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SOIL GAS SURVEYS - INTERPRETATION OF ANOMALIES


Modern geochemical methods, such as Petro-Find soil gas surveys, are important tools in the search for hydrocarbon reservoirs because they are direct indicators of hydrocarbons. Because no reservoir seal is perfect, all hydrocarbon reservoirs leak light hydrocarbons as microseeps to the surface. These microseeps travel vertically to the surface where they can be sampled and analyzed by the gas chromatograph. Concentration patterns or surface anomalies of these microseeps can be reliably related to petroleum/gas accumulations at depth. While individual hydrocarbon concentrations are important, the ratios of various light hydrocarbons are also necessary for a thorough soil gas evaluation.

As shown by the accompanying table, two ratios and percent methane of total light hydrocarbons provide the main bases for reservoir content predictions i.e. whether anomalies represent oil, gas condensate or natural gas. This scheme can also differentiate between biogenic and thermogenic gas on the basis of its wetness i.e. the amount of C2+ (total ethane, propane and butane concentrations). Some use the compositional ratio C1/C2+C3 as a measure of wetness. Biogenic gas is very dry (i.e. consists entirely of methane). In contrast, thermogenic gas can be dry, or can contain significant amounts of wet gas components. Methane stable carbon isotope analysis provides the only means to differentiate between biogenic and thermogenic dry gas.  per cent methane and light alkane ratios determines type of reservoir anomaly represents

It should be noted that absolute boundaries indicated in the table may vary between regions due to factors such as local geology, source type and thermal maturity. Condensate is in a gas phase in the reservoir and in a liquid phase at the surface. Stacked reservoirs will result in more complex anomalous patterns including overlapping.

The vertical migration can be short-circuited by faults, which can cause the anomaly or surface projection of the oil reservoir to shift laterally towards the fault trace. The question of lateral versus vertical migration is very important to the interpretation of geochemical data. Our present knowledge indicates that both exist. The extent of either depends on the geometry of the sediments and the tectonics. Empirical data indicate that in most places here is enough vertical permeability for a seep to exist directly over the reservoir. However, some lateral migration (particularly near the surface) generally occurs, so the shape and location of the surface anomaly will not exactly match that of the prospective reservoir.

Of major importance to the explorationist is the surface location and pattern of the anomalous areas. From experience, scattered data points are best contoured with a triangular, non-gridded computer program such as provided by Scientific Computer Applications, Inc, Tulsa, Oklahoma (http://www.scaitul.com/SCA-Contour-Mapping.aspx).  correlation of seismic structures and geochem anomalies

The identification by soil sampling of areas with little or no potential also provides very useful information for the explorationist. Although soil gas surveys are the most useful when they provide drilling targets, the elimination of areas with little or no potential is important to decisions on e.g. acquiring and disposing of leases; and the early re-deployment of financial and human resources to areas with the greatest exploration potential.

Once a reservoir begins production, the near-surface anomaly decreases in intensity near a well head because micro-seeps migrate more easily to the well bore instead of vertically through the subsurface. The amount and timing of decreasing intensity depend, to a large extent, on the type of reservoir drive and the rate of production.  correlation of seismic structures and geochem anomalies

Petro-Find soil gas surveys are used in conjunction with seismic surveys to identify drilling targets. However, ideally soil gas surveys should be conducted prior to seismic because of their relatively low cost and ability to rule out at an early stage areas of little interest. Seismic surveys can find structures that appear to be excellent traps but are most often found to be dry when drilled. Dry holes are usually due to either lack of charging or breaching of previously charged reservoirs allowing hydrocarbons to escape. Another main advantage of geochemistry over seismic is that it is not limited by the type of trap in which the hydrocarbons have accumulated. Soil gas surveys are especially useful in prospecting for stratigraphic pools, which are not associated with easily discernable structural features.

Depicted in the accompanying picture is a correlation of 2D seismic structures with geochemical anomalies. The drilling of a major seismic structure that was coincident with a C2+ anomaly discovered a stacked petroleum reservoir. A low-grade anomaly with a low-grade structure resulted in a marginal producer. A well drilled on a medium-grade seismic structure with no anomaly was found to be dry.

Petro-Find soil gas surveys are used in conjunction with seismic surveys to identify drilling targets. However, ideally soil gas surveys should be conducted prior to seismic because of their relatively low cost and ability to rule out at an early stage areas of little interest. Seismic surveys can find structures that appear to be excellent traps but are most often found to be dry when drilled. Dry holes are usually due to either lack of charging or breaching of previously charged reservoirs allowing hydrocarbons to escape. Another main advantage of geochemistry over seismic is that it is not limited by the type of trap in which the hydrocarbons have accumulated. Soil gas surveys are especially useful in prospecting for stratigraphic pools, which are not associated with easily discernable structural features.  changes carbon isotope ratios with thermal maturation

Soil gas surveys can predict whether a petroleum basin is gas- or oil- prone by the C2+ or the gas wetness, which is the proportion of hydrocarbons heavier than methane. Such surveys can also determine from compositional ratios whether the light hydrocarbons represent oil, gas condensate or natural gas reservoirs. If methane correlates with ethane in a linear fashion, the methane is thermogenic because ethane is almost always thermogenic. If ethane is found to be absent the gas is considered dry but it is not known whether the methane is biogenic or thermogenic because thermal cracking of oil at high temperatures can also produce a dry gas. A poor correlation indicates a mixing of dry and wet gas. Thus compositional ratios cannot differentiate between biogenic dry gas and thermogenic dry gas. This is the role of methane stable isotopes. "…gases with similar ratios, regardless of absolute value, can indicate genetically related families of gases …." (Source: Dietmar Schumacher; Soil gas Hydrocarbons and their Exploration Significance; Geo-Microbial Technologies, Inc.)

The fact that soil gas survey samples contain hydrocarbons in gaseous form allows the explorationist to differentiate between biogenic and thermogenic dry gases by cross-plots of carbon and hydrogen stable isotopes in methane versus composition and compositional ratios. Two distinct processes-biogenic and thermogenic - produce three types of gas:

Knowing whether the soil gas is either biogenic or thermogenic has major implications for the presence or absence of liquid hydrocarbons in a basin. If the gas is found to be bacterial in origin it is unrelated to whether or not petroleum could be present in the basin. If thermogenic in origin, different interpretations come into play.

Basically, surface geochemical prospecting is a source rock tool applied at the surface. This technology can provide information on the maturity of source beds in a basin as well as the composition and patterns of subsurface hydrocarbons. When used in conjunction with geophysical and geological information, geochemical data can refine subsurface models of hydrocarbon trapping and possible migration configurations.


PETRO-FIND GEOCHEM LTD

215 Mallin Crescent
Saskatoon, Saskatchewan, Canada S7K 7X3
Contact: Paul Lafleur, President
View Paul Lafleur's profile on LinkedIn
Phone: (306)931-3156 Fax: (306)931-9773
E-Mail: plafleur@sasktel.net