Iodine data help focus Williston Red River search

Steven A. Tedesco
Atoka Geochemical Services Corp.
Englewood, Colo.

The Red River reservoirs of the Williston basin have been the subject of intensive exploration the past few years in southern Saskatchewan, eastern Montana, and western North Dakota (Fig. 1 [79,323 bytes]). The reservoirs are of two types: the conventional stratigraphic and structural reservoirs that are typically one to five wells and the basin centered play targeting the Red River "B" that is found in the southern part of the Williston basin. The Red River "B" is a marginally economic engineering play that requires minimal geologic exploration in defining the target and will not be discussed here.

The historical approach for the conventional Red River reservoirs has been to explore in areas where existing production is present, offset wells with shows, project structural highs into undrilled areas, and/or use 2D or 3D seismic to define new areas.

Recently, the use of 3D seismic has become more prevalent, and the success rates of finding Red River structural highs are increased. However, the production from these new discoveries is highly variable, and in many cases the reservoirs tend to be either dry or marginal.

The use of surface geochemistry provides another tool or filter in evaluating areas for Red River production by being more site specific for seismic and also locating the more productive part of the reservoir. Red River structural highs that do not have an associated surface geochemical anomaly, where surveyed, have either been dry or sub-economic.

Surface geochemistry

Surface geochemistry is based on the concept of vertical migration. Petroleum migrates from a reservoir through the cap rock into the overlying rock strata via microfractures, micro-unconformities and micropores. The mechanism of vertical migration is not clearly understood, and most of the evidence for the concept is based on empirical data.

Petroleum migrates into the soil section and causes a variety of chemical and biological changes. Some of the more common changes are the presence of petroleum and sulfur consuming bacteria, sulfur compounds, Eh, pH, increases in iodine, changes in radioactivity, increases in trace and major metals, and increases in carbonates as well as the presence of petroleum gases.

Some methods of collection of surface geochemical data can be arduous and difficult tasks while other methods can be very easy. Determining which method to use is typically based on what is repeatable and reliable and sometimes personal preference.

Some methods such as radiometries, microbial, Eh, and pH are dependent upon soil chemistry, moisture variations, and the time of the year. This can limit their application and reliability. Soil gas methods have been used extensively in the basin and have a good track record depending upon the contractor. However, soil gas acquisition is limited from April to October and when the ground is not frozen or water saturated.

Iodine surface geochemistry provides an excellent repeatable and reliable tool in surveying large areas as a first pass filter. Iodine sampling is neither limited to time of year nor related to soil chemistry and does not require complicated methods of collection. Iodine increases are the result of the presence of petroleum. Decaying organic matter in the soil seems to be unable to accumulate iodine in any significant amounts. Data interpretation is straightforward and can either be contoured or presented as a posted map.

The data do not require integration of several dependent values, pattern recognition techniques, and/or consideration of several independent variables in relation to each sample site. Most surface geochemical methods have been applied in the Williston basin with varying success and failure.

Red River geology

Four recognized geologic cycles have been defined in the Red River formation and in ascending stratigraphic order are: D, C, B and A (Fig. 2 [45,127 bytes]) as adapted from Hunter and Schalla, 1995.¹ Correlation and identification of each cycle across the basin can be difficult, and specific identification can vary from geologist to geologist.

The Red River strata were deposited in subtidal, supratidal, intertidal, and marsh environments. The cycles represent a series of alternating limestones and dolomites capped by anhydrites. The reservoirs are defined by growth structures that can and do exhibit structural reactivation through time.

Reservoirs that are bound by faults have more fracturing and subsequently seem to have better production. The reservoirs are characterized by rapid changes in porosity and permeability, which affects production. The apexes of the structures are not necessarily productive, and production can be found on the flanks of highs.

Field examples

Skjermo field was discovered in 1982 and produces from the Red River "C" at approximately 10,500 ft in 163n-102w, Divide County, N.D. (Fig. 1).

The field has produced approximately 490,000 bbl of oil and 500,000 bbl of water from four wells as of 1996. The field is three separate Red River reservoirs in 9, 20, 27, and 28-163n-102w.

