Organic geochemistry and organic petrology of Oligocene and Miocene lignites, subbituminous and high volatile bituminous coals and carbonaceous shales in northern and central Trinidad

Abstract

The so far unstudied Oligocene and Miocene organic matter–rich deposits of the Cunapo and Nariva Formation of northern and central Trinidad, western tropical Atlantic Ocean have been investigated using organic geochemical and organic petrological techniques to assess the thermal maturity and paleoenvironmental conditions during peat accumulation. Twenty samples were collected along various sections within the Cunapo and Nariva formations. Evaluation based on organic petrological analyses and Rock-Eval pyrolysis reveals that the coals and carbonaceous shales are thermally immature in the lignite and subbituminous rank for Cunapo Formation and mature in the subbituminous to high-volatile bituminous rank for Nariva Formation. The respective maturities correspond to high volatile matter contents.

Petrographically, the Cunapo Formation coals are dominated by huminite with minor amounts of inertinite and liptinite, and the Nariva Formation coals contain high amounts of vitrinite, with moderate to low liptinite and low inertinite contents. The detailed petrographic data for Cunapo and Nariva coals indicate predominantely limnic conditions and a herbaceous origin of organic matter. Further conclusions are deduced from petrographic ratios such as gelification-, tissue preservation-, vegetation- and groundwater_AC indices, ash- and sulfur contents, iso- and n-alkane distribution, and 17α(H)-homohopane ratio. In summary, the data supports that both the Cunapo and Nariva coals formed in a tropical peat environment, partly influenced by very high microbial activity and ± in case of Nariva coals - partly grading into ombrogenic mires. The Cunapo coals were influenced by the development of the Orinoco River system during the Miocene and exposure to marine water during transgressive phases in a tidal, coastal environment.