The Middle Palaeocene Beda Formation, penetrated by wells in block NC74F in the SW Sirt Basin, is dominated by lagoonal, intertidal, and supratidal rocks including evaporites originally deposited in a restricted shallow lagoon. In the area studied, about 60% of the formation has been dolomitized. Dolomite occurs predominantly replacing precursor carbonate mudstones, although pore-filling cements line moulds and vugs. Replacive crystals are euhedral to subhedral, finely, to very finely crystalline, and generally less than 0.03 mm in diameter. They are non-ferroan, and show a uniform dull-to-moderate orange luminescence. Crystals forming cements are coarser up to 0.35 mm in diameter and are characteristically baroque. Geochemical data indicate that the Beda dolomites have high strontium, low iron, and low manganese contents. They are also depleted in O-18, with negative delta(18)O values averaging -3.63 parts per thousand (PDB). The C-13 values show a more marine delta(13)C isotopic signature, with an average delta(13)C value of +3.33 parts per thousand (PDB). The petrographic and geochemical characteristics of the Beda dolomites and their association with evaporites supports a model of early, near-surface, dolomitization resulting from hypersaline brine refluxion.