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Fine-grained (<63 μm) carbonate from Plio-Pleistocene exploration wells, offshore Louisiana and Trinidad, has δ13CPDB signatures of about -50 to -10 ‰. In this paper we explore two hypotheses that appear to account for these negative δ13C anomalies. The first hypothesis is that the negative δ13C signatures observed in Green Canyon block, offshore Louisiana, are due to authigenic subsurface precipitation of fine-grained carbonate as a record of past migration pathways of hydrocarbons through subsurface Plio-Pleistocene sands. The second hypothesis is that the authigenic δ13C signatures record precipitation at or near the sediment-water interface and thus record the past locations of hydrocarbon seeps on the northcentral margin of the Gulf of Mexico. Preliminary geochemical and petrographic evidence is presented to examine these hypotheses. Within the limitations of not knowing past pore fluid chemistries, we also attempt to use the δ18O signatures of the <63 μm carbonate to estimate the paleotemperatures, and perhaps paleodepths and timing, of emplacement of the negative δ13C signatures.

The potential that this isotopically negative carbonate acts as a geochemical recorder or tracer of hydrocarbon movement makes documentation of this phenomenon important to offshore exploration efforts.

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