Extractable and pyrolyzed hydrocarbons in shallow-water carbonate sediments, Florida Bay, Florida

Abstract

A three-dimensional organic geochemical survey conducted in Florida Bay, a subtropical carbonate environment with multiple sources of organic matter, illustrates the hydrocarbon source potential of shallow water carbonate sediments and the effect of multiple biochemical sources on the organic sedimentary imprint. Organic carbon (TOC) in the sediments averages about one per cent. Concentrations of extractable organic matter (EOM) and hydrocarbons are slightly higher, and of organic carbon slightly lower, in sediments of the marine-dominated part of the bay. Hydrocarbon distribution, which is a function of the type of organic matter, also varies across the bay. The C 15+ n-alkanes in sediments of the terrestrially-influenced portion exhibit a bimodal pattern, with maxima at C 27 or C 29 and C 18 and a marked odd/even carbon-number predominance in the C 21C 13 range. In contrast, C 15+ n-alkanes in sediments of the marine-dominated portion have maxima at C 17 or C 18 and C 23 or C 24. TOC decreases, but EOM and hydrocarbon content increase, with depth in the sediment. Hydrous and anhydrous pyrolysis of bulk sediments and individual carbonates generates a new suite of n-alkanes with an even carbon-number predominance and a maximum at C 22. Yields of EOM and hydrocarbons are comparable for both types of pyrolyzates and are 6 to 8 times greater than in situ EOM and hydrocarbons. Carbonate sediments deposited in low energy, shallow-water, environments have the potential to be good oil source rocks.