Hydrocarbon Migration Pathways and Fluid Flow Histories Around Gulf Coast Salt Structures from Carbon Isotope Anomalies

WILLIAMS, D. F., I. LERCHE, and Z. YU, University of South Carolina, Columbia, SC

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 in an unconsolidated siliciclastic matrix with negative carbon isotopic signatures act as geochemical recorders or tracers of hydrocarbon movement. A flux of hydrocarbons through an aquifer then produces a "mixed" d13C signal, impacted on by flow speed, microbial degradation, and the presence of biogenic or detrital calcite. Mapping of d13C anomalies could be used to determine hydrocarbon migration pathways. By measuring the mixed signal as a function of position along the aquifer, it should be possible to extract both the amount and speed of the hydrocarbon flow by "inverting" the measurements-due allowance being made for thermal effects which "kill" microbes o alter their biodegradation rate and for the thermal degradation of oil to gas. 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 such an "inversion" is a pragmatically realizable problem. In this paper we explore a model which predicts the speed of motion of hydrocarbons in the aquifer, based on the measured "mixed" d13C data and temperature profile along the aquifer.

In particular, several wells a few miles apart are used to suggest that the flow speed has been reversed above and below the overpressure top in nominally continuous sands. Flow speeds corresponding to hydrocarbon transit times of the order of 10,000 to 20,000 years between the well locations are indicated by the analysis. These results conform to the idea that buoyant flow of hydrocarbons together with the degradation of isotopic signals by bacterial action are the main causes for observed isotopic anomalies.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)