National Driller's Buyer's Guide, December 1995 (Richard B. Wells.)
Sampling the soil for its natural gas content is a way of outlining prospective blocks of acreage for petroleum exploration. This method continues to be used, and is generally successful. Two other useful tools that can also be applied on the surface involve mapping iodine concentrations and low-level radiation. There are traces of iodine in most all soils, from the weathering of minerals in the soil or forming from the atmosphere. Background iodine concentrations are typically between 0.1 and 2 ppm and vary from one area to another. The presence of petroleum in the subsurface is indicated by iodine anomalies at least 50% higher than the background, or one standard deviation above the mean. Over known oil fields, iodine concentrations are typically between 2 and 20 times greater than background.
This increase in iodine concentration is due to the reaction of trace amounts of hydrocarbons seeping upwards from source rocks or traps. (All traps leak to some extent.) The hydrocarbon molecules react with iodine to form insoluble inorganic compounds. This occurs only in the upper part of the soil where the reaction is triggered by sunlight.
To identify iodine anomalies, soils are sampled in a regular grid pattern over as broad an area as possible. The grid should be at least three times as large as the expected size of the anomaly in order to establish a reliable range from background iodine content and allow reasonable contouring.
The soil samples are analyzed for iodine and the concentrations are plotted on a base map and contoured. Iodine anomalies typically have a ring or halo pattern centered over the area of maximum seepage. The intensity of the anomaly is apparently related to the amount of petroleum seeping to the surface.
Radiometric mapping can be used to indicate subsurface oil accumulations because of the masking effect that hydrocarbon has on natural radiation emanating from basements. Most crystalline basement rock is radioactive to some degree, depending on the potassium, thorium and uranium content. Uranium is the most mobile of these elements, and as it migrates to the surface it creates low-level radiation nearly everywhere. The resulting background radiation can be detected and when mapped it generally shows random patterns of concentration.
The fully-oxidized hexavalent uranium ion released from basement rock is water soluble and highly mobile, and it tends to migrate vertically up through the groundwater towards the surface. If the radionuclides enter a reducing environment (due to the presence of hydrocarbons or other organic matter) the ion is reduced, becomes tetravalent, insoluble and immobile. Its upward movement ceases.
Radionuclides can also move laterally across an area with groundwater flow. If these waters enter a chemically reducing zone, the uranium precipitates at the zone's boundaries causing a high-radioactivity halo to form.
Subsurface hydrocarbon deposits cause a distortion in the random pattern of background radiation. They react to form complex uranium compounds, which are relatively stable and no longer migrate upwards. This results in non-random low-level radiation patterns over petroliferous terrain. Hydrocarbon accumulations are indicated on the surface by haloes of high radioactivity values surrounding central areas of lower-than-background values.
The U.S. Bureau of Mines investigated the role of mapping gamma radiation in petroleum exploration and concluded that it is an exploration tool that is worth considering. For a copy of their report obtain Information Circular 8579.
These methods do not reveal the size or depth of the oil trap, or even whether a trap exists at all. They do indicate minute quantities of hydrocarbon seeping upwards from source rocks or traps, and thus they can separate oil-bearing areas from sterile zones.
In contrast, seismic lines, when properly processed and interpreted on time-structure maps, can reveal structures that could be oil and gas traps but do not imply the presence of oil at depth. As such, the two kinds of exploration tools complement each other. Seismic surveys are much more expensive, it is usually best to run near-surface geochemistry surveys first, and use the results of these surveys to plan seismic surveys.
Surface-geochemistry maps can effectively limit the size of areas to be shot seismically and help focus further exploration efforts, with significant savings in the cost of seismic acquisition and processing.
These methods and others are described in the proceedings of the annual symposium: Unconventional Methods in Exploration for Petroleum and Natural Gas, held at Southern Methodist University in Dallas.