Hydrocarbon generation from deltaic and intermontane fluviodeltaic coal and coaly shale from the Tertiary of Texas and Carboniferous of Nova Scotia

Abstract

Selected samples of coal and coaly shale from southeastern Texas (depositional environment: deltaic; Age: Paleocene-Eocene; R o: 0.25–0.75%) and Nova Scotia (depositional environment: intermontane swamp, delta and lacustrine; Age: Carboniferous-Westphalian A-D; R o: 0.67–0.70%) were analyzed by fluorescence microscopy, Rock-Eval pyrolysis, NMR, anhydrous pyrolysis (temperature programed and flash) and hydrous pyrolysis to evaluate the hydrocarbon potential and hydrocarbon types. Based on maceral assemblages, palynofacies and hydrocarbon potential, these coal and coaly shale samples are classified as humic, mixinitic (mixed), and sapropelic compared to humic and sapropelic as defined by Stach et al. ( Coal Petrology. Gebruder Borntraeger, Berlin, 1982). Mixinitic (or mixed) coal and coaly shale containing mainly humodetrinite and lipodetrinite generated more than 10% pyrolysate (expelled oil + oil in wash fraction + remaining coal extract) and show a bimodal n-alkane distribution (derived from cutinite, suberinite and alginite). Pyrolysis of humotelinite/humocollinite-rich and liptinite-poor humic coals generated low amounts (<10%) of pyrolysates, mainly consisting of low molecular weight hydrocarbons. A modified model of oil-prone and non-oil-prone coal types, related to depositional environments, is proposed. A new concept of primary migration is proposed: the humodetrinite-liptodetrinite network of mixinitic (or mixed) and sapropelic coal (excluding exsudatinite) acts as a migration pathway for liquid hydrocarbons which migrate as a hydrocarbon phase. According to pyrolysis, NMR, organic petrography and the primary migration concept, mixinitic (or mixed) coal or coaly shale are considered as likely source rocks for liquid hydrocarbons within deltaic and intermontane basins.