Hydrocarbon Accumulation in Basal Pennsylvanian Fluvial Sandstone Near Hardinville, Crawford County, Illinois: a Model Paleogeomorphic Trap: ABSTRACT

Abstract

The surface of the Mississippian-Pennsylvanian unconformity in the Illinois basin is characterized by an anastomosing pattern of paleovalleys eroded by the ancient Michigan River system. Fluvial sandstones deposited within these valleys commonly were buried by transgressive Pennsylvanian marine shales, creating the potential for stratigraphic entrapment of hydrocarbons. One such trap was discovered accidentally on the northeast flank of the Hardinville dome in 1955. The exploration significance of a linear sand body within a paleovalley was not recognized at that time and only four producing wells resulted. Not until 1974 was this reservoir again encountered, 1.5 mi to the southwest, on the dome's southwest flank. Correct log correlations led to its identification in 1976 as a 3-mi-long, 5-45-ft-thick conglomeratic sand body along one side of the paleovalley floor. Sixteen producing wells in the southwestern portion of the reservoir will recover an estimated 1.2 million bbl of 36/sup 0/ oil. This reservoir illustrates the difficulties and rewards in developing exploration models for stratigraphic traps. Traps associated with paleovalleys at the Mississippian-Pennsylvanian unconformity in particular could prove to be important targets for future exploration. Maps showing (1) paleogeology and paleotopography of the sub-Pennsylvanian surface, and (2) structure of overlying Pennsylvanian coals can bemore » used to delineate paleovalleys on the unconformity surface and potential hydrocarbon reservoir strata associated with sedimentary fill. Seismic data may also be useful, but models will require careful computer modeling and field testings.« less