This GeoDigital Information study of the Cambrian and Ordovician sandstones of the Illinois Basin is designed to assist exploration and production geologists in their understanding of factors that control hydrocarbon distribution and production potential in these reservoirs. This research is a logical compliment to the GDI study of the deep Ordovician gas reservoirs in the Michigan Basin. These lower Paleozoic rocks of the Illinois Basin are similar in many ways to those in the Michigan Basin and, thus, offer similar reservoir potential. This fact has created interest in the deep potential of this region. Adding to the reservoir potential of the Cambro-Ordovician rocks in the Illinois Basin is the fact that many large hydrocarbon reservoirs occur in equivalent age rocks in nearby parts of the U.S. Midcontinent such as eastern Indiana, northwestern Ohio, Kansas, and Oklahoma.

     This study integrates a variety of data sets including wireline logs, cores, seismic lines, source rock analyses, and published literature to provide a comprehensive summary of the stratigraphy, lithologies, depositional environments, hydrocarbon source potential, and reservoir possibilities in the Cambro-Ordovician section.

     Lower Paleozoic rocks in the Illinois Basin occur at depths ranging from 1,000 feet along the basin margins to more than 13,000 feet in the center of the basin. Approximately 20 Ordovician oil fields, located mainly near the basin margins, provide evidence that deeper fields may be present in the basin center. This regional geological study focuses mainly on the exploration potential of the deeper portion of the Illinois Basin, but studies of these relatively shallow fields were used in combination with the extensive core control from the basin margins to provide a framework for future exploration of these deep reservoirs. Special emphasis was placed

on recognizing and mapping potential hydrocarbon source rock intervals and on defining thermal maturity trends of these source rocks within the basin.

     All or portions of 115 counties in southeastern Illinois, southwestern Indiana, and western Kentucky lie within the study area. Approximately 360 wells penetrating more than 100 feet below the Trenton Limestone (Galena Group in Illinois) were included in the stratigraphic database. While this is an average of only one well per 60 miles (and much less than this for the deep part of the basin), it reflects the relatively untested nature of the Cambro?Ordovician section in the study area.

     This study defines successfully the regional stratigraphic markers and intervals in the Cambro-Ordovician section of southern Illinois, southwestern Indiana, and western Kentucky. It also generates a comprehensive and internally consistent stratigraphic database for all study wells. This includes the definition and correlation of stratigraphic markers that can be traced laterally through parts or all of the 50,000 square mile study area.

     In addition, the study describes the major lithologies within each stratigraphic interval based on core examination and log interpretation. It also examines the petrography of reservoir and source rocks with emphasis on mineralogy, lithology, types and origin of porosity, and the types of organic matter (in the source rocks). From this information, depositional models are generated for predicting the regional distribution of the various lithologies, particularly the source and reservoir lithologies. These various data sets are integrated to define promising plays and trends for future exploration and development of Cambro-Ordovician reservoirs in the Illinois Basin.