扬子地区古生代主要烃源岩有机质富集的环境动力学机制与差异

在扬子地块古生界发育了3套区域性的优质烃源岩,即:下寒武统、上奥陶统一下志留统、二叠系(中、上二叠统).下寒武统烃源岩主要由硅质岩(留茶坡组)和上部的硅质泥岩、碳质泥岩(或石煤)(牛蹄塘组)组成.早寒武世在台一盆转换带(或台地边缘)广泛发育的海底热液活动,喷流出大量的富硅流体,使大量硅岩得以在台缘外侧-盆地区沉淀;同时...

Environmental dynamics of organic accumulation for the principal Paleozoic source rocks on Yangtze block

On Yangtze block，there are three regionally distributed high quality source rock horizons：the Lower Cambrian，Upper Ordovician Lower 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 silica rich 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 large scale rise of sea level.In this case，nutrient rich 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 Ordovician Lower 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 variable order 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. 