Dolomitization in the Trenton Limestone (Ordovician) of Indiana: A Tale of Two Trends

Keith, Brian D.
Indiana Geological Survey, Indiana University, Bloomington, IN

Significant hydrocarbon production from the Trenton Limestone (Ordovician) and equivalent units in eastern North America has led to renewed attention from industry in recent years. The most significant areas of Trenton hydrocarbon production are related to areas of localized dolomitization and/or fracturing. In Indiana there are two distinct trends of dolomitization: one trend that covers approximately one-third of northern Indiana has been recognized as part of the Lima-Indiana hydrocarbon province since the late 1800s. A different trend in southwestern Indiana covers less than half of the areal extent of the northern trend.

An estimated 100 million barrels of oil were produced from the northern trend, within which two types of dolomite have been described. The upper few feet of the Trenton consists of an extensive nonporous, generally ferroan dolomite. Beneath this dolomite is a nonferroan, generally porous dolomite that is locally highly variable in thickness with the dolomite extending downwards in places through the entire thickness of the Trenton. This dolomite has generally less than 8 percent vuggy porosity with low permeability. Production from this trend was of a considerably lower quality compared to areas in Michigan and Ohio where hydrothermal dolomitization along fracture systems produced significant hydrocarbons.

Less is known about the dolomite trend in southwestern Indiana. There have been numerous hydrocarbons shows in the Trenton, but no production. Based on the only core available, this dolomite is finely crystalline, calcareous (10-40% calcite), and quite permeable. The dolomite forms lenses at different levels within the Trenton that are laterally discontinuous.

AAPG Search and Discovery Article #90031©2004 AAPG Eastern Section Meeting, Columbus, Ohio, October 3-5, 2004