LeSchack, L. A., and D. R. Van Alstine, 2002, in Surface exploration case histories: Applications of geochemistry, magnetics, and remote sensing, D. Schumacher and L. A. LeSchack, eds., AAPG Studies in Geology No. 48 and SEG Geophysical References Series No. 11, p. 67–156.

High-resolution ground-magnetic (HRGM) and radiometric surveys for hydrocarbon exploration: Six case histories in Western Canada

L. A. LeSchack, Hectori Inc.
D. R. Van Alstine, Applied Paleomagnetics, Inc.

In Western Canada, and probably elsewhere around the world, “magnetically enhanced zones” above microseeping hydrocarbon reservoirs can exhibit distinctive magnetic signatures that are characteristic of the reservoir. These distinctive magnetic signatures have proven to be invaluable for hydrocarbon exploration, and we have achieved 85% exploration success using ground-based magnetic and radiometric techniques in Western Canada. Differences in timing and duration of microseepage and differences in composition and pressure of the microseeping hydrocarbon gases from separate petroleum systems probably control the magnetic mineralogy, magnetic grain-size distributions, magnetic susceptibility, and natural remanent magnetization (NRM) directions in the magnetically enhanced zones. Together, these differences can yield diagnostic “residual” (remanent + induced) short-spatial-wavelength magnetic anomalies above different reservoirs.

Whereas our magnetic surveys are measuring fossil anomalies at depths of about 150 m, our radiometric surveys are measuring modern geochemical alterations at depths <25 cm. Thus, finding both magnetic and radiometric anomalies at the same location implies not only that a microseeping hydrocarbon reservoir once existed below, but also that it is still there and still leaking. In this study, we present six case histories from Western Canada in which our combined magnetic and radiometric surveys were effective for hydrocarbon exploration.

Our high-resolution ground-magnetic (HRGM) surveys have sufficiently high resolution that residual magnetic anomalies commonly appear to be dipolar in Western Canada. Nearly equal intensities for the positive and negative lobes of the anomalies, and major departure of the dipole axes from present magnetic north, imply that (1) about half the intensity of the residual anomalies represents remanent, rather than induced, magnetization; and (2) a significant proportion of the remanent magnetization is “reversed polarity” and hence is older than the most recent geomagnetic reversal at 0.78 Ma.

In the prolific Devonian reservoirs of Western Canada, much of the reversed-polarity magnetization probably dates from a strong “reversed-polarity-bias interval” that prevailed during the early Tertiary, from 63 to 41 Ma. At that time, generation of hydrocarbons, rapid subsidence, and the regional topographic hydrodynamic drive created high pore pressures that facilitated regional vertical fracturing of the Laramide foreland. Above reservoirs where oil was trapped during early Tertiary migration, buoyant hydrocarbon microbubbles began to rise along the regional, vertical microfractures. At higher structural levels, the microseeping hydrocarbons caused magnetic minerals to precipitate (by inorganic and/or biogenic processes) in magnetically enhanced zones, thereby recording early Tertiary, reversed-polarity remanent magnetization. Later in the Tertiary, a second generation of magnetically enhanced zones probably was created after maximum burial, at peak overpressure, and when methane began to exsolve by pressure reduction during isostatic uplift.

In Western Canada, the strongest HRGM anomalies occur above the deepest, most prolific reservoirs at the highest pressures, and the weakest HRGM anomalies occur above shallower, less-productive reservoirs at lower pressures. In the Alberta Basin, the HRGM anomaly intensity decreases monotonically, from highest values over prolific Leduc Formation (Upper Devonian) pinnacle-reef reservoirs, to somewhat lower values over Nisku Formation (Upper Devonian) biostrome reservoirs, to still lower values over less-productive Cretaceous blanket/channel-sand reservoirs, to lowest values over dry and abandoned (D&A) wells. In the Williston Basin, strong HRGM anomalies occur above Mission Canyon Formation (Mississippian) limestone cuesta reservoirs, whereas no HRGM anomalies (only radiometric anomalies) occur above shallower lower Amaranth Formation (Triassic?) channel-sand reservoirs. The stronger HRGM anomalies above the deeper Devonian and Mississippian reservoirs may reflect (1) higher concentrations of authigenic magnetic minerals in the magnetically enhanced zones; (2) more focusing of vertically ascending microbubbles by the more nearly point-source pinnacle reef and cuesta reservoirs, compared with more spatially diffuse blanket/ channel-sand reservoirs; and (3) shallower depths of magnetically enhanced zones as a result of higher pressure within the deeper reservoirs.

For hydrocarbon exploration, the distinctive magnetic signatures revealed by high-resolution ground-magnetic surveys have an important practical application: We find that the HRGM anomaly intensity and the residual magnetic-anomaly azimuth can identify the reservoir that is causing the anomaly. We illustrate this principle in three case histories in the Williston Basin and three case histories in the Alberta Basin. Although all six of these case histories are from Western Canada, ground-magnetic surveys would probably be equally successful worldwide, especially where hydrocarbon microseepage has occurred during the Tertiary (65 to 1.8 Ma), when the geomagnetic field exhibited reversed-polarity bias.

Case histories 1 and 2 document three new oil-field discoveries, based on magnetic and/or radiometric anomalies over lower Amaranth and Mission Canyon reservoirs near Pierson, Manitoba. Case history 3, at the Waskada field, Manitoba, is an after-drilling study, in which the HRGM survey delineates Mission Canyon limestone reservoirs and the radiometric survey delineates productive channels in the overlying lower Amaranth sand. Case history 4, another after-drilling study, documents that an HRGM survey and a 3-D seismic survey are equally effective in targeting a Leduc pinnacle reef at the Rumsey field, Alberta. Case histories 5 and 6 cover 10⁴ ha in central Alberta, including 55 Cretaceous producers, 15 Nisku producers, and 22 abandoned wells. After-drilling comparison of the magnetic data with the production data reveals that the HRGM surveys could have been used to predict the producers and to avoid the dry holes.

Statistical comparisons of high-resolution ground-magnetic (HRGM) with high-resolution aeromagnetic (HRAM) data and verification with ground data of a specific HRAM anomaly in central Alberta reveal that airborne and ground-magnetic surveys can be used together, cost-effectively, for hydrocarbon exploration. Reconnaissance HRAM surveys are especially useful in targeting prospects for further, more-detailed evaluation by HRGM/radiometric surveys. In Western Canada, combined HRGM and concurrent radiometric surveys have been highly successful in finding hydrocarbons, and the total cost, including permitting, is about 20% the cost of a 3-D seismic survey over the same area. These surveys complement traditional exploration methods, substantially reduce finding costs, and significantly increase the probability of exploration success.