Abstract: A horizontal vapor tight oil battery assembly includes a single, large size, combination separation and production tank for connection to a well-head. The separation of gas and water from oil and holding of the production oil all occurs within the same chamber. In one form, the combination separation and production tank is mounted on skids, and in another form, is adapted to be installed underground. The separation and production tanks are preferably adapted from surplus pressurized tank cars. Treaters may be used in association with the combination separation and production tank.