Page 203 - Geochemical Remote Sensing of The Sub-Surface

180 V.T. Jones, M.D. Matthews and D.M. Richers

A second phase of in-depth, total scanning fluorometric analysis is often performed
on selected anomalous samples identified by the field fluorescence, adsorbed-gasdata or
interstitial-gas data. These samples undergo freeze drying followed by a thorough cyclic
extraction in hexane to optimise recovery of associated sedimentary aromatics (Brooks
et al., 1986). The oil type is then determined by total scanning fluorescence which
employs step-wise scanning of excitation and emission wavelengths to produce a three-
dimensional fingerprint fluorogram (Fig. 5-28).

SAMPLING STRATEGY

Spatial pattems of near-surface hydrocarbon composition and concentration are
prime factors when interpreting the survey results. Results from a poorly-designed or an
uncontrolled survey can be difficult or impossible to interpret, and can lead to a
completely erroneous assessment of the hydrocarbon potential of an area. An
improperly-spaced grid with sample spacing in excess of target size can result in only the
most cursory assessment of potential, with anomalous areas appearing as localised
single-point anomalies.
The distribution of sample sites in a geochemical survey is largely governed by the
purpose and budget of the survey. For regional surveys a sampling density of one sample
per 2-5 km 2 seems adequate. Such a density still allows for the discrimination of regional
ambient backgrounds from secondary backgrounds. Detailed diagnostic work requires a
close-spaced grid, sometimes with a sample interval of only a few tens of m. Regional
sampling is generally performed using a modified grid because a regular grid, on which
samples are taken at the intersections of a straight lines, does not minimise cost or
maximise information. We recommend that sample positions be chosen within grid cells
according to ease of access (minimum cost) and along zones of known or inferred
fracturing and faulting (maximum information). Satellite imagery, aerial photography,
seismic data and other data are useful when attempting to site samples on or near
fractures and faults. The analytical results from a regional survey should yield some
indication of compositional and/or magnitude "sweet-spots", either as isolated data
points or small clusters. If the objective is merely to evaluate whether a basin has a
source section, and general trends of where it is mature and focused to the surface, a
regional study may be all that is required. A more detailed follow-up survey, however, is
recommended if the objective is to highlight the zones of higher hydrocarbon potential.
One method commonly employed for detailed surveys is to sample seismic shot
holes, further providing a means to easily tie the geochemistry to subsurface structure.
Because seismic lines are not normally placed on a close-spaced grid, infill sampling
between seismic lines is usually recommended. It should be emphasised that in order to
define a target adequately, approximately 70% of the data should be collected in
presumed background areas beyond the immediate target area. An embarrassingly large
number of surveys have been performed in which sample locations do not extended