G. H. Swei1 and Maurice E. Tucker*2

1 Earth Science Department, Nasser Nation University, Tripoli, Libya.

2 Department of Earth Sciences, Durham University, Durham, DH1 3LE.

* Author for correspondence, email:

The reservoir quality of Middle Eocene carbonates in the intracratonic Sirt Basin is strongly influenced by depositional facies and various diagenetic processes. Based on data from core samples and well logs from five boreholes in the Assumood and Sahl gasfields in the subsurface of the north-central Sirt Basin, six major carbonate facies (and fourteen microfacies) are distinguished in the Middle Eocene Gialo Formation (thickness ~1100 ft / 335 m). Wackestones-packstones dominate the Gialo Formation with abundant grains of larger benthic foraminifera, especially nummulites, in a matrix composed mostly of finely comminuted bioclastic material. Sediments were deposited as a mosaic of facies on a broad carbonate ramp in moderate-energy nummulite-packstone banks, with locally restricted back-bank lagoons and fore-bank areas of foraminiferal debris passing to offshore lime mudstones.

Marine diagenesis was minor with micritization of bioclasts and rare vadose marine fibrous cements. Sparry calcite cements are ubiquitous and were precipitated during shallow to moderate burial from seawater and/or meteoric water, the latter largely unaffected by surface-pedogenic processes. This is indicated by the fabrics and pre- and post-compaction precipitation of drusy spar and echinoderm overgrowths, and supported by their bright-to-dull luminescence and low negative d18O signatures (-2.68 to -4.16 PDB). Some early neomorphic alteration of calcitic bioclasts is suggested by bright CL and marine to low negative d18O values (-1.06 to 3.93 PDB). Bioclastic grains have d13C values ranging between 0.76 and 1.19 PDB, interpreted as marine signatures. Similar low positive d13C values of the cements (0.9 to 1.05 ) indicate a source of carbonate from dissolution of grains and/or seawater/meteoric water, but without any near-surface/soil effects. Dissolution of grains and matrix, notably originally aragonitic grains but also the originally high-Mg calcite nummulites, was a major porosity-enhancing process, and took place from shallow to moderate burial depths. Fractures may locally have increased porosity too.

Shallow-water packstones/rudstones containing both primary intergranular and secondary biomouldic porosity have the best reservoir quality and these are concentrated in the upper parts (top ~100 ft / 30 m) of the wells, with porosity-permeability decreasing downwards. This trend relates to the broad, large-scale facies pattern of more mud-dominated facies giving way upwards to more grainy, nummulite-dominated facies as a result of an overall shallowing of the depositional environment. In addition, the influx of meteoric waters in the upper part of the Gialo platform, before drowning and deposition of the overlying Augila Shale, increased porosity through dissolution. This integrated study has helped in understanding the reservoir heterogeneity and hydrocarbon potential of the Gialo carbonates.

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