Source rock evaluation of Middle Jurassic coals, northeast Greenland, by artificial maturation: aspects of petroleum generation from coal

摘要

Paralic liptinite-enriched coals and carbonaceous mudstones in northeast Greenland constitute potential highly oil-prone source rocks, whereas the humic coals may be marginal source rocks. The liptinite-rich coals are dominated by resinite or fluorescing amor-phous organic matter and alginite, resulting in hydrogen index (HI) values generally above 300 and reaching up to 728. During artificial maturation up to 330 deg C/72 hr, the coals follow the maturation paths of kerogen types I and II on an HI vs. T_max diagram, and calculations show that upon passage through the oil window, roughly 85% of their generation potential is realized. Activation energy (E_a) distributions with prominent principal E_a values centered around 60-62 kcal/mole and frequency factors from 5.855 X 10~15 s~(-1) to 3.249 X 10~16 s~(-1) strongly influence the generation characteristics from 300 to 330 deg C/72 hr artificial maturation. Important changes include marked loss of liptinite fluorescence and increase in resinite reflectance; small change in T_max; significant decrease in HI; pronounced increase in extract yields; increased generation of saturates; and generation of labile bitumen with low E_a values. These observations indicate significant bitumen/petroleum formation from the coals during a relatively narrow temperature range, which, together with the petrographic composition, may facilitate expulsion of a waxy crude oil. The coals demonstrate that under certain depositional conditions, highly prolific coal source rocks can form with the capacity not only to generate but also to expel liquid petroleum.