Environmental Dynamics Of Organic Accumulation In The Oil Shale Bearing Layers In The Upper Cretaceous, Southeast Songliao Basin (NE China)

Thick dark organic rich mudstones (OM) deposited in the Qingshankou Formation and NenjiangFormation, Upper Cretaceous, and especially in southeast of this basin, higher quality thick oil shale aredeveloped in the lower part of each formation. The oil-shale bearing layers are characterized by the highorganic matter content and with low maturity, which indicating lots of palaeo-environmental information iscontained in OM, and it should be the best layers to study the environmental dynamics of organicaccumulation.To reveal the sedimentary environment of study area is firstly before to research the environmentaldynamics. On the basis of core and field profile observation, well log analysis and systematic geochemicaltesting, lacustrine is the mainly sedimentary facies in this eare, and in can be divided into lakeshore shallowlake and semi deep-deep lake further. In addition, there are4types of event deposits such as storm deposits,volcanic ash deposits, rapidly ostracoda unloading deposits and seismites, and the front delta is alsodiscovered in this area. The sedimentary-organic-geochemical facies of different sedimentary evlution stagesof the basin is established combined with geochemical data. The main facies type of oil shale is semideep-deep subfacies-A-B(aquatic organisms)-brackish-salt water with reducing environment(K2qs1) and semideep-deep subfacies-A-B-brackish water-fresh water with reducing environment(K2n1-2).The established isochronous sequence frame is the basis of studying horizontal and vertical sedimentaryenvironmental evolution. During the depositional period of Qingshankou Formation and Nenjiang Formation,semi deep-deep lake deposits are mainly developed in the study area, and the deep water sedimentary systemsare most difficult to interpret by using principles of sequence stratigraphy. The organic matter in mudstonescontains a wealth of information of lake level changes. Based on the changing of TOC content, the highresolution sequence stratigraphic framework is established from boundary to deep water area of the basin.According to the oil shale distribution characteristics in the sequence stratigraphic framework, in the end ofTST mainly develops poor quality thin oil shale with gradually increased distribution area, and in the HST, thearea reaches maximum and develops high quality thick oil shale, and the thin poor quality oil shaledistribution area reduced gradually in the RST.Organic matter sources analysis is the premise to study the environmental dynamics. Based on the polishsection observation, and combing the alative amount of each n-alkanes, specific carbon isotope value andsterane/hopane as well as organic element ratios, the organic matter of Qingshankou and Nenjing Formation is mainly from aquatic organisms. The constituent of aquatic organisms is mainly lamalginite with a smallnumber of telalginite, methanobacteria, cyanobacterium and chloracea etc. Meanwhile, there is a smallamount of terrigenous organic matter of sporophyte, chloroplast and phosphor in the mudstone.Systematically use the indices of HI, S2, Tmax and OI, as well as biomarker compounds parameters, itturned out that the oil shale developed layers(Sequence I,V,VI) have high abundance of organic matter and thekerogen type is I. With the lake level regression, the type of organic matter in Sequence II,III,VII,VIIIgradually change from type I to type III, and in each sequence from LST to RST, the organic matterabundance and type turn to better first and then to worse. Through horizonal contrast, in the deep water area,the organic matter type and source are stable, and changed greatly in the shallow water. Polish sectionobservation and biomarker parameter characteristics reflect that the lower HI ratio organic matter type mainlyresults from the poorer preservation conditions but not the terrigenous organic matter, because of thedestroyed organic hydrogen by oxidation.To carry out high-resolution inversion environmental dynamics organic accumulation, accurate timescale need to match to sedimentary strata, strata cycle analysis is an efficient measure to determine thesedimentary age. Organic matter abundance in mudstones could directly reflect paleoclimatic evolution, so it’sthe most efficient data to study cyclostratigraphy. Based on the TOC cycle changes，the marjor TOC cylce ismainly controlled by long eccentricity, and its corresponding chronological interval is about405Ka， and theminor TOC cycle is mainly controlled by short eccentricity, and its corresponding chronological interval isabout100Ka. Based on the age of each chronological interval，using “volume fraction of rock matrix”principle to calculate the deposition rate (V) of different sedimentary facies and basin evolution stages. Indeep water area, the deposition rate makes no influence on organic matter accumulation when V＜7cm/Ky,when V>7cm/Ky, with the increasing rate of terrigenous clastic input, the organic matter is destroyed, thusaffecting the organic matter abundance. In shallow water area, the organic matter abundance is tapering offwith the increasing deposition rate, and when the V is range from5to10.6cm/Ky, the organic matter contentis high.Based on the polish section observation, the terrigenous organic matter content is about8.0%in semideep-deep lake, but in shallow water area, the algae content is logarithmic to TOC, thus the terrigenousorganic matter content could be excluded based on that relationship. Using the “HI” method to restore theorganic matter which destroyed by the oxidation. The palaeo-lacustrine productivity was calculated by oganiccarbon method, and the oil shale formed in deep lake when the palaeo-lacustrine productivity of is greaterthan2210C/m~2a, and in the semi deep lake it is greater than1977C/m~2a and it is greater than1773C/m~2a inthe shallow lake. According to the evolution characteristics of productivity in the sequence framework, the oilshale formed in super eutrophic water. The water body in semi deep-deep lake turns from super eutrophic tomedium eutrophic from bottom to top of Qingshankou Formation. Perhaps because of the mudstone nutrients releasing or terrigenous nutrients input, the shallow water body has higher degree of nutrition in the top ofQingshankou Formation, but the organic matter abundance is relatively low due to the poorer preservationconditions.Based on organic matter sources, palaeo-lacustrine productivity, deposition rate study, Combing withcore and geochemistry data to study the evolution and correlation of paleoclimate, paleogeography and waterconditions. Through analyzing the interaction of each organic matter enrichment conditions, the models oforganic matter accumulation environmental dynamics were established. There are mainly2models: salt watermodel (model I) and brackish-fresh water model (model II).Model I responds to the oil shale bearing layers of K2qs1, it’s formed in warm and humidpaleoclimate(the paleotemperature is19-27℃), superhigh productivity developed in the salt water and deepeuphotic zone, providing a lot of organic matter. With the uplift of lake level, the basin environment is unfilledto fill. In semi deep-deep lake area, the salt water environment becomes salinity stratification and leads toanaerobic environment, and it’s the best preservation condition. The lower deposition rate has prevented thedilute and damage of organic matter, therefore, the medium-thick high quality oil shale accumulates here.Model II responds to the typical oil shale bearing layers of K2n1-2, it’s formed in warm and humidpaleoclimate(the paleotemperature is18.7℃), the terrigenous nutrients supplement promotes excellent lakeproductivity and producing a large number of organic matter. With the large-scale uplift of lake level, thesedimentary environment becomes unfilled to fill. The brackish-fresh water body increase the depth of oxygenzone, and the oxidation destroyed part of the organic matter. However, the rapid deposition promoted the rapidsubsidence, burial and preservation of organic matter, and forms the medium quality thick oil shale. The waterbody of K2qs2-3and Kn3is mainly brackish-fresh, but it has lower organic matter abundance because of thedecreased lake eutrophication and worse preservation conditions. The paleoclimate of Yaojia Formation is hotand water body is brackish and there is almost no organic matter. It is because the water is not so deep that thestrong oxidation nearly damaged all the organic matter.