Fluid inclusions in quartz and dolomite cements were used to determine temperature, timing, origin, and function of fluids in the diagenetic evolution of the Lower Cretaceous Upper Sarir Sandstone (Sirt basin, Libya). Transmitted light, backscattered electron, and scanning electron microscope (SEM) cathodoluminescence analyses were carried out to determine the paragenesis. In quartz overgrowths, fluid inclusions along the grain-overgrowth boundary yield homogenization temperatures from 130 to 141degreesC and salinities between seawater and freshwater. Aqueous fluid inclusions in later quartz overgrowths have homogenization temperatures from 114 to 129degreesC and salinities between IS and 21 wt. % NaCl equivalent. Fluid inclusions in dolomite have variable homogenization temperatures (120-141degreesC) and high salinities (2022 wt. % NaCl equivalent). This trend in homogenization temperatures of fluid inclusions is interpreted as the result of quartz cement precipitation in the presence of hot, low-salinity, connate fluids connected with an increased heat flow related to regional rifting. Later quartz overgrowths precipitated during regional cooling concurrent with the end of the rifting and after invasion of saline fluids, possibly brought in from adjacent areas or refluxed downward during or after times of evaporite deposition, This diagenetic history linked to a decreasing heat flow could be common in other aborted-rift basins, which are important targets for oil exploration.