Bjoroy, Malvin1, Ian L. Ferriday1, Wallace G. Dow2
(1) Geolab Nor AS, Trondheim, Norway
(2) Consultant, The Woodlands, TX
ABSTRACT: Optimum Surface Geochemical Methods Applied to the Faerows-Shetlands Region, N.E. Atlantic Margin
Optimum methods for sea floor core sample collection, preservation, analysis, and data
interpretation are described and applied to a geochemical study in the Faeroes-Shetland
region, N.E. Atlantic margin. Proper application of these parameters is essential to
obtain the most meaningful and useful results at the lowest possible cost. Sampling sites
should be selected from sea floor, seep-related geomorphic anomalies whenever possible.
The best sample collection method for soft bottom conditions, especially in deep water, is a relatively simple, 4 meter gravity corer. Piston and vibro-corers and longer core barrels have some applications, but the added cost is not often justified. Samples should be collected from the bottom, anoxic zone of the core, placed in l liter cans, flushed with nitrogen, immediately frozen to -80 C., and shipped to the laboratory packed in dry ice. Bactericides and lower freezing temperatures do not effectively inhibit bacterial activity in the cans.
In the laboratory, gas chromatography and carbon isotope analysis of adsorbed gas best defines thermogenic hydrocarbons, with headspace and occluded gas analysis showing primarily biogenic methane. Most recycled lithic fragments are removed by wet sieving. Thermally generated, oil-like components are most reliably defined with extract gas chromatography and GC/MS (biomarker) analysis.
These techniques were applied in a 670 core study in the Faeroes-Shetland region, N.E Atlantic margin. Here, the geographic distribution of geochemical anomalies coincides well with the drilling results. Seeped marine-sourced oil and gas were identified in sectors where oil has been found such as near the Foinaven and Schiehallion fields and near an oil discovery well (6004/16-1Z) in the southern area. In the northern UK sector, only dry gas has been discovered where the sea floor cores indicated primarily biogenic methane. These results demonstrate that sea floor geochemical studies can be a valuable aid to exploration in offshore basins.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.