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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 74 (1990)

Issue: 3. (March)

First Page: 345

Last Page: 351

Title: Source-Rock Evaluation of Devonian Lacustrine Carbonates, Northern Scotland (1)

Author(s): J. PARNELL (2) and M. RAHMAN (3)

Abstract:

A Rock-Eval pyrolysis data set from Devonian lacustrine rocks from Scotland shows that the hydrocarbon yields per unit weight of organic matter (hydrogen indices) are greater from carbonate-rich rocks than from mudrocks. Higher yields are recorded from both primary carbonate laminites and from rocks in which the carbonate is a diagenetic precipitate.

The distinction in yield between carbonates and mudrocks is related to both type of organic matter and lithological differences. The carbonate laminites (deposits of a stratified lake) contain a higher proportion of type I kerogen than the mudrocks (deposits of shallower water). Oxidation of kerogen was not important in either rock type. Samples of both rock types represent a similar range of thermal maturities, and this is not an overriding control on the pyrolytic yield.

The transformation indices for the mudrocks are higher than for the carbonate rocks, indicating that the mudrocks contain a greater proportion of migrated bitumen. Both primary and diagenetic carbonates are relatively impermeable and lack migrated bitumen. After solvent extraction, mudrock samples yield hydrogen indices more comparable with those from the carbonates.

The carbonate rocks also have higher total organic carbon (TOC) contents than the mudrocks, consequently their pyrolytic yields (S[2]) are much higher.

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