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Geochemical
Exploration Surveys in the
Geochemical exploration for petroleum is the search for
surface or near-surface occurrences of hydrocarbons and their alteration
products. Geochemical and microbiological surveys document that hydrocarbon microseepage from oil and gas accumulations is common and
widespread, is chiefly vertical (with obvious exceptions in certain geologic
settings), and is dynamic (responds quickly to changes in reservoir
conditions).
Geochemical exploration techniques can be direct or indirect, and
measurements can be instantaneous or integrative. Direct techniques analyze
small quantities of hydrocarbons that occur in the pore space of soil, are
adsorbed onto clay minerals, or are incorporated in soil cements. Indirect
methods detect seepage-induced changes to soil, sediment, or vegetation.
Bacteria and other microbes play a profound role in the oxidation of migrating
hydrocarbons, and are directly or indirectly responsible for many of the suface manifestations of petroleum seepage.
Onshore hydrocarbon microseepage surveys require
careful planning and implementation. Microseepage
data are inherently noisy data and require adequate sample density to
distinguish between anomalous and background areas. The sampling pattern and
sample number must reflect survey objectives, expected size and shape of the
target, expected variation in surface measurements, and probable
signal-to-noise ratio Defining background values adequately is an essential part
of anomaly recognition and delineation. Undersampling
and/or the use of improper sampling techniques is a major cause of ambiguity
and leads to interpretation failures. This presentation will be illustrated
with examples from
Geochemical exploration data have found their greatest value when integrated
with subsurface geological and geophysical data. Properly applied, the
combination of surface
and subsurface exploration methods leads to better
prospect evaluation and risk assessment.