Justin C. Reuter1,
Keith R. Johnson2,
Mark J. Lutz2
(1) Central Michigan University, Mt. Pleasant, MI
(2) Dominion Midwest Energy, Grand Rapids, MI
Abstract: Imaging pre-pleistocene geomorphology via gravity surveying as an aid in the identification of drilling locations in the Antrim Shale, Alcona County, Michigan
Thousands of shallow wells have been drilled into the Late Devonian Antrim Shale in the northern portion of the Michigan Basin. Gas production established from these wells lies along a relatively continuous fairway, which extends from Antrim County in northwestern Michigan to Alcona County in northeastern Michigan, covering a widespread distance approximately 90 miles in length by 26 miles in width. While Antrim gas production rates vary along this fairway, there are few dry holes.
In Alcona County, dry holes have been drilled in areas where the Antrim Shale was unexpectedly absent in the geologic section. A 12 square mile study area was selected in Alcona County where the Antrim Shale was missing in localized areas due to abrupt changes in the pre-pleistocene paleo-topography. In this study area, the glacial till ranges from 229 to 580 feet in thickness and the top of the Antrim Shale ranges from 357 to 656 feet measured depth from surface. Incised paleo-river valleys and erosional surfaces scour into the Paleozoic bedrock, totally removing the Antrim Shale Formation in places. Since the surficial glacial till and the Paleozoic bedrock provide a stark density contrast, a gravity survey was proposed as a tool for imaging the pre-pleistocene geomorphology. The processed and interpreted results of the gravity survey show a strong spatial correlation between bedrock structure lows and low gravity values observed on residuals derived from third order polynomial trends. The application of inexpensive gravity surveys provides a compliment in the selection of drilling locations in this area of northern Michigan and may provide benefit in other areas where similar problems are encountered.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana