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.