae in the lake,and the algae bloom leads to high lake productivity.Many hydrothermal activities have also prompted abundant reducing gases(H2S,SO2 and CH4)to be released into the lake basin along the hydrothermal overflow channel,thus creating an anaerobic and salinity environment for the bottom water and effectively promoting the development of organic-rich mudstone.Based on the study of tectonic evolution,sedimentary evolution,magmatic and hydrothermal activity,paleoclimate and paleo-sedimentary environment evolution in the study area,the organic matter enrichment model of lacustrine mudstone of Wenchang Formation in the eastern Yangjiang sag was established by analyzing the interaction relationship of various factors(tectonic evolution,paleoclimate,paleoweathering,paleosalinity,redox conditions,organic matter source,bioproductivity,terrigenous clastic input,magmatic and hydrothermal activity).In the early-middle Eocene,the study area was in the background of regional tensile stress,under the joint influence of detachment faults and magmatic diapir,an under compensated deep lake sedimentary environment was developed.The warm-humid paleoclimate and intermittent hydrothermal activity promoted the prosperity of lake algae and surrounding terrestrial plants,providing sufficient sources for organic matter enrichment.The high lake level caused the anoxic environment of bottom water,and the reducing gas released by hydrothermal activity further promoted the preservation of organic matter.Therefore,the warm-humid paleoclimate interval and intermittent hydrothermal activity jointly controlled the formation of lacustrine organic-rich mudstone in the Eocene Wenchang Formation.