Organic Matter Enrichment And Paleoclimate Records Of Lacustrine Shale In The Hetaoyuan Formation,Biyang Sag,Nanxiang Basin,East China

Nanxiang basin is a typical Cenozoic continental rifting basin in eastern China,which is rich in petroleum and alkali resources.Taking a shale oil well in the central deep depression of Biyang Sag as an example,the macro core and micro thin section observations,field emission scanning electron microscope observation,cathodoluminescence observation,element geochemistry,XRD mineralogy analysis,TOC content analysis,rock pyrolysis analysis,pyrite framboid morphology,pyrite sulfur isotope,and molybdenum?Mo?isotope of shales,carbon and oxygen isotopic analysis of carbonate in shale are used to reconstruct the paleo sedimentary environment and regional paleoclimate in the third member of the Hetaoyuan Formation in Biyang Sag.The lake evolution driven by the middle Eocene climate is discussed and the mechanism of organic matter enrichment under different lake hydrological conditions is analyzed.For the core of the third member of Hetaoyuan Formation in well C2,we have identified six lithofacies types: laminated calcareous mudstone,laminated dolomitic mudstone,massive siliceous clay mixed mudstone,massive siliceous mudstone,massive dolomitic mudstone,and siltstone.The ratio of Ni/Co and U/Th of redox sensitive metals can reflect the relative change trend of redox in sedimentary water,but the specific redox state of lacustrine sedimentary water can not be accurately identified based on the criteria of redox conditions of marine mudstone.Siltstones and adjacent massive siliceous mudstones are obviously formed in oxygen-bearing water body;the morphology and size statistics of pyrite framboid show that carbonate-rich laminar mudstone?laminar calcareous mudstone and dolomitic mudstone?are mainly formed in the bottom water which consists of anoxic,occasionally sulfurized,while the massive siliceous clay mixed mudstone is formed in dysoxic-oxic water body.The sulfur isotope fractionation degree of carbonate-rich laminated mudstone is relatively small and the larger ?⁹⁸Mo value indicates that the oxygen content of lake sedimentary water is lower.This is consistent with the indication of pyrite morphology and size.Based on the lithofacies assemblage,combined with paleoclimate index ln?Al₂O₃/Na₂O?and paleo-salinity index?B/Ga ratio?,he lake sedimentary stages recorded by the core can be divided into six stages,and the change of lithofacies is due to the changes of mineralogy and sedimentary structure of mudstone caused by the change of lake hydrological conditions in different stages.The first member is dominated by laminated calcareous mudstones.Strong chemical weathering in the lake basin indicates a relatively warm and humid climate.At that time,the lake was a large deep lake,which was composed of oxygen bearing low salinity fresh water and anoxic high salinity bottom water.Ln?Al₂O₃/Na₂O?shows that the climate of the lake basin changes from humid to arid.Laminated dolomitic mudstone and laminated calcareous mudstone are formed under similar sedimentary conditions.The difference is that limited rainfall in relative arid climate increases lake water evaporation,and elevated Mg/Ca ratio promotes primary dolomite precipitation.The massive siliceous clay mixed mudstone was formed in the lake basin when the climate was relatively dry and the water level was lower.The chemical stratification caused by salinity was weakened and disappeared due to less fresh water input,and the oxygen level of water body increased.The third member is mainly composed of siltstone and massive siliceous mudstone,with local massive dolomitic mudstone;the lake is in a more arid climate,and the weak chemical weathering intensity leads to the retention of a large number of fresh provenance feldspar,and the lake water level is lower,and the delta go toward the center of the lake.The presence of feldspathic siltstone and wavy cross bedding between siltstone and mudstone indicate delta front facies deposition.The fourth member is mainly composed of laminated dolomitic mudstone,with laminated calcareous mudstone and massive siliceous clay mixed mudstone locally,reflecting the rebound of climate drought.The sedimentary water body is a deep lake.The fifth member is similar to the third member,which is mainly composed of feldspathic siltstone.The drought is aggravated again,and the lake water level drops again,which is pushed forward by the Delta in the second act.The VI member is mainly composed of laminated dolomitic mudstone.In this stage,as in the fourth stage,the climate humidity increased,the lake water level rose,and the layered saline lake was formed again.The relative contents of dolomite and calcite,Mg/Ca ratio,anhydrite relative content and stable carbon and oxygen isotope of shale indicate that the climate in the lake basin has changed from relatively warm and humid to relatively cold and dry,and the evaporation of lake water gradually becomes stronger?with some fluctuations in some parts?,which is basically consistent with the global Eocene cooling trend.In the relatively high water stage,the accumulation of organic matter in carbonate rich laminar argillaceous facies is mainly controlled by paleoproductivity and redox conditions,and is independent of sedimentation rate.That is to say,in the period of relatively low water level,the enrichment of organic matter in massive siliceous clay mixed mudstone is mainly controlled by redox conditions and sedimentation rate.Nanxiang basin is similar to other Meso Cenozoic continental rift basins in eastern China,that is,organic rich shale often coexists with evaporite.Therefore,a controversial issue has been discussed,that is,whether the Paleogene in eastern China was transgressive and provided salt source for evaporite.In the past,many scholars mainly used paleontological fossils?foraminifera,coccolith,dinoflagellates,red algae,brown algae?and finger minerals?glauconite,apatite,dolomite?to demonstrate the possibility of transgression.these fossil and mineral evidences cannot exclude the possibility of lake water deposition.We collected organic mud shale and co-existing evaporates from Dongying depression and Dongpu Depression in Bohai Bay Basin,Biyang Depression and Wuyang Depression in Nanxiang Basin,Qianjiang Depression in Jianhan basin,gaoyou Depression and Hongze Depression in Subei Basin,and used elemental geochemistry and isotopic composition to identify salt sources.The content of B and Br in all the fault-depression salt samples in the east is lower than that of Marine facies salt.The Sr isotope ratios of these samples were significantly higher than those of the paleogene seawater.The sulfur isotope ratios of stone salt,gypsum and anhydrous mirabilite were also different from that of seawater.The Mo isotope values of the maximum Mo isotope values of the dark organic sediments co-existing with the evaporite rocks were lower than those of the seawater in the early Eocene,indicating that the saline water did not originate from the transgression related to the PETM event.The characteristics of rare earth elements in the eastern fault basin are obviously different from those in seawater.These geochemical signals indicate that the salt source of evaporite rocks may be less correlated with transgression.The paleogene of many fault depression in the east is sometimes drilled into the sedimentary tuff and volcanic rock,and some rocks contain a certain amount of zeolite,and the evaporite is generally abnormal in Eu.These characteristics may indicate that the deep jet from the late chalk-Cenozoic extensional tectonic background in the east supplied the salt source.In addition,considering that the Eocene was a warm and humid climate,mesoproterozoic and Lower Paleozoic mixed gneisses,marble and granulites,as well as volcanic rocks at the edge of the basin were prone to weathering and flowing into the lake as solutons,which was also a possible source of salt.