Open versus closed mesogenetic systems in Cretaceous fluvial and tidal sandstones, Sirt Basin, Libya

Abstract

This study constrains factors controlling the distribution of diagenetic alteration and their impact on reservoir quality of the Cretaceous sandstones from the Al-Bayda Platform, located in the southern Sirt Basin (Libya). These factors include the presence of early cements as well as the influx of hot basinal brines. The studied samples come from two blocks in the Khalifa Field, which are dislocated by a major normal fault. The deep-burial (mesogenetic) alteration includes the partial to pervasive replacement of early (eogenetic) dolomite and calcite cements by ferroan-dolomite, ankerite and siderite, precipitation of grain-coating chlorite, and cementation by quartz overgrowths, barite and anhydrite, particularly in the downthrown block. The association of quartz overgrowths with barite suggests that deep burial was influenced by the influx of hot basinal brines through faults. Conversely, deep-burial alteration in braided fluvial deposits of the Nubian sandstones of the upthrown block include: illitization of eogenetic smectite, quartz cementation and formation of chlorite. This study shows that deep burial of the studied sandstones did not occur in a closed system, but was affected by the influx of hot basinal brines through faults, which formed during basin rifting. This interpretation is supported by the relatively high homogenization temperatures (100–110°C; corrected to 110–125°C) of primary fluid inclusions within quartz overgrowths, which exceed the maximum burial temperatures experienced by the Cretaceous succession, and by the high salinity of these inclusions.