Reference

Illinois API county codes

Every Illinois API well number starts with 12, the state code. The next three digits are the county code, one of the 102 listed below, so a number beginning 12-001 belongs to a well in Adams County.

To decode a whole number, including the sidetrack and completion digits, paste it into the API well number decoder.

How to read a Illinois API number

12State: Illinois
001County: Adams
12345Well, unique within the county

The first ten digits identify one well. Two more digits can follow for a sidetrack, and two after that for a later completion. The county is the one where the well starts at the surface, so a horizontal well that crosses a county line keeps its surface county's code. A few state regulators assign the producing county instead, so the state record is the final word on a well near a boundary.

All 102 Illinois counties

County code County API numbers start
001 Adams 12-001-
003 Alexander 12-003-
005 Bond 12-005-
007 Boone 12-007-
009 Brown 12-009-
011 Bureau 12-011-
013 Calhoun 12-013-
015 Carroll 12-015-
017 Cass 12-017-
019 Champaign 12-019-
021 Christian 12-021-
023 Clark 12-023-
025 Clay 12-025-
027 Clinton 12-027-
029 Coles 12-029-
031 Cook 12-031-
033 Crawford 12-033-
035 Cumberland 12-035-
037 DeKalb 12-037-
039 De Witt 12-039-
041 Douglas 12-041-
043 DuPage 12-043-
045 Edgar 12-045-
047 Edwards 12-047-
049 Effingham 12-049-
051 Fayette 12-051-
053 Ford 12-053-
055 Franklin 12-055-
057 Fulton 12-057-
059 Gallatin 12-059-
061 Greene 12-061-
063 Grundy 12-063-
065 Hamilton 12-065-
067 Hancock 12-067-
069 Hardin 12-069-
071 Henderson 12-071-
073 Henry 12-073-
075 Iroquois 12-075-
077 Jackson 12-077-
079 Jasper 12-079-
081 Jefferson 12-081-
083 Jersey 12-083-
085 Jo Daviess 12-085-
087 Johnson 12-087-
089 Kane 12-089-
091 Kankakee 12-091-
093 Kendall 12-093-
095 Knox 12-095-
097 Lake 12-097-
099 LaSalle 12-099-
101 Lawrence 12-101-
103 Lee 12-103-
105 Livingston 12-105-
107 Logan 12-107-
109 McDonough 12-109-
111 McHenry 12-111-
113 McLean 12-113-
115 Macon 12-115-
117 Macoupin 12-117-
119 Madison 12-119-
121 Marion 12-121-
123 Marshall 12-123-
125 Mason 12-125-
127 Massac 12-127-
129 Menard 12-129-
131 Mercer 12-131-
133 Monroe 12-133-
135 Montgomery 12-135-
137 Morgan 12-137-
139 Moultrie 12-139-
141 Ogle 12-141-
143 Peoria 12-143-
145 Perry 12-145-
147 Piatt 12-147-
149 Pike 12-149-
151 Pope 12-151-
153 Pulaski 12-153-
155 Putnam 12-155-
157 Randolph 12-157-
159 Richland 12-159-
161 Rock Island 12-161-
163 St. Clair 12-163-
165 Saline 12-165-
167 Sangamon 12-167-
169 Schuyler 12-169-
171 Scott 12-171-
173 Shelby 12-173-
175 Stark 12-175-
177 Stephenson 12-177-
179 Tazewell 12-179-
181 Union 12-181-
183 Vermilion 12-183-
185 Wabash 12-185-
187 Warren 12-187-
189 Washington 12-189-
191 Wayne 12-191-
193 White 12-193-
195 Whiteside 12-195-
197 Will 12-197-
199 Williamson 12-199-
201 Winnebago 12-201-
203 Woodford 12-203-

County names are the US Census names for these codes. The API standard does not promise the two match, so we checked them against state well records. Across 35 states, where this table names a county it matched the one the agency recorded on 85,091 of 85,100 wells. Offshore state and federal waters use their own pseudo-county codes, which are not listed here.

Finding the well itself

The API number stops at the county. It does not hold the well's coordinates, lease, survey section or operator. Those sit in the well record, which you find by searching the full number in the state agency's system. Search it at the Illinois oil and gas regulator.

Frequently asked questions

What is the API state code for Illinois?

12. API state codes run alphabetically from 01 for Alabama to 49 for Wyoming, then 50 for Alaska and 51 for Hawaii. They are not the Census FIPS state codes, which is a common source of wrong lookups.

How do I find the county from a Illinois API number?

Read digits three to five and find them in the table on this page. In 12-001-12345, the county code is 001, which is Adams County. If the code is missing from the table, it is usually an offshore area rather than an onshore county.

Does a Illinois API number give the well's exact location?

No. The number stops at the county. Coordinates, the lease and the survey location are in the well record held by the state agency, which you search with the full API number.

State codes: American Petroleum Institute Bulletin D12A well numbering standard. County names: US Census county FIPS names, checked against state well records. Reference generated .

How to cite this page

Sources: American Petroleum Institute Bulletin D12A well numbering standard; US Census county FIPS codes.

TetraSoft. (2026). Illinois API county codes. Reference generated 2026-09-24. https://tetrasoftco.com/tools/api-county-codes/illinois/