Eocene spheroidal dolomite from the western Sirte Basin, Libya

Scanning electron micrographs of crypto-crystalline (< 4 μm) dolomites found in cores taken from the lower Eocene Facha Member reservoir of the Zella and Aswad fields in Libya show that they consist of tiny globular bodies mixed with anhedral crystals, producing a chalky texture. The sedimentary sequence indicates deposition in upper intertidal/supratidal arid environments and in saline lagoons. The crypto-crystalline dolomite is facies-controlled and probably formed as a primary precipitate, similar to those reported from present-day subaqueous environments in the Coorong of South Australia and Holocene supratidal crusts off Florida. Such chalky-textured dolomites could form local fluid-migration barriers within hydrocarbon reservoirs.