Case Studies - Oil & Gas Exploration

Definition of Hydrocarbon Alteration Plumes using ALTREX, Denver Julesburg Basin, Colorado, U.S.A.

Recent work has shown that in some cases hydrocarbon reservoirs are overlain by an associated alteration plume. In some cases, the alteration plume has been shown to have anomalous magnetic susceptibilities, as documented by Machel and Burton, (1991) and by Reynolds et al., (1991). The effect of man-made culture (wells, pipes) can also be great, e.g., in the Cement oil field (Reynolds et al., 1991). Electrical and induced polarisation (IP) measurements have also been discussed (Sternberg, 1991), with the strongest anomalies occurring where the near-surface rocks are porous and iron rich. Again culture (fences, pipes and well casings) can be significant (Holladay and West, 1984). Electrical methods are sensitive to conductivity, as are electromagnetic (EM) methods.

Fugro Airborne Surveys, has performed a number of airborne transient electromagnetic (GEOTEM) surveys over oil fields with known alteration plumes. The surveys were flown to test de hypothesis that airborne EM methods can detect alteration in cases when the conductivity of the altered material varies from background.

The ALTREX System

The ALTREX airborne system is installed in a CASA-212 STOL aircraft (Figure 1) and consists of a cesium vapour magnetometer and a GEOTEM TEM (Time-domain ElectroMagnetic) system (Smith et al., 1996), comprising a large vertical-axis three-turn transmitter loop generating a dipole moment of approximately 450,000 Am². The digital EM receiver is an induction coil which is housed in a towed bird. The surveys were designed with flight lines oriented parallel to the larger dimension of the oil field at 500 metre intervals and with tie lines planned at 3,000 metre intervals. The areas were sufficiently large to sample the region surrounding the oil field.

Figure 1: CASA C-212 survey aircraft.

Geochemical Model

Hydrocarbons dissolved in water are continuously moved to the surface due to basin pumping. The water in the vicinity of a hydrocarbon reservoir is reducing (oxidation-reduction potential) relative to other waters in the basin. This water reacts with the rock-water environment to create alteration features that can be mapped by geophysical methods.

Geophysical Target

Typical alteration plumes have a carbonate-silica cap (0-30 metres) close to the surface. Below this conductivity low is a much more electrically conductive unit. The low conductivity at the surface is caused by carbonate and silica deposited in the pore spaces while the high conductivity underneath is caused by clay alteration within the plume. Diagenetic magnetite may also be formed in the alteration zone. To be detected, the respective anomalous responses must be significantly different from background.

Field Example- Pollen Field, Colorado 

Overview

The Pollen Field, as described by Ziegler (1983), is located in the south-central portion of the Denver Julesburg Basin approximately 3 kilometres south of Strasburg in Arapahoe County, Colorado. Production, at approximately 2500 metres, is from a J sandstone marine-bar complex which is quite extensive in the area. The same complex produces at the Quill and Fairway Fields. The conductivity (tau) image identifies a series of approximately north-south trending conductivity highs that correspond extremely well with oil production as indicated by the well control information. It is very interesting to note that there is an extension of the conductivity high north of the Pollen field, which indicates the alteration plume extends in this direction. Other excellent associations are noted corresponding with existing well control and other leads are also visible in the data. In this area the aeromagnetic data failed to show a correlation with the hydrocarbon production. In other areas, there is a better correlation.

Conclusions

Airborne TEM data can effectively map surface conductivity contrasts and high-resolution aeromagnetic data can identify anomalous patterns which may be related to hydrocarbon alteration plumes. The airborne application will be successful where the conductivity and/or magnetic susceptibility are anomalous. The advantage of airborne geophysical techniques is that large areas can be covered increasing the possibility of traversing an oil field with an alteration signature. As these geophysical methods are geological mapping tools it must be noted that other near-surface geological features will also be mapped which may not relate to an alteration plume.

References

Holladay, J.S., and West, G.F., 1984, Effect of well casings on surface electrical surveys: Geophysics, 49, 177-188.

Machel, H.G., and Burton, E.A., 1991, Chemical and microbial processes causing anomalous magnetisation in environments affected by hydrocarbon seepage: Geophysics, 56, 598-605.

Reynolds, R.L., Fishman, N.S., and Hudson, M.R., 1991, Sources of aeromagnetic anomalies over Cement oil field (Oklahoma), Simpson oil field (Alaska) and the Wyoming-ldaho-Utah thrust belt: Geophysics, 56, 606-617.

Smith, R.S., Annan, A.P., Lemieux, J., and Pedersen, R.N., 1996, Application of a modified GEOTEM system to reconnaissance exploration for kimberlites in the Point Lake area, NWT, Canada: Geophysics, 61, 82-92.

Sternberg, B.K., 1991, A review of some experience with the induced-polarisation/resistivity method for hydrocarbon surveys: Successes and limitations: Geophysics, 56, 1522-1532.

Ziegler, J.R., 1983, Pollen Field, in Crouch, M.C. 111, Ed., Oil and Gas fields of Colorado/Nebraska and adjacent areas 1982, Rocky Mountain Association of Geologists, 396-399.