EXPLORATION Results of the Middle Proterozoic Midcontinent rift frontier play along Lake Superiors south shore

Albert B. Dickas
University of Wisconsin-Superior
Superior, Wis.

A 10 paper volume reviewing the history of exploration for hydrocarbons along the Midcontinent rift trend of Wisconsin, as well as detailed analyses of the geology, geophysics, and logging data associated with the 7-22 Terra-Patrick borehole, is being prepared under the co-editorship of the author of this article. This volume will be published by the Wisconsin Geologic and Natural History Survey as part of its Miscellaneous Paper Series. Publication is planned for late 1996.

Two very significant wildcats were completed in the midwestern section of the U.S. between October 1987 and April 1992. At the time information regarding these ventures was sought by a wide circle of observers ranging from corporate scouts to geologists. Because of the secrecy surrounding both boreholes, little to no information was available until very recently. These operations share several common characteristics, as follows:

• Both were drilled as a test of hydrocarbon possibilities associated with the Midcontinent rift trend that extends from northeastern Kansas to central Ohio by way of the Lake Superior basin.
• Both were sited along the south shore of Lake Superior, one in Wisconsin and the other in the adjacent Upper Peninsula of Michigan.
• Both were a test of hydrocarbon source, reservoir, and trap potential of clastic sedimentary rocks belonging to the Middle Proterozoic Oronto group.
• In both boreholes Amoco Production Co. played a key role, thus maintaining its earlier interest in the exploration for oil and gas in Precambrian strata of the Midcontinent extensional terrane.

Michigan wildcat

Amocos 1-29 and 1-29R St. Amour, one a sidetrack of the other, were drilled under confidentiality established by the Mineral Well Act 315, P.A. of 1969 (Sec. R 299.2273). The act states that "a log of an exploratory test well shall be kept confidential until released by the owner or operator". As originally established, confidentiality was effective until Sept. 1, 1998. However, Amoco on Mar. 3, 1995, released all information pertaining to these wells. The Amoco 1-29 St. Amour, with a surface location in SE NE SW 29-46n-18w, Munising Township, Alger County, Mich., was spudded Oct. 18, 1987. After drilling 90 ft of glacial drift, Amoco cored 283 ft of dolomitic sand belonging to the Ordovician Au Train formation. This well was declared a lost hole at a depth of 373 ft. After skidding the rig 42 ft, Amoco on Oct. 26, 1987, spudded the 1-29R St. Amour borehole with the same location as stated above. According to interpretation of a 30 mile, east-west seismic reflection profile, the borehole location is approximately 10 miles east of the extension of the Keweenaw fault southeasterly from its classic exposure along the spine of the Keweenaw Peninsula (Fig. 1)(24574 bytes). In the vicinity of the St. Amour test, this reverse fault forms the western limit of the central structure of the Midcontinent rift. To the west in Wisconsin, this central feature, known as the St. Croix horst, is composed of several thousand feet of Oronto group sedimentary rock overlying thousands of feet of basalt. A total depth of 7,238 ft was reached on Jan. 1, 1988, after which this well was plugged and abandoned as a dry hole. Below 110 ft of glacial till, the borehole was 100% cored from 160 ft to TD.

A generalized stratigraphic section for the 1-29R borehole reads as follows:

• 0 (surface)-110 ft: glacial drift, Pleistocene age.
• 110-385 ft: Au Train formation, Ordovician age.
• 385-521 ft: Munising formation, Upper Cambrian age.
• 521-6,523 ft: Oronto group, Middle Proterozoic age.
• 6,523-73 ft: brown basalt.
• 6,573-6,645 ft: medium grained redbeds.
• 6,645-91 ft: gray to black basalt.
• 6,691-6,873 ft: brown to gray fine/medium grained clastics.
• 6,873-6,923 ft: dark gray basalt.
• 6,923-7,238 ft (TD): fine to medium grained redbeds.

