Tolmount is one of the largest gas prospects in the UK. Credit: Premier Oil.
The prospect lies in the UK’s southern North Sea. Credit: Dana Petroleum.
The Tolmount reservoir is a part of the lower Lemon Sandstone Formation Play Fairway. Credit: RISC Operations.

The Tolmount discovery lies in Block 42/28d of the P1330 license, approximately 40km east of the Yorkshire coast in the southern North Sea, UK.

Block 42/28d was first awarded to Dana Petroleum Exploration & Production (E&P) in December 2005.

E.ON E&P farmed into the project in 2010 buying 50% equity, and operatorship of the project.

E.ON sold its entire UK exploration and production (E&P) assets to Premier Oil (Premier) in April 2016, transferring ownership and operatorship of the Tolmount field to the latter. Premier Oil merged with Chrysaor in 2021 to form Harbour Energy.

The gas field is owned by operator Harbour Energy (50%) along with Dana Petroleum (50%).

First gas from the Tolmount field was achieved in April 2022, while the Tolmount East field came online in December 2023. The Tolmount area has a production capacity of 13,000 barrels of oil equivalent per day.

Tolmount is one of the UK’s biggest gas prospects since Breagh and has significant upside potential.

Discovery and appraisal

Dana Petroleum initiated the drilling of the first exploration well, 42/28d-10, in 2008. However, the endeavour was thwarted as they could not reach the reservoir due to mechanical issues with the rig.

E.ON subsequently undertook the drilling of another well, 42/28d-11, and a side-track, 42/28d-11z, but these attempts were also unsuccessful owing to the collapse of the borehole.

In 2011, after a comprehensive phase of planning and redesign, well 42/28d-12 was drilled using the Ensco 92 jack-up drilling rig.

The well was aimed at the Permian Leman Sandstone Formation and triumphantly encountered a gas column that measured 200ft within the reservoir.

Following the drilling, a well test was conducted, yielding a remarkable flow rate of 50.2 million standard cubic feet of gas per day.

Tolmount was appraised with the drilling of two appraisal wells, 42/28d-13 and 42/28d-13z, in 2013.

The wells confirmed the presence of high-quality hydrocarbons in the Lower Leman Sandstone Formation reservoir.

Tolmount prospect geology and reserves

Tolmount lies in the Lower Leman Sandstone Formation Play Fairway, which is characterised by Aeolian dunes, fluvial sands, and local sabkha. The reservoir is linear with parallel faulting.

The Greater Tolmount Area is estimated to contain up to one trillion cubic feet of gas, with the main field containing approximately 540 billion cubic feet and the Tolmount East and Tolmount Far East fields estimated to hold a further 220 billion cubic feet and 150 billion cubic feet of gas resource, respectively.

Tolmount East prospect

The Tolmount East gas field lies in the southern part of the North Sea, approximately 37km east of Flamborough Head and 152km from the UK/Netherlands median line.

The Tolmount East development well was completed in late 2022 and placed into production in December 2023. It is the first tieback to the Humber Gathering System.

Tolmount prospect development details

E.ON carried out predevelopment activities such as drilling studies, offshore surveys, and subsurface activities to design the development plan for the field, before transferring ownership of the field to Premier.

In February 2017, Premier selected the development concept for the Tolmount field, which comprised a standalone normally unmanned installation and a new gas export pipeline to shore.

The design of the platform includes 1,456-tonne topsides and six slot facilities installed at a water depth of 52m.

A 48km-long, 20-in-diameter pipeline along with a 3in-diameter methanol line transports the produced hydrocarbons to an onshore terminal.

The Tolmount field is developed using four platform wells. Vertical/low-angle deviated wells were drilled on Tolmount reservoir sands, considering the complex geology of the reservoir.

Contactors involved

Rosetti Marino, an engineering company, was awarded the engineering, procurement, construction, installation, and commissioning contract for the Tolmount platform.

Construction and engineering company, Costain was awarded the front-end engineering design (FEED) study contract in September 2017 for the subsea pipeline to the onshore plant.

Wood Group was selected to provide FEED services for the project in April 2017.

Offshore construction and services provider Hereema was awarded the contract to transport and install the Tolmount platform. The installation was completed in October 2020 by the semisubmersible Sleipnir.

Saipem was subcontracted by Humberside Gathering System for pipeline EPCI works for Tolmount.