Plummer Shoal – A Model for Middle Mississippian Salem Limestone Exploration

By

PARTIN, THOMAS H.

Citizens Gas and Coke Utility, Worthington, IN

     The Plummer Oil Field, located in Greene County, IN, was discovered in 1969 and has produced over six million barrels of oil, mostly from the Salem Limestone at an approximate depth of 700 feet.  It is the most productive Salem oil field in Indiana.

     The total reservoir consists of four skeletal grainstone beds and one skeletal oolitic grainstone bed separated by impermeable carbonate mudstones.  These porous compartments thin from over thirty feet in the bottom “E” zone to a maximum of eight feet in thickness in the top “A” zone.  The highest recorded log porosity in each of the zones is more than 20%.  The geometry of the lower four skeletal grainstones conforms to an arcuate shape of an underlying Silurian reef.  The uppermost skeletal-oolitic grainstone is elongated into northeast-southwest possible tidal bars.  Four cores in the upper portion of the Plummer Field indicate depositional settings ranging from subtidal shoal, intershoal, restricted lagoon, and tidal flat. 

     Although the subtidal shoaling conditions existed in a wide range across the inner ramp of the eastern portion of the Illinois Basin, the Plummer Shoal was a dominant seafloor feature. The present structural closure on top of the Salem is about 80 feet.  The numerous individual shifting shoals were deposited in a structurally simple intracratonic basin where relative sea level changes and storms produced variable sediment packages and stacked geometries.

     A working knowledge of structure, depositional environments, facies, porosity-permeability distribution, and an understanding of lateral-vertical relationships are a must to Middle Mississippian Salem exploration.