Palaeogeographic Characteristics And Hydrocarbon Source Rock Evaluation Of The Late Paleozoic In Western Greater Higgnan Mountains

The western region of Greater Higgnan Mountains is an important resource area which is located in the west of Daxing'anling orogenic belt.The oil and gas enriched areas were discovered in the Bell-Wulson Sag of Hailaer Basin and the Damoguaihe,Nantun and Tongbomiao Formations were source rock formations.A large number of oil and gas resources were discovered in the Erlian Basin which is also located in western Greater Higgnan Moutains.The source rock formations were concentrated in the Tengger and Aershan Formations.Most of the main oil and gas reservoirs and source rock formations in this study area are Mesozoic strata.The evaluation of the resource potential of the Mesozoic strata is also systematic.However,the oil and gas resource potential of the Paleozoic strata is often neglected.The Paleozoic stratas have the characteristics of wide distribution,large thickness,deeply burial,less drilling and strong weaterhing at the surface.The source rock evaluation work on the basis of these difficulties is a problem that geologists need to solve.Based on the geological characteristics of the outcrop rock,sampling analysis of Late Paleozoic outcrop samples,geophysical exploration date collection and organic geochemistry experiments,this paper combines geology,geophysics,organic geochemistry and lithofacies palaeogeography to evaluate the source rock of Late Paleozoic Carboniferous-Permian in the western part of Great Higgan Moutains.The lithofacies paleogeographic characteristics were determined and the original organic matter abundance was restored in this paper.The favorable areas for the development of source rocks were also predicted.The geophysical analysis results of heavy,magnetic and electric fields in the study area indicate that the late Paleozoic potential sedimentary rock strata in the Erlian Basin and adjacent areas have the macroscopic geophysical characteristics of low resistance,high density and weak magnetic.Based on electrical prospecting data,the sedimentary rock formations are predicted to have a depth range of3-5 km.The residual gravity anomalies in this range are extracted and the potential distribution of sedimentary rocks is predicted.The potential sedimentary rocks are in the east of Erenhot and south of East Ujimqin Banner,and the thickness range of the Deposition center is about 1000m-2000m.Based on the characteristics of exposed rocks of Late Paleozoic stratas and previous studies of the entire lithofacies and palaeogeographic features in the study area,the analysis of entire lithofacies and palaeogeographic were carried out.We also identified the sedimentary environment of the Late Paleozoic sedimentary rock in the study area.The favorable original depositional environment of the marine mudstone of the Linxi Formation is the shallow marine shelf facies in a clastic rock system.The favorable original sedimentary environment of the rocks in the Zhesi Formation,the Amushan Formation and the Benbatu Formation is the platform facies in a carbonate system,and the distribution of source rock thickness planes in each group is also predicted based on the sedimentary rocks.The assessment results of organic matter abundance indicated that the LinxiFormation mudstone has relatively high residual organic carbon content,which can meet the criteria of the middle marine mudstone source rock,but the residual hydrocarbongeneration potential is low.The residual organic carbon content of limestone in the Zhesi,Amushan,and Benbatu Formations was relatively low.Weathering correction and original organic matter abundance were applied to the residual TOC and hydrocarbon generation potential in this study,and the original hydrocarbon generation potential after restoration was high.The results showed that the late Paleozoic strata Hydrocarbon generation andhydrocarbon expulsion are obvious and the generated hydrocarbons are discharged in large quantities.The restored TOC of mudstone in the Linxi Formation can meet the above-average standard,and the original organic carbon content of the limestone sample can also meet the criteria for gas source rock.The evaluation results of organic matter types indicated that the type of Linxi Formation mudstone is type II₁ with high sapropel group content.The limestone of Zhesi Formation is type III with low sapropel content,and the relative contents of the vitrinite and inertinite components are relatively high.The types of organic matter in the Amushan Formation and Benbatu Formation were Type II₂ and Type III respectively,and the high-content components were Saprolite and Inertinite.The vitrinite reflectance of mudstones in the Linxi Formation is generally greater than 1.2%,which belongs to the stage of high maturation-over maturation.The OEP values of the limestones also indicated a high maturity.The marine mudstones and carbonates in the study area have different characteristics of hydrocarbon generation.The kerogen of mudstone has a high degree of metamorphism,and there is basically no potential for hydrocarbon generation.The hydrocarbon-generating materials are mainly from the carbonization of organic matter and partially dispersed methane gas.Affected by its complex diagenesis,the organic matter inside the carbonate rocks can generate hydrocarbons in various forms.Therefore,not only kerogen can generate the hydrocarbons,dispersed organic matter outside the kerogen also generated hydrocarbons.The hydrocarbon generation model of carbonate rocks has some differences with mudstone in the end of evolution.The kerogen microstructure and gas chromatographic results of saturated hydrocarbons show that the source matter of late Paleozoic in the western part of Greater Higgan Moutains is better.In the mudstone,higher plants formed a large amount of kerogen components with high degree of thermal evolution.Bacteria,algae and other aquatic organisms in limestone and terrestrial higher plants provided large numbers organic matter in limestone with highly contaminated by land resources.The sedimentary environment of source rocks is mainly in the weak reduction-reduction environment,which is beneficial for the generation of hydrocarbons without large amounts of organic oxidation.The favorable areas for the development of the source rocks in the western Greater Higgan Moutains were predicted.The favorable area of the source rock in the Linxi Formation is located in Xilinhot-Xiwuzhumuqin Banner-Linxi County areas.The favorable areas of Zhesi Formation were in the south of Erenhot and the Sunite Zuoqi-Xilinhot-Xiwuzhumuqin Banner.The Amushan Formation is in the west of the East Ujumqin Banner and the Dora Town-Sunite Right Banner-Linxi County borders the southern region.