Between the base of the Paleozoic section and 6,523 ft the drilled section is classified on the basis of lithology as "undifferentiated Freda formation" overlying (below 5,850 ft) Copper Harbor conglomerate. These formations, plus the Nonesuch formation, define the Oronto group along the south shore of Lake Superior. The Nonesuch, deposited under lacustrine conditions and containing a total carbon content approximating 1%, is considered the source for Precambrian indigenous oil seeping from the ceiling of the White Pine copper mine of the western Upper Peninsula of Michigan (Fig. 1)(24574 bytes). The absence in the 1-29A borehole, between "undifferentiated Freda formation" and the Copper Harbor sequence, of any strata suggestive of Nonesuch lithology might suggest the presence of a disconformity forming a modified Oronto sequence. In the Lake Superior district, the base of the type Oronto group is defined by the presence of either massive basalt flows or the presence of one or more individual flows separated by coarse, immature, polymictic, red-bed interflow clastic rocks. The lithology of the sedimentary rock sequence underlying the oldest and youngest of the three basalt flows drilled by the 1-29R is reminiscent of interflow units. The lithology of the remaining sedimentary unit, at 6,691-6,873 ft, however, is similar to that associated with the Nonesuch formation. As there is no evidence for reverse faulting in this borehole, the presence of Nonesuch-like strata below and between basaltic flows suggests that lacustrine sedimentation took place earlier (i.e., pre-Copper Harbor time) in this portion of the Midcontinent rift than further west in Michigan and Wisconsin (i.e., post-Copper Harbor time). Because coring was continuous below 160 ft no mud-log was employed for this operation. Nevertheless, no hydrocarbon shows were reported, resulting in the well being classed as dry.

Wisconsin wildcat

Prior to the spudding of the 7-22 Terra-Patrick borehole in the predawn hours of Mar. 9, 1992, an earlier, related wellsite was announced by Amoco Production in May 1985 at NE 13-46n-7w. This planned 12,000 ft test location is approximately 4.5 miles south-southwest of the 7-22 Terra-Patrick site. Due to the lack of existing oil and gas exploration legislation and the decline in hydrocarbon prices that took place during 1985, eventual activity at this initial site was delayed and finally canceled. Six years later, following legislative approval of rules and regulations covering drilling operations for hydrocarbons, several standing room only public hearings regarding the issuance of a drilling permit by the state of Wisconsin, the approaching cancellation of 10 year options on the active leases, and recovery in oil prices, Amoco began seeking a venture partner. With positive responses from Terra Energy Ltd. and, in turn, Patrick Petroleum Co., the 7-22 Terra-Patrick borehole was sited in SW SW NE 22-47n-6w (Fig. 1)(24574 bytes). As interpreted by reflection seismology, a vertical borehole at this location would test the southeast flank of an elongate east-west oriented anticline, apparently formed by drag folding during the compressive phase of tectonic activity along the Midcontinent rift system. This drag-fold anticline was defined by structural closure to the east, south, and west and by fault closure to the north. Fault closure is against the Douglas fault, which defines the northern edge of the St. Croix horst, the central structure of the Midcontinent rift as traced from southern Minnesota to the Wisconsin-Michigan border along the south shore of Lake Superior. The Terra-Patrick borehole was projected to drill to a measured depth of 6,000 ft, but after penetrating all potential reservoir beds the decision was made to abandon the project after reaching a total depth of 4,966 ft. Plugging operations were completed on Apr. 1, and the well was declared dry and abandoned. Mud-log analysis indicates that a sedimentary rock sequence directly correlatable to Oronto group strata regionally found overlying the St. Croix horst had been drilled. The stratigraphic sequence at the Terra-Patrick site, as indicated on the mud-log, is as follows:

• 0 (surface)-290 ft: glacial drift, Pleistocene age.
• 290-3,732 ft: Freda formation, Middle Proterozoic age.
• 3,732-4,168 ft: Nonesuch formation, Middle Proterozoic age.
• 4,168-4,966 ft (TD): Copper Harbor conglomerate, Middle Proterozoic age.

While minor gas shows were reported throughout the central section of the Nonesuch formation, on-site wireline log analyses proved to be discouraging. As a result no drillstem testing was conducted and no sidewall or conventional cores were collected. n

The Author

Albert Dickas is vice chancellor for research and professor of geology at the University of Wisconsin-Superior, where he has taught and held various administrative positions since 1966. Before 1980 he served as the founding executive director of the Lake Superior Research Institute, the universitys applied environmental analyses branch. During the past 12 years, in association with university cooperative extension, his interests have focused on the Precambrian Midcontinent rift system, resulting in numerous publications and presentations regarding the tectonic development and economic potential of this structure. He has a BA and MS from Miami University (Ohio) and a PhD from Michigan State University.