Anomaly shifts indicate rapid surface seep rates

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

Surface geochemistry in petroleum exploration relies on the process of vertical migration. One of the key elements in the theory of vertical migration of hydrocarbons is the rate at which petroleum migrates from the reservoir to surface.

The rate of hydrocarbon migration has been subject to debate for a number of years.

Several theories have been proposed that cannot adequately explain or quantify the mechanism of vertical migration.1 Some authors have postulated, based on computer modeling and general calculations, that the movement is slow, taking a minimum of 10 years to go only 1,000 ft.2 Others propose even longer migration rates of several hundred to thousands of years.

One would then argue that fossil surface geochemical anomalies should exist. These fossil anomalies are postulated to represent petroleum accumulations that temporarily remained static, then moved at some later date. However, a surface geochemical anomaly would have been established and remain in place regardless of the loss of the petroleum accumulation.

Conversely, recent unpublished work by both contractors and exploration companies suggests that the rate of migration is very rapid. This disparity led to an investigation of the surface geochemical data from several existing fields and the development of the case histories presented below.

Surface anomaly theory

The surface geochemical anomaly that exists above a petroleum reservoir in the soil can only be a temporary manifestation of chemical changes caused by the presence of hydrocarbons.

The hydrocarbons and their byproducts, even though they are causing a variety of chemical reactions due to their presence, are in turn subject to a variety of ongoing biological activity and weathering processes. These processes can quickly eliminate or modify the hydrocarbons or cause minor chemical changes in the soil. These changes can be quantified as variation in moisture content, erosion, temperature, biological activity, chemical changes, increase or decrease in organic acids, and barometric pressure changes that can effect the stability of these chemical modifications due to hydrocarbons in the soil.

Hydrocarbons can invariably be temporarily flushed from the soil, thus making detection difficult at times. Some of the byproducts caused by the presence of hydrocarbons, such as iodorganics, trace elements, and major metal and carbonate compounds are relatively stable. However, these compounds are dependent upon the continued presence of hydrocarbons to remain in anomalous quantities in the soils.

Other transient effects, such as petroleum eating bacteria, changes in Eh, pH, and conductivity are not always present and have other causes. The soil strata consists of a dynamic system which suggests that replenishment and migration of hydrocarbons from reservoir to the surface is a much more rapid and real-time event than previously defined in the petroleum exploration literature.

Case histories

During the past 14 years the author's organization has conducted surface geochemical studies over a broad range of environments, some of which subsequently became discoveries.

These studies involved reservoirs at depths of 400-9,600 ft in the U.S. and Canada. Several of the studies were long term and included yearly repeat surveys after production was established.

In addition, in 1988-89 the underground natural gas storage facility located near the town of Keota, Iowa, was sampled as part of an ongoing petroleum exploration program. Because the storage facility's reservoir is well defined, it provided an ideal model to test the concept of vertical migration. What was not expected at the time was the rapid variation in leakage over a year in relation to injection and withdrawal of natural gas.

Dolley field

Dolley oil and gas field, in 6n-61w, Weld County, Colo., was discovered in 1990 using iodine surface geochemistry and seismic that found a Cretaceous D sand reservoir at 6,850 ft.3 The pre-discovery iodine surveys correlated with a seismically defined D sand channel and delta mouth bar system ( Fig. 1A [241,538 bytes]).

Subsequent drilling as of January 1994 demonstrated a close correlation with the iodine anomaly (Fig. 1B). The field was sampled several times over an 8 year period, and an iodine survey was carried out in 1994 (Fig. 1C).

The anomalous area that existed previously (Fig. 1B) has disappeared in several parts of the survey. A survey in early 1998 (Fig. 1D) indicates most of the anomalies seen earlier (Fig. 1A) are gone, and production is presently at stripper level.

The author has seen this effect over several other fields through time. Note that the anomalous area in the northeast quarter of Sec. 28 has remained, suggesting part of the reservoir has not been efficiently drained and may be an isolated reservoir.

