Abstract: Sampling Techniques and Interpretation of Surface Geochemical Studies in Offshore Petroleum Exploration

Michael A. Abrams

Almost every offshore petroleum province in the world has evidence of hydrocarbon seepage. High resolution seismic can detect "active" seeps where gas bubbles, bright spots, etc., are present. Bottom sediment geochemistry is needed to measure low flux or "passive" seepage.

Geochemical data around seep zones indicate leakage can be very site specific. Leakage points are usually related to faults and diapirs. High resolution seismic profiles, in association with conventional deep seismic data, provide an excellent tool for locating leakage points where there is increased opportunity to sample migrated thermogenic hydrocarbons.

Hydrocarbon concentrations are strongly related to sample depth in sediments. Increased hydrocarbon concentration with depth appears to be related to various processes such as bacterial activity, oxidation, and pore water flushing occurring in surficial marine sediments. This is a shallow zone called the "Zone of Maximum Disturbance" or ZMD. Shallow non-permeable barriers such as hydrates, permafrost, and tight clays may prevent movement of migrated hydrocarbons into the very near surface marine sediments. Therefore, sediment sampling devices must be able to obtain samples below the ZMD and/or shallow barriers in order to detect anomalous migrated hydrocarbons. In areas of active seepage, shallow sediment or water column sampling devices such as conventional gravity corers, sniffers, nd airborne detectors (airborne laser fluorescence), should be able to detect active subsurface hydrocarbons seepage. Deep coring devices such as vibracorers, jet corers, and rotary core systems can obtain samples deep enough to detect passive hydrocarbon seepage.

Analytical programs for seep characterization should include several different light hydrocarbon techniques (headspace, cuttings, and acid-extraction) and a method (e. g., total scanning fluorescence) for screening the presence of high molecular hydrocarbons. Detailed gas chromatography and GC-MS are reserved for testing locations of probable migrated hydrocarbons.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994