Determination of Vertical and Lateral Extent of Porous Zones in Subsurface Mississippian Monteagle Limestone, Rugby Quadrangle, Morgan, Scott, and Fentress Counties, Tennessee

John R. Isham

The Monteagle Limestone of the Cumberland Plateau in north-central Tennessee has produced more than 8.5 mmcf of gas from shallow, stratigraphically controlled reservoirs through 1985. The examination of over 800 geophysical logs (gamma-ray and compensated density or porosity) indicates that a maximum of nine porosity zones are present. Subsurface cross sections can be used to correlate zones between wells. Early Ordovician orogenic events created northeast-trending thrust belts that produced uplifting and shoaling along basin-margin arch systems. Current reworking of oolitic material along these margins caused large linear shoal deposits to form perpendicular to the northward-prograding shoreline. Shifting ebb tidal currents were the dominant force shaping the orientation and geometries of the shoal deposits. Secondary porosity may have developed in response to periodic sea level changes, which subaerially exposed tidal bars and spill-over lobes to atmospheric conditions. Solid oolitic material could then be leached by meteoric waters, creating a stratigraphic zone of porosity. Sea level change may have occurred by eustatic changes and/or local tectonism. By mapping the vertical and horizontal extent of these zones, we may better understand the constraints on Monteagle production.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.