Keota dome

The Keota Natural Gas Storage Facility in Keota, Washington County, Iowa, stores gas in the St. Peter sandstone at approximately 1,000 ft defined as a northwest trending anticlinal structure.

The dome or anticline produced minor amounts of oil in 1963 from the overlying Petaconica and McGregor carbonate formations before the reservoir was converted to gas storage. All three formations are Middle Ordovician in age.

Based on the limited oil production from the Keota Dome, a group of petroleum companies undertook an active exploration program in the area from 1985-91. This led to the drilling of several test wells and the completion of one producer. During the course of exploration the gas storage field was surveyed as a model for surface geochemical exploration.

Keota gas storage field encompasses 2,500 acres and covers most of the mapped anticline. The area of potential oil production covered less than 100 acres as based on evaluation of logs and geologic samples.

The gas is injected into the dome in summer through fall for winter withdrawal. The Keota dome was first sampled in the summer of 1985 using the iodine method4 and indicated remnant anomalous values indicative of a depleted reservoir.

Iodine and soil gas samples were collected over a 9 month period from July 1988 to April 1989. The soil gas method used was headspace, and samples were acquired from a depth of 3-4 ft.

The percentage of ethane at each sample location was measured on four occasions (Fig. 2 [203,032 bytes]). Only a few minor anomalies were present in July 1988, but sampling in October 1988 showed a large anomalous population of total ethane as a percentage of the total gas that reflected the injection of gas into the storage reservoir.

Similar anomalous conditions also existed in January 1989. However, the anomalous conditions were essentially gone as of April 1989 reflecting the drawdown of the reservoir by spring due to withdrawal of gas for winter sale.

The iodine results presented in Fig. 3 [200,608 bytes] exhibit similar characteristics to the soil gas data. Iodine indicates a similar rapid change to the presence or absence of petroleum gases and their byproducts in the soil. The absolute rate of migration is unknown, but it can be construed to be rapid.

Dramatic changes in the presence of petroleum gases and iodine in the soil in relation to a gas storage facility were recently confirmed in a lawsuit by a private landowner against Public Service Co. of Colorado's Leyden Natural Gas Storage Facility southwest of Boulder in Jefferson County, Colo.

It was found that the Leyden facility, which stores gas in abandoned coal mine workings at 800-1,000 ft, had caused natural gas contamination on adjacent properties due to the lateral migration of gas during injection that was followed by vertical migration of gas to the surface.

The presence or absence of gas on the adjacent property was directly related to pressure and amounts of gas present in the storage facility. The actual rate of migration both laterally and vertically is still not quantified and requires more study.

Conclusion

The data as presented indicate the rapid nature of vertical migration and how "real-time" the process is.

In addition, it becomes clear that sampling of "old fields" as models in an exploration program is not a valid tool. Therefore, models that are developed prior to discovery are critical to a successful exploration program.

The rate and mechanism of vertically migrating hydrocarbons is still not specified, and more work is needed. However, previous computer modeling and academic calculations obviously are too conservative and have not included a number of variables and mechanisms that have a clear impact upon the process of vertical migration.

Understanding that the development of surface geochemical anomalies is rapid, definitive, time sensitive, and subject to weathering and biological processes will help in the development of a more accurate and effective exploration tool.

References

1. Tedesco, S.A., Surface geochemistry in petroleum exploration, Chapman and Hall, 1994, 206 p.
2. Klusman, R., Soil gas and related methods for natural resource exploration, John Wiley & Sons, 1993, 483 p.
3. Hart Publications, Lariat partners make knowledge of basin pay, Western Oil World, 1991.
4. Allexan, S., Fausnaugh, J., Goudge, C., and Tedesco, S., The use of iodine in geochemical exploration for hydrocarbons, APGE Bull., Vol. 2, No. 1, 1986, pp. 71-93.

The Author