HOW IODINE SURVEYS HELP LOCATE S.E. COLORADO MORROW RESERVOIRS

Steven A. Tedesco, Chuck K. Goudge
Atoka Exploration Corp.
Denver

Surface geochemistry has become a more successful and widely accepted exploration tool through better interpretation, more sensitive analytical equipment, and more detailed surveying methods.

The results of using iodine surface geochemistry in the search for Pennsylvanian Morrow sandstone reservoirs along the Las Animas arch indicates it is a useful tool to provide more focused target areas for seismic and thus for reducing exploration costs.

This article will present the use of iodine surface geochemistry supplemented by soil gas surveys in the search for Morrow channels in southeastern Colorado.

SURFACE GEOCHEMISTRY

Iodine is a halogen and shows a strong affinity for hydrocarbons.1 2 3 4 Surface geochemistry was aggressively pursued in the former U.S.S.R., which developed numerous geochemical techniques including the iodine method.

Vertically migrating hydrocarbons found in soil above a petroleum accumulation concentrate iodine in anomalous amounts. This reaction is very unique and very specific.

Migrating hydrocarbons are very different from the natural organic matter (humic substances) that occurs in differing amounts in all soils.

These humic molecules are continually exposed to oxygen which, being a more effective oxidizer than iodine, replaces all the available sites that iodine might fill.

This is supported by empirical evidence that shows little correlation between soil humic substances and iodine content. Surveys in diverse environments (ranging from the rain forests of Belize and Indonesia, highly organic soils such as in northern Michigan and Louisiana, and desert environments like Arizona and Nevada) indicate that all have similar background soil iodine.

The specificity of this reaction makes iodine the ideal tool for tracking petroleum hydrocarbons. Iodine is a natural integrator of hydrocarbon seepage, and it does not exhibit the short-term fluctuations related to atmospheric pressure, moisture, and temperature that plague soil gas methods. For more details on iodine see Tedesco 1994.5

CASE HISTORIES

The eastern Colorado Morrow reservoirs are a part of an elusive valley channel-fill system of Pennsylvanian age (Fig. 1).

The valley system was developed upon beveled Mississippian strata and extends southward from the eastern edge of the Denver basin. It crosses the Las Animas arch and continues into the Anadarko basin.

There are numerous fluvial systems within the Morrow, but it has been divided into lower, middle, and upper sandstone units. Not all of the channel sandstones lie in advantageous positions to accumulate hydrocarbons. Seismic has difficulty identifying the Morrow sandstone, and its current application is to locate the Morrow channel scouring of the Mississippian surface.

However, the upper and middle sands may not display a channel scour on the Mississippian and therefore are difficult to identify with seismic. The play in southeastern Colorado and southwestern Kansas has less than a 2% wildcat success rate using seismic.

Jace field was discovered in July 1989 with the drilling of the 1A Leathers, in NE NE 1-18s-42w, Kiowa County, which pumped 161 b/d of oil with 25 Mcfd of gas.

Immediately after the discovery well was announced, iodine and soil gas surveys were conducted (Figs. 2, 3). The iodine survey has a background average of 1.0 ppm, and anomalous values start at 1.6 ppm.

The soil gas (pentane) survey correlates well with the iodine survey, and values are in parts per billion. The anomalous area in the southwest was not drilled until 1991 by Savant Resources. The initial well was put on pump for 300 b/d of oil.

Fig. 4 shows the wells drilled since discovery until December 1993, and a strong correlation to both the iodine survey and the hydrocarbon survey is apparent. In 1993 a second iodine survey and a second soil gas survey were conducted across Jace field (Figs. 5, 6). Both surveys indicate that the hydrocarbon seepage is diminishing as production proceeds. These "real time" changes in hydrocarbon flux have been observed by many geochemical contractors.

Recent theoretical calculations by Klusman6 and Saeed7 support a real time relationship based on a computer model, but indicate a longer time (10 years from source to surface) than empirical data suggest. For the explorationist this "real time" loss of signal means that old anomalies should not interfere with surveys attempting to find undiscovered reserves.

Moore-Johnson field was discovered in October 1989 with the drilling of the 1 Moore-Johnson, in N 1/2 NE NW 14-18s-43w, Greeley County, Kan. It swabbed 522 b/d of oil (Fig. 1).

An iodine survey was conducted in this area a short time after the discovery well (Fig. 7). The survey indicates the iodine anomalies correlate reasonably well with existing production. The survey was controlled by the inability to obtain access to all lands. Thus, a reasonably spaced grid was not obtainable at that time.

In summer 1993 a second survey partially resampled the first survey and covered sections to the north and south (Fig. 8). Again the survey correlates well with existing production. The second survey indicates the preferred form of grid for these types of reservoirs: 1/10 mile spacing to provide as close a target location for seismic as possible.

CONCLUSIONS

Surface geochemistry has become an essential tool in the search for difficult to find stratigraphic accumulations.

Iodine geochemical surveys integrated with a targeted seismic program can greatly reduce the finding costs for Morrow reservoirs. There are many Morrow channels that do not contain hydrocarbons and therefore would not have a surface geochemical expression even though they may be identified seismicly.

These areas where seepage is not present could be avoided which would eliminate some of the dry holes. The areas where microseepage is occurring would focus an exploration program's seismic and drilling efforts.

Microseepage as measured at the surface cannot identify economic or subeconomic petroleum reservoirs but merely provides a target for seismic. The best use of surface geochemistry is to eliminate those areas that have no chance of production (no microseepage) and focus on areas that do (microseepage present).

This article has also demonstrated that as fields are depleted by production through time, their surface expression diminishes. This indicates that if surveying is done at a later date in a productive area, any intense anomalies found probably represent undiscovered reservoirs rather than existing production.

The disappearance of the surface anomaly in conjunction with depletion suggests that it is not wise to use old fields as models in surface geochemical exploration.

REFERENCES

Tedesco, S.A., Exploring for dolomite chimney reservoirs using iodine surface geochemistry, Ontario Petroleum Institute Proceedings, London, Ont., Vol. 32, 1993 (in press).
Alexan, S., Fausnaugh, J., Goudge, C., and Tedesco, S., The use of iodine in geochemical exploration for hydrocarbons, Association of Petroleum Geochemical Explorationists Bull., Vol. II, No. 1, 1986, pp. 71-93.
Gallagher, A.V., Iodine: a pathfinder for petroleum deposits, in Unconventional Methods in Exploration for Petroleum and Natural Gas, Southern Methodist University Press, Dallas, 1984, pp. 148-159.
Kudel'sky, A.V., Prediction of oil and gas prospects, Geologiya Nefti i Gaza, No. 4, 1977, pp. 45-49.
Tedesco, S.A., Surface geochemistry in petroleum exploration, Chapman and Hall, New York, 1994 (in press).
Klusman, R.W., Soil gas and related methods for natural resource exploration, John Wiley & Sons, New York, 1993, 488 p.
Saeed, M., Light hydrocarbon microseepage mechanism(s): theoretical considerations, master's thesis, Colorado School of Mines, Golden, Colo., 1991, 93 p.

BIBLIOGRAPHY

Wheeler, D.M., Scott, A.J., Coringrato, V.J., and Devinee, P.E., Stratigraphy and depositional history of the Morrow formation, Southeast Colorado and Southwest Kansas, in Sonnenberg, S.A., Shannon, L.T., Rader, K., von Drehle, W.F., and Martin, G.W., eds., Morrow sandstones of Southeast Colorado and adjacent area, Rocky Mountain Association of Geologists, 1990, pp. 1-8.