An iodine survey was done over the reservoir in 27 and 28 (Fig. 3 [157,663 bytes]). The background iodine data vary from 0.1 to 3.0 ppm, and anomalies start at 3.5 ppm and are as high as 10.5 ppm.

The map also shows porosity feet of the productive Red River "C." Using a cutoff of 6%, there are 12 ft in the well in Sec. 28 and 14 ft in the well in Sec. 27. The well in NW NW 20 had less than 7 ft of porosity and was a poor producer. No iodine anomaly is associated with this well, suggesting that reservoir has been effectively drained. Structure on the Red River is indicated, and the contour interval is 20 ft.

The well in SW NE 29 tested the Mississippian. The iodine anomaly indicates a strong signature despite the age of the field. The Red River reservoir in this part of the basin is typified by low permeability and partially plugged pore throats.

The presence of such a strong anomaly would indicate the reservoir is not being effectively drained. This is consistent with work done by Tedesco² indicating that when a reservoir is effectively drained, within 5 years the surface manifestations of increase in iodine, changes in Eh, pH, petroleum consuming bacteria, etc., will be gone.

Cold Turkey field in 130n-102w and 103w, Bowman County, produces from approximately 9,600-9,700 ft (Fig. 1). The field is again several separate Red River reservoirs; specifically this article deals with wells in 22 and 27-130n-102w.

The iodine survey (Fig. 4 [105,101 bytes]) and the 2D seismic lines were acquired by different companies prior to drilling. The iodine survey indicates the presence of an anomaly prior to drilling that overlies a thinning from the Red River to the Interlake (Fig. 5 [99,116 bytes]). Subsequently, three wells have been drilled and completed in the Red River.

The background iodine varies from 0.3 to 2.0 ppm, and anomalies start at 2.5 ppm. There is a strong iodine anomaly associated with the structural high and subsequent stratigraphic thin. The producing wells that are present, but have no associated iodine anomalies, were drilled years before the iodine survey was acquired.

An unique aspect of surface geochemistry is that after discovery, migrating petroleum gases diminish through time, once production has been established. The changes the petroleum caused in the soil also disappear as less and less hydrocarbons migrate up from depths. The current concept of why this is happening is that as reservoirs are depressurized by production, the hydrocarbons cannot exceed the capillary pressure to enter the cap rock and migrate upwards. The function of how quickly this phenomenon occurs is a function of how effectively the reservoir is being drained.

The Canadian Coastal well is in 33-13-20w2, southwestern Saskatchewan (Fig. 1) and was drilled near the end of 1997 based on a structural high identified on a 2D seismic line. The iodine survey was carried out along existing roads and utilizes a classed post format (Fig. 6 [43,806 bytes]). The survey clearly indicates the presence of anomalies associated with the well. No subsurface data are available, and the area is presently under development.

The Talisman Bures well is in 9-21w2, Saskatchewan (Fig. 1) and targeted the Red River based on seismic defining a structural high. The iodine survey was done prior to drilling and indicates background data (Fig. 7 [72,451 bytes]). The well may have encountered minor shows and was plugged and abandoned.

Conclusion

The use of iodine surface geochemistry can provide an additional exploration method to focus seismic surveys and leasing programs more effectively in targeting the Red River reservoirs. This in turn minimizes exploration costs and increases success rates and reduces and-or eliminates the number of dry holes that are drilled.

The proper implementation of iodine surface geochemistry is in utilizing grids with an idea, prior to collection, of the areal extent of the project. In addition, the iodine, as with most surface geochemical methods, is not specific to any reservoir but merely indicates the presence of petroleum in the soil.

References

Hunter, L.D.V. and R.A. Schalla, Stratigraphic column on inside cover in the 7th International Williston Basin Symposium, Montana Geological Society, 1995, 445p.
Tedesco, S.A., Iodine geochemistry reduces risk, American Oil & Gas Reporter, September 1997, pp. 105-108.

The Author

Steven A. Tedesco is owner of Atoka Geochemical Services Corp. He worked for Mobil Oil Corp., Shell Oil Co., and several independent oil and mining companies and has more than 20 years of experience with surface geochemistry in exploration for natural resources. He is author of Surface Geochemistry in Petroleum Exploration, Chapman & Hall, 1994. He has a BS degree from Northeastern University and an MS degree from Southern Illinois University.