A SEISMIC REFLECTION STUDY OF A MISSISSIPPIAN MUD-MOUND NEAR ADEN, ILLINOIS

In 2011 and 2012 a Vibroseis™ seismic reflection survey, consisting of one westeast and one north-south seismic reflection profile, was conducted near the community of Aden (Hamilton County), Illinois, for hydrocarbon exploration purposes. Previous interpretations of the west-east seismic line (Ranaweera, 2015), as well as drill cuttings (Cantrell, 2011) from a nearby well (Webb #1), indicated the existence of a mud-mound within the Ullin Limestone. This thesis studied the shape and dimensions, as well as the seismic velocity properties, of the Aden mud-mound by re-processing and interpreting the Aden seismic reflection data. The interpretation process was aided by constructing two-dimensional synthetic seismograms from well log data from the nearby Broughton and Johnsonville oil fields, which are known locations of Waulsortian mud-mounds similar to the one inferred to exist at Aden. The results of this investigation indicate that the Aden mudmound has a longitudinal extent of approximately 0.6 miles (0.97 km) wide across the west-east ADEN #1 profile. From north-to-south on the ADEN #2 profile, the feature is approximately 0.7 miles (1.13 km) long. The geologic feature is located within the Ullin Limestone (Lower Mississippian) and is only manifested by the draping of younger strata overlying the mud-mound as observed on the seismic reflection images. Results from the 2D synthetic seismograms indicate that the seismic reflections generated across the known mud-mounds at Broughton and Johnsonville are similar to the seismic reflections seen at the location of the interpreted mud-mound on the Aden seismic reflection data. The presence of the Aden mud-mound is inferred by the draping of overlying strata above the mud-mound. This is interpreted to be caused by the increase in stratigraphic relief at the top of the Ullin, which becomes thicker at the mud-mound locations. Geologic and well log data from drill holes at Aden revealed that the interpreted Aden mud-mound formed in the Harrodsburg Member of the Ullin Limestone. The Harrodsburg is approximately 90 feet (27 meters) thick at Aden but becomes thicker at the location Webb #1 (up to 152 feet) where the mud-mound is located, indicating that the Aden mud-mound is at least 152 feet thick, when measured from the base of the Harrodsburg Member. A study of Root-Mean-Squared (RMS) and interval velocities, derived from sonic and pseudo-sonic log data from wells from the Aden, Broughton, and Johnsonville locations, indicated that RMS and interval velocities from the upper member (Harrodsburg Member) of the Ullin Limestone are higher than RMS and interval velocities present in the lower member (Ramp Creek Member) of the Ullin. Stacking velocities derived from the Aden seismic reflection data matched the RMS velocity values derived from sonic log data from drill holes at Aden with percent differences of less than 5%. This study indicates that stacking velocities at this location are accurate enough (within approximately 5%) to perform velocity studies when well log data from drill holes are not available. Additionally, there were no significant variations in stacking velocity profiles in the Ullin Limestone atthe Aden mud-mound location when compared to stacking velocity profiles derived at CMP locations away from the mud-mound. Likewise, there were no significant variations between RMS and Interval velocity profiles of the Ullin Limestone, derived from wells that were drilled into mud-mound locations at Aden, Broughton, and Johnsonville, and RMS and interval velocity profiles derived from wells that were drilled away from the mud-mound locations. The conclusion of this study is that RMS and interval velocity profiles, as well as stacking velocity profiles from seismic reflection data, are not good indicators for pinpointing the existence and location of carbonate mud-mounds. The best method to identify such carbonate structures is by interpreting their structure observed on seismic reflection records, including the draping effect on shallow reflections above the mudmound as observed in the recorded seismic reflection data, and by analyzing drill cuttings if available