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.