ORIGIN OF THE MID‐YPRESIAN JIRANI DOLOMITE — A MAJOR RESERVOIR ROCK IN THE NW LIBYA OFFSHORE

Abstract

The Jirani Dolomite was formed during a major relative fall in sea‐level in middle Ypresian (early Eocene) times, in shallow, hypersaline lagoons on a restricted shallow platform, where supretidal and intertidal and semi‐arid to arid conditions prdominated. Localisation of the shallow logoonal facies to the region of the Jifarah Trough suggests that restriction was caused by downwarp relative to mild uplift of the Jifarah Platform to the north.Two main facies are recognised: and anhydritic dolomite facies, composed of dolomite and dolomitic limestone with anhydrite nodules; and a non‐anhydritic dolomite faices, composed of dolomite and dolomitic limestone only. Lithologic and petrographic evidence leads to the conclusion that anhydrite was also present in the non‐anhydritic dolomite facies initially, but was removed by dissolution processes. Petrographic studies show that dolomitisation proceeded in three stages. Stage I involved penecontemporaneous/early8 diagenetic dolomitisation of the precurosr limestones with gypsum/anhydrite under hypersaline seepage reflux conditions. Stage II dolomitisation, mainly confined to the non‐anhydritic dolomite facies, was probably formed in the mixing zone between meteoric and sea water, probably at shallow depths of burial. Stage III dolomitisation occured at depth in the late stage of basin evolution, causing some infilling of mouldic and vuggy porosity by coarsely crystalline, saddle dolomite. Evidence of mixing zone dolomitisation indicates that the Jirani Dolomite was exosed subaerially. Studies of the Metlaoui Group show that this event can be related to a major relative fall in sea level in early Lutetian times following deposition of the Jdeir Formation. Exposure of the deposits to flushing by meteoric waters explains the dissolution of the anhydrite from the non‐anhydritic dolomite faices, and the accentuation of the well‐development porosity in this facies, which make it one of the most important reservoir rocks in the offshore region.