Keywords
Sirt Basin,
facies,
diagenesis,
porosity,
stable isotopes,
fluid inclusions,
Abstract
The Selandian/Thanetian succession in the western Sirt Basin, Libya, is mainly composed of carbonates with lesser amounts of shale. They were deposited on a platform-homoclinal ramp with inner, mid and outer ramp facies, each with distinctive microfacies, ranging from mud-supported to grain-dominated carbonates. Two phases of dissolution, near-surface and burial, affected the late Paleocene succession. Marine and meteoric cements are minor, but early dolomite is locally developed. Burial compaction is widespread, associated with calcite and dolomite cements. The average δ13C values in the Dahra and Zelten/Harash formations are 2.3‰ and 3.2‰, respectively. Both show no significant change up through the section, which suggests a stable carbon isotope composition of seawater through Selandian/Thanetian time, with little latter diagenetic alteration through organic matter decomposition. On the other hand, the δ18O data show more negative values than most signatures reported for the Paleocene; this is largely the result of meteoric water influx and/or burial cementation-neomorphism under increasing temperature; this is supported by fluid-inclusion results.
The best porosity is recorded in grainstones of the Dahra Formation, whereas, the Zelten and Harash formations have much lower porosity. The highest porosity is developed in bioclastic foraminiferal grainstone, bioclastic foraminiferal packstone-packstone/grainstone facies and, less important, foraminiferal nummulitic packstone. The porosity evolution in the Selandian/Thanetian succession is controlled by original depositional texture andsubsequent diagenesis.