35 ILCS 450/2-40
The Department shall have the power to require any operator, producer, transporter, or person purchasing any oil or gas severed from the earth or water to furnish any additional information deemed to be necessary for the purpose of computing the amount of the tax, and for such purpose to examine the meter and other charts, books, records, and all files of such person, and for such purpose the Department shall have the power to issue subpoenas and examine witnesses under oath, and if any witness shall fail or refuse to appear at the request of the director, or refuses access to books, records, and files, the circuit court of the proper county, or the judge thereof, on application of the Department, shall compel obedience by proceedings for contempt, as in the case of disobedience of the requirements of a subpoena issued from such court or a refusal to testify therein.
35 ILCS 450/2-40