The Enrichment Characteristics Of Organic Matter In Sequence Stratigraphic Framework Of Oil Shale-bearing Series Of Upper Cretaceous In The Southeastern Part Of Songliao Basin

This paper focused on the southeastern part of Songliao basin where abundant oil shaleresources develop, and mainly aimed at revealing the favorable enrichment zones and theregularities of distribution of organic matter to provide theoretical basis for exploration of oil shaleresources of the study area. The oil shale-bearing series of the study area was the sedimentaryproduct of the depression stage of Songliao basin in Late Cretaceous Period, and showed littlevertical change of lithology characteristics, corresponding to the two periods of the greatest lakeflooding. Therefore, the method of applying lithology to studying the sequence stratigraphiccharacteristics has already not met the needs of the research into the sequence of the study area.This paper made a synthesis of the core, well-logging and geochemical data, and did the researchon the sequence stratigraphic characteristics of the study area, which had important significance toperfect scientific research on the Upper Cretaceous strata sequence of Songliao basin.On the basis of observing the important cores in great detail and well-logging analysis, plusanalyzing the Vertical change trend of the TOC and pyrolysis data qualitatively, the QingshankouFormation to the1stand2edmembers of the Nenjiang Formation could be devided into7third-order sequences. The Qingshankou Formation was composed of third-order sequence I, IIand III; The Yaojia Formation was composed of third-order sequence IV and the lower V whichcorresponded to the ascending half cycle; the1stand2edmembers of the Nenjiang Formation weremainly composed of third-order sequence VI、VII and the upper V which corresponded to thedescending half cycle.The Qingshankou Formation to the1st and2ed members of the Nenjiang Formation of the study area mainly developed lacustrine facies through the core observation, which could be furtherdevided into5microfacies, there were offshore lake mudflat, offshore lake sand beach, sandyshallow lake, argillaceous shallow lake and semi-deep lake to deep lake. Other than that, there wasa certain scale event deposit, such as dolostone, Ostracoda limestone and seismites.Through the comprehensive analysis of TOC and pyrolysis data of NGN1, NGN2andZK0833, the conclusion could be drawed that the sequence I, V and VI showed the most organicmatter abundance and the best organic matter type which mainly consist of Ⅰ-type andⅡ-typekerogen; the sequence II took second place and developed Ⅱ-type kerogen; the sequence III andVII showed the least organic matter abundance and the worst organic matter type which mainlyconsist of Ⅱ-type and III-type kerogen. And generally, HST manifested the most organic matterabundance and the best organic matter type within the sequence of the study area; TST and RSTtook second place; LST took the last.By the research on the distribution characteristics of oil shale in the study area, the oil shalemainly developed in the sequence I, V and VI, and corresponding to HST, TST and RST ingeneral. Furthermore, the oil shale developing in HST showed higher quality, more layers andlarger thickness and distribution, and corresponded to semi-deep lake to deep lake microfacies;however, the oil shale developing in TST and RST showed lower quality, fewer layers and smallerthickness and distribution, and corresponded to semi-deep lake to deep lake and argillaceousshallow lake microfacies.The enrichment conditions of organic matter mainly included warm and humid paleoclimate,semi-deep to deep lake depositional environment corresponding to high base level and largeaccommodation, anoxic and reductive water condition, through the comprehensive analysis of thesedimentary evolvement characteristic of the study area and the vertical distribution characteristicsof TOC, trace element, rare-earth element of the Qingshankou Formation and the1stand2edmembers of the Nenjiang Formation of NGN1. The fresh water environment and brackish to saltwater environment both could form organic matter enrichment. Furthermore, the type of theorganic matter developed in brackish to salt water environment was better than the type of theorganic matter in fresh water environment in general. Therefore, two kinds of oil shale formingconditions which were distinguished by their water quality could be summarized. The first kindcorresponded to brackish to salt water environment and was represented by the oil shale at the bottom of the1stmember of the Qingshankou Formation and the1stmember of the NenjiangFormation. The second kind corresponded to the fresh water environment and was represented bythe oil shale at the bottom of the2edmember of the Nenjiang Formation.