Interpretation of the magnetic anomaly over the Omaha Oil Field, Gallatin County, Illinois

Abstract

A 40 nanoTesla (nT) magnetic anomaly identified in an aeromagnetic survey over southern Illinois contours as a localized magnetic high on the west flank of a regional magnetic low. This magnetic anomaly is generally coincident with the Omaha Oil Field in northwest Gallatin County, Illinois. It was initially assumed that cultural sources of steel associated with this oil field were the primary source of the magnetic feature; however, similar oil fields overflown by the survey do not exhibit magnetic anomalies in the data set. The Luther Rister et ux ♯1 well, drilled near the apex of the Omaha structural dome, encountered two zones of ultramafic intrusive rock containing 9.0 percent by volume magnetite. These intrusives were identified to be alnöites which are a class of mantle‐derived ultramafic rock that can be associated with the incipient stages of crustal rifting. A ground magnetic survey verified the presence of the anomaly, and provided detailed data for 3-D modeling of the source. Petrophysical evaluations, magnetic susceptibility measurements and thin section modal analysis were made on drill cuttings from the ultramafic intrusives encountered in the Luther Rister ♯1 well. These measurements were made to constrain the 3-D magnetic modeling by the petrophysical characteristics of the source. After removal of the regional magnetic field, the resulting 140 nT residual magnetic anomaly was successfully modeled using two ultramafic sills with an igneous feeder plug. The two igneous sills adequately account for the structural closure exhibited in the Omaha Oil Field and raise the interesting possibility of other hydrocarbon trapping structures generated by intrusives emplaced into the sedimentary section.

Fine-scale structure of the Precambrian beneath the Illinois Basin

“…For example, Krassay et al (2013) demonstrated the utility of forward magnetic anomaly modeling of interpreted mafic igneous sills or tabular intrusions within sedimentary basins when high-resolution magnetic intensity data and a coincident grid or network of seismic profiles are available. In the southern Illinois Basin, occasional positive magnetic anomalies have been shown to be related to shallow ultramafic igneous sills intruded into Paleozoic sediments by forward modeling of high-resolution magnetic intensity data (Sparlin and Lewis, 1994). In both of these cases, the sills or series of sills, which individually ranged in thickness from 72 m to a few hundred meters, produced magnetic anomalies of 100-250 nT.…”

Investigating Possible Earthquake-Related Structure Beneath the Southern Illinois Basin from Seismic Reflection

“…Mafic sills are occasionally penetrated in drillholes in the surrounding area of the U.S. midcontinent (e.g., Lucius and Von Frese, 1988;Wickstrom etaL, 1992) with ages of 1.3-1.0 Ga, and ultramafic dikes of Permian age crop out or are exposed in coal mines in southern Illinois 10-20 km south of the seismic profile (Clegg and Bradbury, 1956). The "Omaha Dome," located several kilometers south of the seismic profile, is an uplift caused by the intrusion of ultramafic sills or laccoliths into the Upper Devonian New Albany Group and younger strata (for review, see Nelson, 1995); however, deeper units appear to be unaffected by intrusion (Stevenson et al, 1981), an observation supported by magnetic anomaly modeling (Sparlin and Lewis, 1994). Gravity anomaly and residual magnetic intensity maps Hildenbrand and Kucks, 1993) indicate a steep, NE-trending gradient crossing the profle in the vicinity of the dipping reflector zone that bounds a negative gravity and magnetic anomaly to the southeast.…”

Boundary Separating the Seismically Active Reelfoot Rift from the Sparsely Seismic Rough Creek Graben, Kentucky and Illinois

“…The fault system of the rift is exposed as the Fluorspar Area Fault Complex of western Kentucky and southeastern Illinois (Figure 3). There, alkaline ultramafic dikes, sills, and complexes of Permian age are abundant, many having intruded along short, northwest-striking cross faults that link longer, northeast-striking faults (see Figure 6; Zartman et al, 1967;Bikerman et al, 1982;Trace and Amos, 1984;Snee and Hayes, 1992;Sparlin and Lewis, 1994;Nelson, 1995;M.L. Sargent, written communs., 1995 andHildenbrand and Ravat, this volume).…”