Environmental dynamics of organic accumulation for the principal paleozoic source rocks on Yangtze block
Chen Daizhao1 Wang Jianguo1 Yan Detian2 Wei Hengye1 Yu Hao1 Wang Qingchen1
1.Key Laboratory of Petroleum Resource Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029|2.Key Laboratory of Tectonics and Petroleum Resources, of Ministry of Education, China University of Geosciences, Wuhan 430074
On Yangtze block，there are three regionallydistributed highquality source rock horizons：the Lower Cambrian，Upper OrdovicianLower Silurian and Permian(Middle and Upper Permian).The Lower Cambrian source rocks mainly consist of the lower black chert(Liuchapo Formation)and upper siliceous shales，carbonaceous shales(or stone coals)(Niutitang Formation).During the Early Cambrian，extensive hydrothermal activities occurred on the Yangtze platform margins extending ca. 2 000～3 000 km long，and released vast amounts of silicarich fluids into the ocean，inducing extensive silica precipitation off the platform margin through basin.Simultaneously，a large quantity of H2S and greenhouse gases(CO2，CH4)and metalliferous anoxic fluids were vented into oceanic waters and atmosphere，leading to an apparent elevation of atmospheric CO2 concentration and largescale rise of sealevel.In this case，nutrientrich deep water masses were carried through the upwelling currents into the shallower shelf，where the primary productivity was increased，which aggravated the anoxic state of water masses and enhanced the organic burial and preservation.
The Upper OrdovicianLower Silurian source rocks mainly comprise black shales and siliceous shales of Wufeng and Longmaxi formations，which formed during marine transgressions under a warm climate during the transitional time.With the accelerated amalgamation between Yangtze and Cathaysian blocks to the east，a large epicontinental sea with subbasins(or sags)linked to a largely increased landmass was developed on the Yangtze Block，forming silled subbasins within which water circulation was restricted and oceanic stratification was intensified，favorable for organic accumulation.The enhanced chemical weathering could have increased nutrient flux into the basins，thereby increasing the primary productivity.However，greatly increased continental clastic flux into basins could have diluted the organic matters in sediments as seen in the upper part of Longmaxi Formation.
The Middle Permian source rocks are mainly present in the Qixia Formation，especially in the lower part； they deposited in the intrashelf depressions(or subbasins).The organic abundance in rocks varies episodically in accordance with the tune of stratal cyclicity，therefore it could have been controlled by the fluctuations of primary productivity triggered by the variableorder sea level fluctuations.The superimposed sea level rises in different order are the most favourable interval for organic accumulation and preservation.The Upper Permian source rocks mainly occur in the Dalong Formation； they also deposited in the intrashelf depressions(or subbasins)，within which water masses were fairly euxinic(sulfidic)，favourable for organic accumulation and preservation.