A GRAVITY STUDY PERFORMED OVER A SILURIAN PINNACLE REEF IN WESTERN INDIANA

The objective of this study is to show whether gravity measurements are able to identify a petroleum-bearing Silurian pinnacle reef structure situated beneath the Indiana State University (ISU) campus in western Indiana. In the 1800’s the campus and downtown areas of Terre Haute was the site of several prolific oil wells that were eventually abandoned. In 2012, Pioneer Oil Company explored the area with the idea that this old oil field, like many others in this part of Indiana, might be associated with the drape of Devonian and younger sedimentary strata over a Silurian reef. As a result of their seismic exploration, several production wells were drilled beneath the campus. The gravity study presented here consists of 44 stations distributed along two crossing profiles (N-S and E-W) centered on campus to examine whether such a reef and its overlying draped strata might exhibit a gravity anomaly associated with the subsurface density variations. Given that the average density of limestone (2.55 g/cm³) is greater than the average density of shale (2.40 g/cm³), the expected gravity anomaly for the region would be positive. The resulting Bouguer anomaly for these profiles after the regional gravity gradient is removed is a ~0.16 mgal positive gravity anomaly, which is very well defined on the N-S profile. The anomaly is similarly developed (but with more variation) on the E-W profile, which was limited to the west by the Wabash River. This result matches the expected gravity anomaly, and its magnitude and scale is very similar to that found across the reef structure of the Terre Haute East Field in an ISU Senior Thesis (Hauser, 1976) and for gravity across the Silurian reef beneath the Wilfred gas storage facility in Sullivan County to the south (Dana, 1980).