Second Derivative Differentiation of Anomalies in Biogeochemical Data

RIESE, W. C., ARCO Oil and Gas Company, Bakersfield, CA, and GLENDA B. MICHAELS, Western State College, Gunnison, CO

Seepage of hydrocarbons alters the surface and near-surface geochemical environment as they move through it. Measurement of these often subtle alterations and implying the presence of reservoirs from them are objectives of petroleum geochemical exploration; the sampling of various biologic constituents is found to be useful in defining anomalous terrain that might be otherwise overlooked by alternative methods. Both higher order plants rooted in these settings and lower order organisms distributed through them are useful for these purposes.

This alteration of near-surface environments may, in the context of the biosphere, be thought of as the introduction of stress to the plant-rock system; and just as the seepage chimney itself is not homogeneous in geochemical character, neither is the stress introduced to the biologic community by its development. Furthermore, the response of individuals from any given species will vary as a function of age, vigor, health, and any number of other biologic parameters. The geochemical patterns that result from the sampling of these stressed populations will often be seen to be extremely noisy or will show extreme sample-to-sample variability. This noise has, in fact, caused many workers to discount the use of biogeochemistry in some settings.

We suggest that this noise, as characterized by calculation of a second derivative function across an area, is itself the signal which should be mapped: it represents a measure of how fast variance in the system changes from sample to sample and describes the extreme heterogeneity that characterizes surface alteration haloes. Expressed another way, the variance of the system (1) describes the slope of a line fit through data that become increasingly noisy as a deposit is approached; the incremental differences in point-to-point variance (2) describes how rapidly that slope changes as we move from background to anomalous terrain.

Utilization of this technique has been useful and effective in describing the extent of geochemically anomalous terrain in data that would otherwise be considered too noisy to interpret. The technique

has been applied to data collected over reservoirs in the offshore Gulf of Mexico with positive results.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)