Sediment gases as indicators of subsurface hydrocarbon generation and entrapment - examining the record both in laboratory and field studies

摘要

Surface geochemistry, the measurement of near-surface hydrocarbons, hasbeen used by the petroleum industry for more than 80 years to explore forsubsurface petroleum deposits. The fact that hydrocarbons generated deep inthe sedimentary section from thermally mature organic rich rocks can migrateto the near-surface in measurable concentrations is well documented but themethods currently used by industry to extract and measure these near-surfacemigrated hydrocarbons have not been rigorously tested. One of the key goalsin this PhD research effort is to determine the best procedures to removemigrated hydrocarbon gases (C1 to C5) from near-surface marine sedimentswith minimal fractionation based on laboratory experiments and fieldcalibration studies. A second key goal is evaluate procedures to evaluate nearsurfacesediment gasoline range hydrocarbons (C5 to C10). The middle boilingpoint range hydrocarbons have been largely ignored in surface geochemistryand could contain valuable information to determine subsurface petroleumgeneration, migration, and entrapment. Lastly, it is extremely important tounderstand how best to evaluate and integrate the near-surface gas andgasoline range measurements into an overall understanding of the petroleumcharge system, specifically the source, maturation, and migration elements forevaluating prospect charge.Empirical observations from the global surface geochemical database andlaboratory experiments demonstrate that several of the surface geochemicalmethods currently used by industry do not accurately remove the nearsurfacemigrated gases thus providing biased and incorrect results.The acid extraction (Horvitz adsorbed method), microdesorption, and ballmill (occluded) bound sediment gas extraction methods all provided gascompositions and isotopic ratios significantly different than the charge gases.The extracted bound gases contain elevated wet gas (C2 to C5) relative to thecharge gases. These results are similar to what was noted in the global surfacegeochemistry database. In addition, the laboratory results indicate we do notfully understand sediment bound gas process. Thus the bound gases may not properly reflect the composition or isotopic ratios of the migratedhydrocarbons.Three interstitial sediment gas methods were examined as part of myresearch efforts, two canned headspace with different preparation andlaboratory procedures; and a new extraction method designated as thedisrupter. One of the headspace methods and the new disrupter gasextraction method provided gas compositions and isotopic ratios very similarto the charge gases. One of the headspace can methods provided highlyvariable gas compositions due to can leakage and preparation procedures. Ingeneral the interstitial hydrocarbon gases when properly collected andevaluated can provide critical information on the presence of mature sourcerock at depth.The gasoline range plus hydrocarbons are rarely examined in surfacegeochemical studies due to the great difficulty in extracting this boiling pointrange of hydrocarbons. The SPME method, in conjunction with the disrupterchamber, has been shown from laboratory evaluation to accurately removeand reflect gasoline range hydrocarbons in marine sediments. Choosing themost efficient fiber, optimal boundary conditions, and limitations is verycritical. Early field testing has shown the gasoline range hydrocarbons areheavily bacterially altered in most near-surface marine. Despite these issues,the gasoline range plus hydrocarbons have great potential in determining thesource and maturity of the migrated hydrocarbons in near-surface marinesediments. Thus near-surface sediment gases and gasoline rangehydrocarbons, when properly collected and extracted, can be used asindicators of subsurface generation and entrapment as shown in theobservation with the global surface geochemical database and laboratoryexperiments.