ABSTRACT

Structural complexities are created in Gulf Coast basins by active growth faulting and salt tectonics. Such complexities require the development of quantitative models for hydrocarbon exploration. Innovative exploration models are especially needed in deep-water basins like the Flex Trend where seismic and well coverage are limited in extent and/or expensive to obtain. Ideally such models should be predictive.

Recent studies of exploration wells from offshore Louisiana and Trinidad suggest fine-grained authigenic carbonate particles with negative carbon isotopic signatures in an unconsolidated siliciclastic matrix act as geochemical recorders or tracers of hydrocarbon movement. A flux of hydrocarbons through an aquifer then produces ¹³C signal impacted on by flow rate, microbial degradation and the presence of biogenic or detrital calcite. Mapping of ¹³C anomalies could be used to determine hydrocarbon migration pathways. By measuring the ¹³C signal as a function of position along the aquifer, it should be possible to extract both the amount and speed of the hydrocarbon flow after due allowance is made for extraneous effects.

Preliminary measurements in offshore Louisiana strongly suggest this process is operative in Plio-Pleistocene sands. A preliminary mathematical framework has been established to indicate that extracting the hydrocarbon signal is feasible. In this paper we explore a model which predicts the speed of hydrocarbon migration in an aquifer, based on the measured ¹³C data and temperature profile along the aquifer. In particular, measurements from several wells a few miles apart suggest that the flow rate has been reversed above and below the overpressure top in nominally continuous sands. Flow rates corresponding to hydrocarbon transit times of order 10,000-20,000 years between the well locations are indicated by the analysis. These results support the idea of buoyant flow of hydrocarbons together with the degradation of isotopic signals by bacterial action being the main causes for observed isotopic anomalies.