API Well Number - Unique Well Identifier

Unique Well Identifier

The sixth through tenth digits (20130 in the example above) of the API number is a unique number within the county where the well was drilled. In states with very few wells, the unique number may be based on the permit number, and may be unique within the state rather than the county. This is an unusual situation.

There are four types of unique well identifiers, and these are called historical, current, reserved, and exempt.

Historical API numbers were assigned by Petroleum Information or other service companies and cooperative groups. In most states, wells drilled before 1967 have historical numbers. These numbers range in value from 00001 to 20000, so 42-501-05095 would be a typical historical number.

Current API numbers are assigned by regulatory agencies, usually the oil and gas commission for the state where the well was drilled. These numbers are assigned as part of the well permitting process, and they may be the same as the well permit number. Current numbers are numbered sequentially beginning from 20001-60000 with some exceptions. In the example above, 42-501-20130 is a current number.

Some states have deviated from this recommended system because of their own needs or previously established systems. Illinois and North Dakota have no break between their historical and current well numbers. Arkansas started its current numbers at 10001, while Texas started at 30001. Colorado, Michigan, and Utah have special numbering systems. The wells in the Federal waters of offshore Texas and Louisiana started at 40001.

Reserved API numbers were assigned by Petroleum Information to various wells drilled after 1967. These were for wells that for some reason were not assigned a number by the regulatory agency. If PI deemed the well deemed "information important," it received a number between 60001 and 95000. There are eight types of wells that may have received a reserved number. These are stratigraphic or core tests, water supply wells, water disposal wells, water or gas injection wells, sulfur wells, underground storage wells, geothermal wells, or prospect tests.

Exempt numbers range in value from 95001 to 99999. These numbers are proprietary, and may not be assigned by any regulatory agency or data vendor like Petroleum Information. This allows the oil company to include information on any wells that are "information important" but confidential.