The Markham Field is a gas accumulation lying within the Southern North Sea and straddling the UK-Netherlands median line. The field is a structural trap comprising a well defined horst in its eastern lobe and a domal feature in the western lobe. The Permian Lower Leman/Slochteren Sandstone reservoir comprises aeolian dune sandstones in the southern part of the field passing northwards and upwards through sabkha deposits into a playa claystone. Diagenesis and reservoir quality are highly dependent on original lithofacies with the aeolian sandstones containing the majority of the net reservoir. The gas, sourced from Carboniferous Coal Measures and sealed by overlying Permian Silverpit claystones, is sweet with 83% methane and a condensate-gas ratio of around 52.2 m 3 /m 3 x 10 6 (9 BBL/ MMSCF). The reservoir is being produced by pressure depletion with wells drilled from two fixed platforms. Recoverable reserves are estimated to be around 19.8 x 109m 3 (700 BCF) of gas.