Sedimentological Characteristics And Paleoenvironmental Implications Of The Early And Middle Jurassic Coals In The Muli Basin

The Muli Basin is an important production area for coal resources in the Qinghai Province.The potential coal resources coalbed methane resources in the Muli Basin is abundant.The understanding of depositional systems,sequence stratigraphic framework and paleogeographic evolution might promote the prediction and assessment of hydrocarbon potentials in this Basin.This work is expected to improve the prediction and exploration of the economically important coal as well as the coalbed methane resources in the research area.The coal-bearing strata of the Early-Middle Jurassic is subdivided into the Lower Jurassic Reshui Formation,the Middle Jurassic Muli and overlying Jiangcang Formations.Based on data from boreholes,well logs,outcrops,rock thin sections and coal macerals,we have investigated the sedimentology,sequence stratigraphy and lithofacies paleogeography of this basin.The distribution and the accumulation pattern of coal seams have been analyzed.The paleoenvironment during the development stage of the peatlands were also discussed.Lithofacies and typical lithofacies associations in the research area were described in cores and outcrops.Five rock types including conglomerates,sandstones,siltstones,argillaceous rocks and organic rocks were identified in the study area,and the above lithofacies types were divided into 20 lithofacies types.Based on the observation of outcrops and sedimentary facies analysis of borehole cores,three sedimentary systems including braided fluvial,braided fluvial delta and lacustrine are identified.The main coal-forming environments include flooded swamp,interdistributary bay swamp and lacustrine-bay swamp.Based on outcrops,cores and well logging data,the key sequence stratigraphic faces of the Early-Middle Jurassic in Muli Basin are identified,including regional unconformity,paleo exposed surface,erosion surface,abrupt color and lithological changes,paleontological markers and logging facies markers.The sequence stratigraphic framework of the whole basin is established,which could be divided into five third order sequences from S1 to S5.Combined with the sedimentological characteristics of typical boreholes and sections,quantitative lithofacies paleogeographic maps were reconstructed based on the isopach maps of strata thickness,sandstone thickness,mudstone thickness,sandstone/mudstone ratio and so on.The subsidence velocity of the basin was uneven from the east to the west during the sequence S1.Most area of the basin was uplift.The sedimentary areas were mainly distributed in the Haider-Reshui-Haider mining areas in the east,as well as some local areas of Juhugeng,Hushan,Leinike and Jiangcang in the west.Braided fluvial environment was mainly distributed in the Reishui and Wailihada mining areas.Delta plains dominated most area of the basin during sequence S1.Shore and shallow lacustrine was in the margin of the basin.Sequence S2 corresponds to the lower Muli Formation in the early Middle Jurassic.A long-term stable subsidence period started since the Middle Jurassic.The sedimentary area during sequence S2 is larger than it was during sequence S1,and the denuded area is gradually covered by sediments.From the north to the south,the paleotopography shows a trendancy from high to low.The total thickness of sequence S2 is thicker in the southwest,with several depositional centers located.The braided fluvial-delta-lacustrine sedimentary system expanded during the sequence S2,and the water was relatively shallow during this period.The basement of the basin continued to subside steadily during the period of sequence S3,and the sedimentary centers migrated northward.The terrain of the basin became more flat.The depositional area of the lower delta plain and lacustrine became enlarged.Another small-scale transgression happend in the Muli Basin during the sequence S4,and the sedimentary system was also braided fluvial-delta-lacustrine.The shallow lacustrine area enlarged and migrated northward,and the depositional area of the braided fluvial and floodplains was limited.Sequence S5 corresponds to the late Middle Jurassic.Large area of oil shales and mudstones was developed in the south.The paleogeography of the basin was mainly covered by shore-shallow lacustrine,delta front and lower delta plain,and the whole basin was covered by water.The coal-accumulating center during the sequence S1 is the Reshui mining areas.The coal seams were mainly developed in the central and southern areas of the basin during the sequence S2 and S3.The coal-accumulating center includes Duosuogongma,Jiangcang,Leinike and Xueholi.Coal seams of equence S4 is much thinner with more layers.The coal accumulation was coming to an end during the sequence S5.From sequence S1 to sequence S3,the coal accumulating area enlarged,with more coal layers and thicker total thickness.While the coal forming area narrowed during the sequence S4 and sequence S5.The coal-accumulation is summarized and analyzed in different types of the lake basin.The Muli Basin experienced a transition from overfilled lake basin to balance-fill lake basin from sequence S1 to S5.There were two main coal-accumulating stages when the basin was an overfilled lake basin.Only thin coal seams were developed during the balance-fill lake period.The change of base level under sequence stratigraphic framework was subdivided into six stages.The sedimentary evolution and the coal accumulation of each stage is different.The early and late stages of transgressive system tract are the main coal-forming stages,and the early stage of lacustrine transgressive system tract is the most favorable one.Coals,as important sediments during the geological history,is very sensitive to the changes of paleoenvironment.Paleoenvironment information was recorded in coals during the development and preservation of peatlands.Geological events will also affect the development of peatlands as well as coals.Based on the study of coal petrology,the maceral characteristics of the Middle Jurassic coals in the Muli basin has been summarized.The coal macerals in the Juhugeng and Jiangcang mining areas are mainly vitrinite and inertinite,and the inertinite content is at a high level.The average inertinite content of the Xia 1 coal and the Xia 2 coal in Juhugeng mining area is 26.1%and 52.5%respectively,while that of the Jurassic coal seam in Jiangcang mining area is 0.7%~30%,averaging 15.6%.The inertinite is exclusively considered the by-product of wildfires.The relationship model between inertinite content and atmospheric oxygen content has been established by predecessors.Based on this theory,the atmosphere oxygen content has been estimated.It is estimated that the oxygen content in the atmosphere of the Middle Jurassic in the Muli Basin ranged from 19.3%to 28%,and the oxygen content of the late Middle Jurassic was lower than that of the early Jurassic.The natural gamma-ray log,density log and gamma-ray log of Xia 1 and Xia 2 coals in Juhugeng mining area were analyzed.The ratio of low-frequency,medium-frequency and high-frequency corresponding periods was 4.8:1.6:1.0,which was similar to the ratio of eccentricity,obliquity and precession of Middle Jurassic.It was considered that the ash content in coal seams were affected by Milankovitch cycle.Based on the Milankovitch cycle,the carbon accumulation rate of Jurassic coal seam in Juhugeng mining area is 59.6-67.8 g/(m2·a)and the corresponding peatlands carbon accumulation rate is 100-113.8 g/(m2·a).The net primary productivity of peatlands is 300-341.4 g/(m2·a)by analogy with that of Holocene.The net primary productivity of peatlands in the same latitude area of Holocene is 150-200 g/(m2·a).The level of net primary productivity during the Middle Jurassic peatlands of Muli Basin is generally higher than that at the same latitude area of Holocene.Higher atmospheric carbon dioxide content is the main factor to improve the net primary productivity of the Middle Jurassic peatlands.Through the study of coals,we could further obtain the records of paleoenvironment changes,which is of great scientific significance.