The USGS Central Region Energy Team assesses oil and gas resources of the United States. The onshore and State water areas of the United States comprise 71 provinces. Within these provinces, Total Petroleum Systems are defined and Assessment Units are defined and assessed. Each of these provinces is defined geologically, and most province boundaries are defined by major geologic changes. The Illinois Basin Province is located in portions of Kentucky, Illinois, Indiana, Missouri, and Tennessee. The main population centers within the study area are Louisville, Kentucky; Peoria and Springfield Illinois; Indianapolis, Indiana; Saint Louis, Missouri; and Nashville, Tennessee. The main interstates, I-80, I-70, and I-40, generally traverse the area from east to west, while interstates, I-55 and I-65 generally traverse the area from north to south. The Ohio River and Mississippi River and their tributaries drain the area. The province boundary was drawn to include the geologic structures generally considered to be in or bounding the Illinois Basin.