Abstract In southern Saskatchewan and Manitoba the interval between the late Paleozoic and late Jurassic epeirogenies is represented in stratigraphic order by continental red beds, evaporites, carbonates, and clastics, all of which are assigned to the Jurassic System. The sequence is in excess of 1,400 feet thick in south-central Saskatchewan and thins to the north, west, and east. Depositional and erosional thinning are complementary and the northern limits are determined by late Jurassic to post-Jurassic truncation. The system was divided (Milner and Thomas, 1954) into the Watrous, Gravelbourg, Shaunavon, and Vanguard formations, each representing widely recognizable lithologic units. Proof of Jurassic age is available for the Gravelbourg, the Shaunavon, and Vanguard formations, which appear to represent the Bajocian to Kimmeridgian time interval. The evaporites of the Watrous formation possibly represent the lower Bajocian or earlier Jurassic stages. A minor unconformity of Middle Jurassic age is indicated by the occurrence of chert on top of the marine carbonates of the Gravelbourg. The evidence for this unconformity is strongest in central and southeast Saskatchewan. There the anhydrite of the Watrous formation thickens markedly and the evaporitic conditions appear to have lasted longer than in western Saskatchewan. In the same area the marine sediments above the unconformity thin and gradually change facies. The carbonates to the west are substituted by clastics, and coarse clastics appear in increasing amounts in an easterly direction. It would appear that a separate basinal area existed toward the end of Middle Jurassic time in Manitoba, receiving clastic sediments from the north and east. The Shaunavon formation cannot be traced lithologically in this facies district, but approximately equivalent picks can be made from electric and radiation logs. The lithologic character of the uppermost Gravelbourg, Shaunavon, and Vanguard formations in Manitoba resembles closely that of the Sundance formation in North and South Dakota. Callovian time is represented by the lower part of the Vanguard formation. A slight unconformity is recognizable between the lower and the middle Vanguard at the basin rim in western Saskatchewan. The middle Vanguard is largely marine in the basin center, but around the basin rims, brackish, or locally continental, sediments represent this member. Most of these sediments appear to be of Oxfordian age. The lithologic character of the upper Vanguard together with the fact that it contains reworked fragments of older Jurassic faunas indicates redeposition of material from the truncated basin flanks into the center. The hiatus between Upper Jurassic and Lower Cretaceous is recognizable only along the basin rims. In the deeper portions of the Jurassic basin a more gradual change from marine to brackish into continental deposition appears to have taken place between Upper Jurassic and Lower Cretaceous time. Truncation in these portions appears to have occurred later, possibly in post-Jurassic time, and, to a lesser degree than on the rims. In the basin center there is no evidence of truncation but merely of a transition from marine into brackish sedimentation.

The Superior Lobe of the Wisconsin glaciation was initially localized by the deep lowlands of the Lake Superior Basin, cut in relatively nonresistant late Precambrian red sandstone. It advanced southwest out of this lowland and crossed a low divide leading to the Minneapolis Lowland, which is underlain by Precambrian and Cambrian sandstones. The conspicuous drumlins of central Minnesota, and the rugged St. Croix Moraine that borders the drumlins on the west and loops across the Minneapolis Lowland, delimit the major stillstand of the Superior Lobe. Discovery of red drift with diagnostic rock types from the Lake Superior Basin (agate, amygdaloidal basalt, red and purple felsite, red sandstone) in southwestern Minnesota indicates that the Superior Lobe once extended farther southwest down the Minneapolis Lowland to the Minnesota River valley and beyond during a pre-Wisconsin or early Wisconsin glaciation. As the Superior Lobe wasted from the St. Croix Moraine, a series of sharp subparallel tunnel valleys were cut into the drumlin plain and even into the underlying bedrock by subglacial streams driven to high velocity by the hydrostatic pressure resulting from the load of many hundreds of meters of active ice. Subsequent thinning and stagnation of the Superior Lobe opened the tunnel valleys to atmospheric pressure and converted the subglacial streams from major erosional streams to small depositional streams, which formed discontinuous eskers along many of the tunnel valleys. After distant retreat the Superior Lobe readvanced three times out of its basin, twice after proglacial lakes had produced a supply of red clay to be overridden. These latter two readvances may represent surges of the ice lobe resulting from the buildup of basal meltwater behind the frozen toe of the ice lobe. The Des Moines Lobe, originating in the Red River Valley of Manitoba and western Minnesota, moved southeastward down the Minnesota River valley and thence northeastward up the Minneapolis Lowland, overriding a segment of the St. Croix Moraine and extending across the state to Wisconsin in the form of the Grantsburg Sublobe. Its drift is characteristically gray to yellowish-brown and highly calcareous. The ice incorporated masses of Superior Lobe drift as it overrode the St. Croix Moraine, stringing them out to produce a complex of foliated red and gray drift. The Grantsburg Sublobe blocked the Mississippi River and other drainage from central Minnesota to form glacial Lake Grantsburg about 16,000 yrs ago. By that time the Superior Lobe had withdrawn completely from central Minnesota, for it supplied meltwater (and red clay) only on the east, down the St. Croix River and its upper tributaries. Meanwhile, the main Des Moines Lobe, which thus far supplied ice only to the Grantsburg Sublobe, thickened sufficiently to spill southward out of the Minnesota Valley across a low divide into Iowa. This produced the lobe that reached Des Moines 14,000 yrs ago. Beheading of the Grantsburg Sublobe in this manner caused the stagnation of the latter, which then wasted to form the Anoka Sandplain in its stead. The St. Louis Sublobe protruded from the Des Moines Lobe in northwestern Minnesota at a later date (about 12,000 yrs ago). Its meltwater flowed down the St. Louis River toward Lake Superior, but it was diverted southward into the St. Croix drainage by the still-existing Superior Lobe. Final wastage of the entire Des Moines Lobe produced glacial Lake Agassiz in northwestern Minnesota and adjacent North Dakota and Manitoba.

Abstract 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 10 4 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.

Abstract The Williston basin is an elliptical cratonic basin located in central and western North Dakota, eastern Montana, and northwestern South Dakota in the United States and in southern Manitoba and Saskatchewan in Canada. Formed in Late Cambrian or Early Ordovician time, the Williston basin was first transgressed by Phanerozoic sediments during Upper Cambrian time. Structures range from sharp fault folds to gentle, broad arches and clusters of small closed structures. Present sedimentary rock thickness approximates 16,000 ft (4900 m). All of the major faults and fault folds have been related to a hypothetical Proterozoic wrench-fault system. Basin rocks are characterized by pre-Pennsyl-vanian carbonate deposits arranged in sequence packages of varying scales and by post-Missis-sippian detrital and evaporite deposits. Large volumes of evaporites are associated with the carbonates. A few siliciclastic deposits are also present in the pre-Pennsylvanian stratigraphic section. Petroleum production started in Montana in the 1920s. Saskatchewan has produced the most oil, approximately 1 billion barrels to date, followed by North Dakota (840 MBO), Montana (622 MBO), Manitoba (166 MBO), and South Dakota (16.5 MBO). Mississippian rocks form the major producing reservoirs. Traps are mostly combination and structural.