Gravity Anomalies in Silurian Pinnacle Reef Trend, Southwestern Indiana

Lawrence L. Malinconico, Jr., Timothy A. Gognat, Patrick L. Scher

Structures produced over the top or along the margins of Silurian Pinnacle reefs have proven to be the source of significant oil production in the eastern Illinois basin. We have been able to refine gravity methods that can assist in the exploration of such reef targets.

A gravity/density model was developed by combining the 1980 work of Dana at the Wilfred pool (Sullivan County, Indiana) with other lithologic and log data in southwestern Indiana. This model includes the density differences between the reef facies and surrounding lithologies as well as density variations that are the result of compaction of the sedimentary sequence above the reef. The density models suggested that positive gravity anomalies with amplitude between 1.5 to 2.5 mgals might occur over the reefs.

From 1985 through 1986, detailed gravity data were acquired in the southwestern Indiana counties of Daviess, Greene, Knox, and Clay, over areas underlain by known structures suspected as being caused by compaction over reefs of Silurian age and that had produced significant quantities of oil from relatively shallow depths. In all cases, a positive gravity model was obtained, and the location of the anomalies correlated well with the location of known subsurface structure as determined from drill-hole information. From these initial studies, however, it was also determined that elevation control at each station was extremely important and that near-surface variations in the thickness of unconsolidated surficial deposits (variations in the bedrock topography) could very easily mask the ravity anomaly of the deeper, significant structures.

Subsequent to the initial study, we have been able to test our models at sites where the subsurface geology was not well known. In several of these cases, valid correlations were found between the gravity anomalies and the presence of reef-induced structures. However, other positive anomalies were found to be the result of buried bedrock highs and facies variations in Mississippian evaporites and carbonates.

The use of gravity data to prospect for pinnacle reefs and reef-induced structures in the Illinois basin although not a foolproof method, appears to have a great deal of merit as an additional tool for identifying small, hydrocarbon-rich targets.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.