Modification of existing oilfield infrastructure could deliver a cost-effective way to extend the economic life of depleted onshore oilfields. Naturally warm connate and injection water contained within these fields could be initially co-produced with remaining oil reserves and used to deliver clean, cheap, non-intermittent heating. The East Midlands Petroleum Province contains over 30 fields with a production history spanning 95 years, and we have chosen to examine the Welton field in detail. Well data for the Welton field have been analysed to ascertain extractable heat within both oil- and non-oil-bearing (water-bearing) strata within the field. Production rates were calculated to be 728 m3 d−1 oil and 854 m3 d−1 water. These values also include productivity of intervening largely water-bearing intervals. Target formation temperature at 1500 m was determined to be 52.5°C, allowing an extractable heat energy calculation to be undertaken for a range of temperature differentials. For a 30°C depletion in temperature, 1.6 MWt extractable heat is available within the Welton field alone. This equates to 14040 MWh of heat energy available for consumption by the domestic market or within commercial greenhouses.