Geochemical Characteristics Of Oil Shale Inyouganwo Formation And Its Aggregation Factors Of Organic Matter,Maoming Basin

Maoming basin is a typical fault basin with abundant oil shale-bearing in southern China,and high quality thick oil shale deposited in Youganwo Formation,Palaeogeneone.Due to its high organic matter abundance and low maturity,and it is considered as the best target layer to study the organic accumulation mechanism of oil shale.Combined with drilling core and outcrop data,the delta and lacustrine facies are identified during Youganwo in the study area.and can be further identified with prodelta,limnology,shallow lake,semi-deep and deep lake four sedimentary subfacies.The research area is mainly developed in semi-deep lake-deep lake and shallow lake sub-facies,the former is the main site of oil shale deposition.Through a series of tests with rock mineralogy,industrial quality,organic geochemistry and elemental geochemistry of oil shale in study area,Dianbai oil shale-bearing area characterized by medium-high oil yield(4.36-8.4%),high clay mineral(75.6-83.0%)and low silica content(17.0-24.4%),high ash content(66.69-82.33%),low-medium calorific value(4.27-7.42MJ/kg),low sulfur(0.08-0.26%),high TOC(8.24-15.35%)and high S1+S2(49.62-107.69mg/g);Besides,high oil yield(4.52-13.07%),containing less clay minerals(53.6-78.2%),containing more quartz(21.8-39.9%),high ash content(58.96-73.11%),high calorific value(6.72-11.90MJ/kg),low sulphur(0.44-2.73%),high TOC(6.75-21.40%),high S1+S2(40.69-174.88 mg/g)presented in Maoming oil shale-bearing area.According to the analysis of HI-Tmax,S2-TOC,and organic microcomponent,shown that both Dianbai and Maoming oil shale have developed type ?-?1 kerogen,which is in the immature stage of evolution.Contains trace elements test showed that Sr and Ba elements presented in loss characteristics,the rest of the trace elements indicated weak enrichment and enrichment.Rich in LREE,loss in Eu and Tm were presented in both oil shale-bearing area.On the basis of polish section,biomarker compounds,and element geochemical analysis,high lacustrine productivity presented in sedimentary period of Youganwo Formation in Maoming basin.Organic matter is mainly from aquatic organisms(algal and bacteria),The constitent of aquatic organisms is mainly layered or enrichment lamalginite with relatively less telalginite,meanwhile,there is a small amount of terrigenous organic matte input of sporophyte,cutinite and vitrinite.But Maoming oil shale-bearing area contains more telalginite,telalginite/lamalginite ratio is more than Dianbai,and the palaeo-lacustrine productivity are also better than Dianbai.Known by the method of element ratios and biomarker compound,oil shale depositional period characterized by low Sr/Ba,high TOC/TS,extremely low content of pregnane and L pregnane sterane in the study area,indicated the Youganwo Formation with environment of fresh water in Maoming basin;Medium V/(V+Ni),high V/Cr,negative Eu abnormity,low Pr/Ph and rearrangement sterane/sterane are indicates that paleo-water redox condition are anoxic in Maoming basin;Otherwise,Combined with the indicate significance of clay mineral,Rb/Sr,Sr/Cu and palynology,responds to the oil shale of Youganwo Formation,it's formed in background of warm and humid paleoclimate.Based on the interaction of organic matter accumulation conditions,the model of organic matter accumulation mechanism was established.With the warm humid climate,the palaeo lacustrine accommodation space can be increase rapidly,because of terrestrial nutrient input,high lacustrine productivity producing a large number of organic matter to the deep water area,meanwhile,the fresh water environment may becomes temperature stratification and leads to anaerobic environment,provide an excellent preservation condition for organic matter,and ultimately developing high quality thick oil shale deposited in the deep lake area(Maximum oil yield:13.07%;Maximum total organic carbon:21.40%).Contrast the differences of organic matter accumulation conditions oil shale with Fushun,Maoming,and huadian basin,found that maoming and fushun oil shale are both deposited in high deposition rate,warm humid climate,paleo-water redox condition of anoxic and freshwater,organic matter is mainly from aquatic organisms(algae and bacteria),temperature stratification could played an important controlling role of save organic matter,leads to high quality thick oil shale deposition.In contrast,the low rate and dry and wet weather conditions in huadian basin resulted in the formation of multi-layer laminated oil shale.And the brackish water caused salinity stratification and leads to anaerobic environment,provide an excellent preservation condition for organic matter,formed a higher quality oil shale than Maoming and Fushun basin(maximum oil rate:24.80%;Maximum total organic carbon:33.3,%).Generaly,the main control factors that caused the difference of the organic matter aggregation mechanism of the oil shale in Maoming,Fushun and Huadian basin were deposition rate,palaeoclimatic conditions and paleo-water redox condition.