It is well known that practically all hydrocarbon reservoirs leak to a certain extent. The microscopic particles of gas and fluid (called hydrocarbon micro-seeps) seep through micro-fractures in the earth's crust, generally in a nearly vertical path to the surface. The surface expression of hydrocarbon seeps, or anomalies, can be reliably related to a petroleum or gas accumulation at depth depending on the complexity of the geology and the interpretation of the geochemical results. Interpretations are more difficult in the case of multiple reservoirs and where structures such as faults re-direct the upward path. Correlation with seismic surveys is especially helpful in these situations.

The leakages of interest are primarily the alkanes (straight chain compounds) comprised mostly of the light gaseous hydrocarbons - Methane, Ethane, Propane and Butane. Pentane+ alkanes, which are liquid at the surface, are not normally measured because of inconsistent results.

For little explored areas, soil gas surveys are more cost-effective if conducted in a two-stage process - reconnaissance followed by high-density surveys. Follow-up high-density surveys define the anomalies in more detail and identify areas where the probability of drilling success is the highest. In areas where the light alkanes in soil gas are either absent or low, the probability of drilling success is considered nil. Higher values require interpretation - what do the anomalies mean? Can they be correlated with one or more Formations? The answer to these questions can be ascertained from either previous experience, seismic or follow-up drilling by the client. Once the anomalies are drilled, the strength of the anomalies can be correlated directly with the drilling results.

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.

The fact that the relative amount of alkanes in soil gas has the same relative composition as the gas in the underlying reservoir provides a reliable indicator of the type of reservoir (i.e. oil, gas condensate, thermogenic gas or biogenic gas). Characterization using light hydrocarbon ratios is the very first step in analyzing the data from soil gas surveys.

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.

Major Ethane plus (total Ethane, Propane and Butane) anomalies correlate with oil, thermogenic gas or gas condensate reservoirs directly underneath. Various ratios such as Methane/Ethane are used to differentiate between them. Petro-Find technology can detect even the smallest amount of light hydrocarbons in soil gas. As the intensity of the signal depends largely on the pressure of the reservoir, interpretation should take into account the type of reservoir drive such as: solution gas or depletion drive; gas cap drive; edge-water drive; and gravity.

High Methane concentrations in soil gas and absence of the other light alkanes indicate a biogenic gas reservoir at depth. Reconnaissance surveys can identify anomalous trends so that development drilling is more efficient in expanding a gas pool to its margins. Follow-up high-density surveys can define the edges or margins of pools with excellent precision. The establishment of margins provides valuable information to an industry that depends on offset wells to guide its exploration efforts in non-conventional pool areas. Once a reservoir is in production, the Methane concentration in the soil gas gradually decreases with depletion, which could take many years. It should be noted that in the immediate vicinity of a producing gas well, the Methane signal is less than for the reservoir as a whole because the gas is drawn horizontally to the well bore. Hint - don't sample too close to a well site.

The light alkanes in soil gas is the sum total of all the light alkanes emanating from all reservoirs directly underneath. Therefore, with the depletion of light alkanes in soil gas emanating from one reservoir, other more deeper reservoirs become apparent. High anomalous values remaining unaffected by production can reveal hidden or bypassed compartments, infill drilling possibilities, and multiple reservoirs.

Analog studies give a good idea on what can be expected from exploration over existing or developing oil pools. For example, the surface signal from a solution drive heavy oil reservoir is immediately affected when production starts. It shows a rapid decline to zero when the solution gas is fully depleted with a concomitant loss of pressure. In the mature part of a developing pool, some of the wells could be inactive by the time the pool is fully developed. The area of depletion as exhibited by low values moves in the direction of the newly developed part of the pool where an apical type of anomaly can be seen. It should be noted that the surface signal from an oil reservoir under water flood is similar to that for a reservoir with edge water drive.

Analog studies find their major application in pinpointing lost compartments, infill possibilities and multiple horizons potential.

PETRO-FIND GEOCHEM LTD
INNOVATION PLACE
L004-410 Downey Road
Saskatoon, Saskatchewan, Canada S7N 4N1
Phone: (306)931-3156 Fax: (306)931-9773
E-Mail: plafleur@sasktel.net