Sediment gases as indicators of subsurface hydrocarbon generation and entrapment - examining the record both in laboratory and field studies
Creators: name: Abrams, Michael Allan
Publisher: name: Imperial College London
Year: 2008

description: Surface geochemistry, the measurement of near-surface hydrocarbons, has been used by the petroleum industry for more than 80 years to explore for subsurface petroleum deposits. The fact that hydrocarbons generated deep in the sedimentary section from thermally mature organic rich rocks can migrate to the near-surface in measurable concentrations is well documented but the methods currently used by industry to extract and measure these near-surface migrated hydrocarbons have not been rigorously tested. One of the key goals in this PhD research effort is to determine the best procedures to remove migrated hydrocarbon gases (C1 to C5) from near-surface marine sediments with minimal fractionation based on laboratory experiments and field calibration studies. A second key goal is evaluate procedures to evaluate nearsurface sediment gasoline range hydrocarbons (C5 to C10). The middle boiling point range hydrocarbons have been largely ignored in surface geochemistry and could contain valuable information to determine subsurface petroleum generation, migration, and entrapment. Lastly, it is extremely important to understand how best to evaluate and integrate the near-surface gas and gasoline range measurements into an overall understanding of the petroleum charge system, specifically the source, maturation, and migration elements for evaluating prospect charge. Empirical observations from the global surface geochemical database and laboratory experiments demonstrate that several of the surface geochemical methods currently used by industry do not accurately remove the nearsurface migrated gases thus providing biased and incorrect results. The acid extraction (Horvitz adsorbed method), microdesorption, and ball mill (occluded) bound sediment gas extraction methods all provided gas compositions and isotopic ratios significantly different than the charge gases. The extracted bound gases contain elevated wet gas (C2 to C5) relative to the charge gases. These results are similar to what was noted in the global surface geochemistry database. In addition, the laboratory results indicate we do not fully understand sediment bound gas process. Thus the bound gases may not properly reflect the composition or isotopic ratios of the migrated hydrocarbons. Three interstitial sediment gas methods were examined as part of my research efforts, two canned headspace with different preparation and laboratory procedures; and a new extraction method designated as the disrupter. One of the headspace methods and the new disrupter gas extraction method provided gas compositions and isotopic ratios very similar to the charge gases. One of the headspace can methods provided highly variable gas compositions due to can leakage and preparation procedures. In general the interstitial hydrocarbon gases when properly collected and evaluated can provide critical information on the presence of mature source rock at depth. The gasoline range plus hydrocarbons are rarely examined in surface geochemical studies due to the great difficulty in extracting this boiling point range of hydrocarbons. The SPME method, in conjunction with the disrupter chamber, has been shown from laboratory evaluation to accurately remove and reflect gasoline range hydrocarbons in marine sediments. Choosing the most efficient fiber, optimal boundary conditions, and limitations is very critical. Early field testing has shown the gasoline range hydrocarbons are heavily bacterially altered in most near-surface marine. Despite these issues, the gasoline range plus hydrocarbons have great potential in determining the source and maturity of the migrated hydrocarbons in near-surface marine sediments. Thus near-surface sediment gases and gasoline range hydrocarbons, when properly collected and extracted, can be used as indicators of subsurface generation and entrapment as shown in the observation with the global surface geochemical database and laboratory experiments.