Bernie B. Bernard¹, James M. Brooks¹, Tommy J. McDonald¹
(1) TDI-Brooks, College Station, TX

Abstract: Use of Surface Geochemical Exploration in deep water West Africa and the Gulf of Mexico

Surface geochemical exploration studies are based on the prospect that upward-migrated petroleum hydrocarbons can be detected in near-surface sediments and their detection may be helpful in reducing risk. Over the last 3 years, we have sampled by piston coring the following numbers of sites for non-exclusive geochemical analyses from deep-water study areas offshore: Nigeria-360, Angola-727, Gabon-118, and the Gulf of Mexico-862 sites. Locations were correlated with surface expressions in 2-D seismic records and typically confirmed with a Chirp subbottom profiler. Three sections per piston core were sampled at 1-meter intervals from the bottom of each core. Typical core recovery was 3 to 5 meters of sediment column.

Analytical determinations on every sample included: (1) the fluorescence intensities of sediment extracts, (2) the C15+ hydrocarbons by GC, and (3) the light hydrocarbon gases by headspace extraction and GC. When concentrations were sufficient, biological markers in extracts and carbon isotope ratios in selected hydrocarbons were determined by mass spectrometry. Results often matched the chemical signatures of regionally produced oils. Indicators of migrated petroleum hydrocarbons were developed and used to qualify cores and distinguish them from background for each study area. Background for each indicator varied substantially with region, as did the general extent of biodegradation of seepage. The results of these studies show that there is significant and widespread seepage of oil in deep and ultra deepwater regions offshore west Africa, but its character is substantially different from seepage in the Gulf of Mexico.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana