Evolution Of The Eocene Sedimentary Environment And Formation Mechanism Of Hydrocarbon Source Rocks In The Weixinan Sag

The lacustrine source rocks have developed in the offshore South China Sea during the Eocene epoch.However,despite many different models for the formation of source rocks,there is little research regarding the sedimentary provenance of the basin,how the sedimentary environment evolved,and how source rocks formed in response to paleoclimatic changes in the basin.This study focuses on the Weixinan Sag and Liushagang Formation in the Beibuwan Basin.The Beibuwan Basin's provenance,as well as the sequence stratigraphy and evolution of sedimentary environments in the Weixinan Sag,has been studied based on seismic profiles,well logs,and core drilling data,as well as experiment analysis.Moreover,we focus on the astrophysical cycles theory and above research to determine how source rocks are formed.Cyclostratigraphy analysis and absolute age make up an astronomical time scale(ATS)for the Eocene.Lacustrine source rocks in the Weixinan Sag were shaped by an astronomically-driven precession climate.Firstly,the provenance of the Beibuwan Basin is investigated through the detrital zircon U-Pb geochronology analysis.Our results show that the source of the Middle-Upper Eocene deposits may have originated from mountain erosion of the Yunkai massif,Lower Xi River,Paleo Red River,and Nanliu River.Lower Oligocene deposits that originated from the Qinfang Basin may be the result of local uplift and denudation.It was believed that provenance shifted from the Beibuwan Basin from the Eocene to the Oligocene responses to tect-palaeogeomorphological evolution.Secondly,a high-resolution 4^th order sequence framework was established based on the drilling data,core photo,and seismic profiles of the Weixinan Sag,and eleven 4th order sequences were divided.The sedimentary system under the 4^th order sequence unit and the evolution of sedimentary environment are discussed.The lower sequence of the L3 is in the initial stage,with a relatively shallow water and enough provenance supply.This stage is characterized by shallow lake-alluvial plain-fan delta-braided river delta sedimentary system.The upper sequence of the L3 is expanded,with a deeper lake water.This stage is characterized by middle deep lake-shallow lake-fan delta–brained delta sedimentary system.The second member can be divided into four 4^th order sequences.In the psl24,the lake basin expanded rapidly the great range of the lake.During the psl23and psl22,the lake basin was shrinking relatively,but still dominated by semi-deep lacustrine deposits.During the psl21,the lake basin expanded again,and the range of lake further extended to the gentle slope area in the south of the Sag.The L1 member can be divided into five 4^th order sequences,the lower sequence include the early and late stage under lowstand systems tract,with a semi-deep to shore-shallow lake deposits.The middle sequence is in the stage of transgressive systems tract,with a relatively enlarged lake basin and the meandering river delta is developed.The upper sequence psl12 and psl11 is in the early and late stage of highstand systems tract.Shallow lake,fan delta and meandering river delta depositional systems were developed in the early stage.In the late stage,the lake basin shrank sharply,and the lake center moved to the southeast.The large fan delta was developed merely in the northwest provenance.Finally,cyclostratigraphy analysis and primary controlling mechanism of the source rocks during Eocene in the Weixinan Sag was conducted under the background of provenance and sedimentary environment.The?28 m long eccentricity cycle,?7 m short eccentricity cycle,and?84 m and?168 m super long eccentricity cycle are identified.The tuned time domain series shows?405 kyr and?100 kyr long and short eccentricity cycles and?41 kyr and?22 kyr obliquity and precession cycles,and the super long cycles of?1.2 Myr and?2.4 Myr.The top boundary of the L1 is about 35.2 Ma or 33.9 Ma of GSSP is used as the boundary of Eocene and Early Oligocene,the bottom age of the L1of?39.4 Ma is used as auxiliary calibration,and then the astronomical time scale is established.Based on XRF elements scaling analysis and sedimentary noise modeling,our results suggest that the nutrients in the bottom water are transported to the lake by monsoon rainfall in a strong reducing environment,which causes algae bloom in the surface water,and a large amount of organic matter is generated after degradation and then deposited into the bottom water for preservation.The content of organic matter is high at the top and bottom of the L2,with a stage of maximum eccentricity and wide range of the lake.Weathering rate is high and nutrient supply is enough at this stage.However,the eccentricity of the other layers of the second member was lower than that of the top and bottom of the second member.The lack of nutrients brought by strong weathering rate to stimulate the bloom of algae,and the weak stratification of the lake led to less preservation of organic matter.