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Pelagites and Organic-Rich Sediments |
Marathon Oil Company, PO Box 269, Littleton, CO 80160, USA
University of Miami, Comparative Sedimentology Lab., Fisher Island Station, Miami, FL. 33139, USA
The organic content of fine-grained carbonate muds deposited in the intraplatform basins and canyons of the Bahamas range between 0.1 and 2.6% total organic carbon (TOC). Geochemical analyses suggest some of these muds are a potential source for hydrocarbon. Interbedded with the carbonate muds are porous carbonate sands and gravels which can serve as local reservoirs or serve as conduits for hydrocarbons to migrate into carbonate slope and platform reservoir facies.
Ingeneral, periplatform oozes are lean (generally less than 0.5 to 1% TOC) whereas green muds, very likely the products of turbidity currents, have higher organic contents (0.5 to more than 2.5% TOC), but values of TOC vary considerably both within and between study areas. Tongue of the Ocean and Exuma Sound are less than 80 km apart but their average TOC values differ markedly, 1% and 0.33% respectively. Higher sedimentation rates in the Tongue of the Ocean are thought, in part, to be responsible for better preservation of organic matter.
In Tongue of the Ocean, cyclic variations in TOC are correlated with fluctuations of sea-level. High stands of sea-level are recorded by aragonite-rich muds with TOC on the average 0.5% higher than the calcitic periplatform oozes deposited during low stands of sea-level. Organic fractions of green muds have
Rock-Eval analysis shows that the periplatform oozes are relatively rich in oxygen; this, along with their low organic contents, results in a poor source rock potential. The green muds range between type II (mixed gas and oil potential) and III (gas potential) source rocks, which, through favourable maturation, should generate hydrocarbons. Gas chromatographic analysis of organic matter evolved at relatively low temperatures (300°C) demonstrated that these immature sediments underwent vapourization and thermal cracking of relatively heat-sensitive or thermally labile material.
13C values (–16.4
to –13.9 vs PDB) which are similar to platform carbonates, whereas lighter 13C values of periplatform oozes reflect mixing of organic material derived from platform and pelagic sources.
