Using Detailed Gravity Surveys to Delineate Buried Glacial Scours in Northern Michigan
The lower peninsula of Michigan is almost entirely covered by Pleistocene glacial drift which ranges in thickness from a few feet to over 1,000 feet. The drift covers Paleozoic and Mesozoic bedrock. Along the basin margins the bedrock can be significantly scoured and channeled by glacial meltwaters. Glacial scouring can reduce bedrock thickness over the productive Antrim shale to less than State-mandated well completion requirements. Due to both the shallowness of the bedrock (i.e. base-of-drift (BOD) surface) and the large density contrast between drift channel fill and adjacent competent bedrock, scours cause high-frequency Bouguer gravity anomalies of sufficient amplitude to be detected. Densely sampled, accurate Bouguer data (microgravity data) is an effective way to map these features and dramatically reduce “scoured-out” wells.
The acquisition of microgravity data requires precise elevation and gravity measurements. Because the scoured areas are often undeveloped and wooded, field activities must be carefully planned. In addition, quality control must occur in tandem with data collection to assure data are adequate before leaving the field.
Following acquisition the data are processed. The Bouguer data are then mapped and used to define the regional gravity (i.e. lower frequency anomalies). The regional is then removed from the Bouguer to isolate the higher frequency anomalies (i.e. the residual gravity) caused by the BOD configuration. Regional definition is an iterative process utilizing both frequency analysis and gravity modeling results. Once calculated, the residual Bouguer is entered into 2½-D gravity models. Model profiles are located where residual anomalies are two-dimensional in nature and where well control constrains the model BOD surface. The model BOD surface is then modified where not constrained to best fit the residual. Lower frequency anomalies which cannot be explained by the model BOD surface indicate adjustments need to be applied to the regional. Once these adjustments are made, the residual is finalized.
Once modeling and the residual are finalized, BOD elevations from the models and well control are plotted on the residual gravity. BOD contours are then interpreted away from the model lines using the residual and well control as guides. The result is a BOD surface interpretation providing operators a much more accurate depiction of the bedrock surface to spot future wellbores.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009