Abrams, Michael A.¹
(1) University of Utah Energy and Geoscience Institute, Salt Lake City, UT

ABSTRACT: Significance of Gas Extracted from Marine Sediments to Evaluate Subsurface Hydrocarbon Generation and Charge

Sediment gases can be collected and analyzed by a variety of methods for regional and prospect evaluation. The most common method; headspace, collects gas sampled through a modified can which contains sediment, degassed water, and inert gas; then heated and shaken. The blender or ball mill method utilizes a blender or metal spheres to mechanically break up the sediment and release interstitial gases into sampler headspace. Sorbed (adsorbed) gas analysis uses heat and phosphoric acid within a partial vacuum to desorb gases within clays or structured water. Comparison of gas compositions from these methods, as well as sediment type and sampling depths, reveals significant differences. Sediment gases rarely match their reservoir counterpart due to several factors: extraction method, coring protocols, seepage type, mixing, partitioning, and secondary alteration (during migration and/or in-situ). The key to effectively using sediment gases for exploration-exploitation is to recognize these differences relative to migrated hydrocarbons and not to use conventional reservoir gas interpretation concepts. Most marine sediment extracted hydrocarbon gases can be placed into four major categories based on total gas concentration (sum C1 - C5), wet gas fraction (sum C2 -C5/sum C1 - C5), and compound specific isotopic compositions: background: low total hydrocarbon concentrations and low wet gas fraction, background-fractionated: low total concentrations and elevated wet gas fraction, anomalous-biogenic: elevated total hydrocarbon gas concentration and low wet gas fraction, and anomalous-thermogenic: elevated total hydrocarbon gas concentration and wet gas. Only the anomalous-thermogenic group, with great caution and supporting data, should be used to evaluate the presence of a working petroleum system.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.