Widespread coal accumulation occurred in Paleogene to Neogene in China.Coals in land area are mainly utilized as solid minerals directly,and coals in sea area act as important source rocks to provide hydrocarbons to adjacent oil and gas fields.The Cenozoic paralic and limnic coal-bearing strata are widespread in the sea areas of China,which were buried underground hundreds to thousands of meters and in relatively high degree of thermal evolution.These Cenozoic coal-bearing strata have very good hydrocarbon systems,including the source rocks,reservoirs and caprocks.Therefore,research on the coal petrology and organic geochemistry of the Paleogene to Neogene coals are of great significance for oil and gas exploration in the sea area of China.This study will characterize the petrology and hydrocarbon generation potential of the Paleogene to Neogene coals based on the coal petrology and organic geochemistry analysis of the coals from typical coal-bearing basins,and make comparison on the coal petrology and hydrocarbon generation potential to the coals in the geological time.This study also tries to present a coal facies classification scheme,which suits to the Paleogene to Neogene coal,based on the maceral composition and maceral origins.Then,this study provides a hydrocarbon generating model for coal facies by summarizing the coal petrology and hydrocarbon generation potential of the coals with different coal facies.This hydrocarbon generating model in combination with the coal facies analysis will provide a new guideline in the hydrocarbon exploration for the coal-generating oil and gas fields.Based on the analysis of vast previous achievements,the Paleogene to Neogene coalbearing basins are distributed in five coal-accumulation areas except the Northwest Coalaccumulation Area in China.Coals in Northeast Coal-accumulation Area are mainly distributed in the Yi-Shu fault zone and Dun-Mi fault zone,and the coal-bearing strata are mainly developed in Eocene.Coals in Offshore Coal-accumulation Area mainly are distributed in East China Sea and South China Sea,and the coal-bearing strata are mainly developed in Eocene to Oligocene.Coals in North China Coal-accumulation Area are mainly distributed in the northeastern Shandong Province,and the coal-bearing strata are of the Eocene to Oligocene age.Coals in South China Coal-accumulation Area are mainly distributed in Guangxi and eastern Yunnan Provinces,and the coal-bearing strata in Guangxi province are mainly developed in Eocene and that in eastern Yunnan province are mainly developed in Miocene to Pliocene.Coals in Qinghai-Tibet Coal-accumulation Area are mainly distributed in Tibet and western Yunnan Provinces,and the coal-bearing strata being mainly of Miocene to Pliocene age.The macrolithotype of Paleogene to Neogene coals in China is mainly semibright coal and semidull coal.Microscopically these coals are characterized by a predominance of vitrinite/huminite group with relatively high contents of liptinite group and only minor amounts of inertinite group.The liptinite group is dominated by resinite.The high contents of liptinite group,especially the resinite,of the Paleogene to Neogene coals are closely related to the high abundance of Coniferopsida of the coal-forming plants,especially the Pinuspollenites,Taxodiaceaepollenites,Tsugaepollenites.The inertinite group in the Paleogene to Neogene coals were formed by forest fires as well as microbial activity such as fungi.A very little content of inertinite group in the Paleogene to Neogene coals is also likely associated with high abundance of Coniferopsida of the coal-forming plants.This type of coal-forming plants are enriched in lipoid and could promote the complete burning of vegetation.The promotion of the forest fire could result in the formation of very little charcoal.Therefore,a very low content of inertinite group were recorded in the Paleogene to Neogene coals.The results from the study of coal petrology and organic geochemistry show that the Paleogene to Neogene coals are excellent source rocks with a very high abundance of organic matter,which is dominated by a mixture of type Ⅱ and type Ⅲ kerogen.Therefore,the Paleogene to Neogene coals could continuously generate and expulse considerable liquid hydrocarbons.These coals are rich in the liptinite macerals such as the resinite and suberinite,which is of the early-stage hydrocarbon generation property,and thus could generate abundant hydrocarbons in the lignite stage.The hydrocarbon expulsion threshold of the Paleogene to Neogene coals is at about 0.65–0.80% in Ro,corresponding to the Tmax of 430–440 ℃.Some secondary components,such as oil drops,oil films,exsudatinite and micrite,are commonly developed in the Paleogene to Neogene coals,which directly prove that the coals can generate liquid hydrocarbons.Biomarkers confirm that the extractable organic matter in the coals are mainly originated from terrestrial higher plants,especially Coniferopsida.The genetic relationship between the organic carbon isotope compositions of the coals of the Pinghu Formation coal in Xihu Sag of the East China Sea shelf basin and the crude oil,saturated hydrocarbon and aromatichydrocarbon in adjacent oil and gas fields directly confirms the existence of coalgenerated oil and gas fields.Some collodetrinite/densinite in the Paleogene to Neogene coals show fluorescence under blue light irradiation and demonstrate a perhydrous property,suggesting that these macerals could generate liquid hydrocarbons.The Thermal Simulation Experiment in Gold Tubes also indicates that the vitrinite/humite group has a good oil generation capacity.Therefore,not only the liptinite group but also some perhydrous collodetrinite/densinite could generate liquid hydrocarbons of the Paleogene to Neogene coals.According to the characteristics of maceral compositions of the Paleogene to Neogene coals and the origins of macerals,this study presents a coal facies classification scheme based on the wetness index(WI).The Paleogene to Neogene coals is classified into four coal facies according to the wetness index,i.e.,dry forest swamp facies,overlying water forest swamp facies,forest margin swamp facies,and open water swamp facies.By summarizing the coal petrology and organic geochemistry of the coals of different coal facies,this study come to the conclude that the hydrocarbon generation potential of the Paleogene to Neogene coals decreases with the increase of water depth of the peat-forming environment(wetness index)in both paralic and limnic sedimentary settings.This means the Paleogene to Neogene liptinite-rich coals characterized by moor are more conducive to hydrocarbon generation.This coal facies model provides a new insight to exploration of the coal-generated oil and gas fields. |