A stock tank is the on-lease storage tank that holds treated crude oil after processing and before it is sold or hauled away. It is also the reference point behind one of the industry's most-used units - the stock-tank barrel. This guide explains what a stock tank is, how oil in it is gauged and measured, and why the concept matters for volumes.
Stock Tank in one line: A stock tank is an atmospheric storage tank on an oil lease that holds treated crude at surface conditions before sale. It is also the reference basis for the stock-tank barrel (STB), the volume of oil measured at standard surface temperature and pressure.
After oil passes through the separator and heater treater and meets BS&W spec, it flows to the stock tanks for temporary storage. These are typically vertical, atmospheric (low-pressure) steel or fiberglass tanks grouped at the tank battery. They buffer production between the continuous flow from the wells and the periodic events of trucking or pipeline transfer, and they serve as the custody-transfer point where ownership of the oil changes.
Because stock tanks vent near atmospheric pressure, light hydrocarbons flash from the oil as vapor. That vapor is either captured by a vapor recovery unit, routed to a flare, or - where allowed and controlled - vented, which is why emissions control at the tanks is a growing focus.
Oil volume changes with temperature and pressure - warm oil with dissolved gas occupies more space than the same oil cooled and degassed at surface. To compare and sell oil consistently, the industry references volumes to standard conditions at the stock tank: a stock-tank barrel (STB) is one barrel of oil measured at defined reference conditions (commonly 60 degrees F and near-atmospheric pressure), after the gas has come out of solution.
This is why reservoir and production engineers speak of STB and of the formation volume factor - the ratio of a volume down in the reservoir to the volume it shrinks to once it reaches the stock tank. Stating rates in STB per day makes them comparable regardless of downhole conditions.
Traditionally a pumper gauges a stock tank by hand - opening the thief hatch and measuring the oil level and interface with a gauge line, or reading a level indicator - to determine how much oil is on hand and when to schedule a haul. Automated tank gauges (level transmitters, radar, or float systems) now do this continuously.
A cloud SCADA such as Merobix reads tank level transmitters over Modbus, so oil inventory, fill rate, and high-level alarms for every stock tank are visible remotely - letting operators schedule hauling efficiently and catch overfills or thief-hatch issues without a site visit.
A stock tank is an atmospheric storage tank on an oil lease that holds treated crude at surface conditions after processing and before it is sold or hauled. It buffers continuous production against periodic transfers and usually serves as the custody-transfer point.
A stock-tank barrel is one barrel of oil measured at standard surface reference conditions - commonly 60 degrees F and near-atmospheric pressure - after dissolved gas has come out of solution. It gives a consistent basis for reporting and selling oil volumes.
Traditionally by hand gauging through the thief hatch with a gauge line to read the oil level and interface, or by a level indicator. Today automated tank gauges - radar, level transmitters, or floats - read it continuously and feed the data to SCADA.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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