Abstract

---

The Lower Acacus Formation of Upper Silurian age in the Ghadames (Hamada) Basin, NW Libya,is an important sandstone reservoir. The formation is buried at a depth of 7500ft in the southern part of the basin and down to 8500ft in the northern part.
The Lower Acacus Formation is characterized by regressive and transgressive cyclic sequences of sandstones, siltstone and shale, whose deposition and distribution were controlled by relative changes in the sea level. The Lower Acacus Formation deposited in a fluvio-deltaic system that prograded northward and into the northwestern flank of the intracratonic Ghadames (Hamada) Basin. A combination of optical microscopy and stable isotopic analyses were used in order to provide detailed constraints on the composition of calcite cements, the evolution of pore fluid types, and the relative timing of calcite cement precipitation.
The principal cements occluding porosity include carbonates, quartz overgrowth, and some subordinated kaolinite and illite. Two types of calcite cements have been distinguished based on texture and manner of occurrence in thin-sections: 1) Patchy calcite cement which regarded as shallow calcite cement, occurs in the southerly shallower portions of the basin characterized the fluvial, iron oxide-rich sandstones (Af2-Af7 units). 2) Poikilotopic calcite cement which regarded as deep calcite cement, occurs in the northerly deeper parts of the basin characterized the deltaic sandstones (A8-A14 units).
Isotopic compositions of calcite cements in the Lower Acacus Formation reflect different regional paleo-fluid regimes from meteoric to mixed waters. Relatively shallow depth patchy calcite-cement is associated with sandstone units of fluvial origin. This cement formed from enriched dδ¹⁸O meteoric waters at low temperature and the lighter negative δ¹³C nature of this calcite suggests an involvement of organically derived CO₂. Deeper depth poikilotopic calcite-cement formed from waters depleted in δ¹⁸O that became progressively hotter, more reducing and saline as they flowed down-dip to mix with the saline waters in sandstone- siltstone units of deltaic origin, with negative d13C values which probably record the increasing importance with depth of bicarbonate production by thermal decarboxylation.
It is important to note that it is difficult to determine the timing (early or late) of the patchy shallow to homogeneously distributed poikilotopic deep calcite cements. However the data suggest that two possible deltaic systems prograded from NE to W and from S to NNW may be inferred, in which both the shallow and the deep calcite cements were formed synchronously. The shallow cement was precipitated in the fluvial sandstones from meteoric waters and the deep calcite cement was precipitated in the deltaic sandstones as the waters flowed down-basin becoming progressively more saline and reducing.