Variations in Composition, Petroleum Potential and Kinetics of Ordovician – Miocene Type I and Type I‐ii Source Rocks (Oil Shales): Implications for Hydrocarbon Generation Characteristics

Abstract

Lacustrine and marine oil shales with Type I and Type I-II kerogen constitute significant petroleum source rocks around the world. Contrary to common belief, such rocks show considerable compositional variability which influences their hydrocarbon generation characteristics. A global set of 23 Ordovician -Miocene freshwater and brackish water lacustrine and marine oil shales has been studied with regard to their organic composition, petroleum potential and generation kinetics. In addition their petroleum generation characteristics have been modelled. The oil shales can be classified as lacosite, torbanite, tasmanite and kukersite. They are thermally immature. Most of the shales contain >10 wt% TOC and the highest sulphur contents are recorded in the brackish water and marine oil shales. The kerogen is sapropelic and is principally . The kerogen type and source rock quality appear not to be related to age, depositional environment or oil shale type. Therefore, a unique, global activation energy (E a ) distribution and frequency factor (A) for these source rocks cannot be expected. The differences in kerogen composition result in considerable variations in E a -distributions and A-factors. Generation modelling using custom kinetics and the known subsidence history of the Malay-Cho Thu Basin (Gulf of Thailand/South China Sea), combined with established and hypothetical temperature histories, show that the oil shales decompose at different rates during maturation. At a maximum temperature of ~120ºC reached during burial, only limited kerogen conversion has taken place. However, oil shales characterised by broader E a -distributions with low E a -values (and a single approximated A-factor) show increased decomposition rates. Where more deeply buried (maximum temperature ~150ºC), some of the brackish water and marine oil shales have realised the major part of their generation potential, whereas the freshwater oil shales and other brackish water oil shales are only ~30-40% converted. At still higher temperatures between ~165ºC and 180ºC all oil shales reach 90% conversion. Most hydrocarbons from these source rocks will be generated within narrow oil windows (~20-80% kerogen conversion). Although the brackish water and marine oil shales appear to decompose faster than the freshwater oil shales, this

A data driven model for studying kerogen kinetics with application examples from Canadian sedimentary basins

“…9), showing a systematic upward shifting of activation energy from "type III" to "type I" kerogen in this study. This is consistent with the kinetics determined using laboratory pyrolysis of various source rock samples (Sundararaman et al, 1988;Jarvie, 1991;Petersen et al, 2010).…”

World‐class Paleogene Oil‐prone Source Rocks From a Cored Lacustrine Syn‐rift Succession, Bach Long Vi Island, Song Hong Basin, Offshore Northern Vietnam

“…Both the narrow oil window and the relatively late initial generation/ expulsion is characteristic of lacustrine source rocks with a relatively homogeneous kerogen composition (e.g. Petersen et al, 2010). Those mudstones in ENRECA-3 which contain a substantial amount of terrigenous organic matter (huminite) can be expected to have different generation characteristics and possibly higher GLRs.…”

Hydrocarbon Potential of Middle Jurassic Coaly and Lacustrine and Upper Jurassic – Lowermost Cretaceous Marine Source Rocks in the Søgne Basin, North Sea

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