DIAGENETIC EVOLUTION OF INCISED CHANNEL SANDSTONES:  IMPLICATIONS FOR RESERVOIR CHARACTERISATION OF THE LOWER CARBONIFEROUS MARAR FORMATION,  GHADAMES BASIN, WESTERN LIBYA

S. Fröhlich^a*, J. Redfern^a, L. Petitpierre^a, J.D. Marshall^b, M. Power^c and P.V. Grech^d

^a North Africa Research Group, School of Earth, Atmospheric and Environmental Sciences, The   University of Manchester,  Oxford Road,  Manchester M13 9PL.

^b Earth and Ocean Sciences,  University of Liverpool,  L69 3BX.

^c formerly Intellection UK Ltd.  Present address: SGS UK Ltd, Rossmore Business Park, Ellesmere Port, Cheshire CH65 3EN.

^d formerly Woodside Energy (N.A.) Ltd. Tripoli, Libya.

Present address:  PETROM S.A., 1 Piata Eroilor, 100316 Ploiesti, Prahova, Romania.

^* Corresponding author. email: sebo.froe@gmx.de   Present address:  Statoil ASA,  Mølnholtet 42,  PO Box 40, 9481 Harstad, Norway.

This paper assesses the diagenetic history of potential fluvial hydrocarbon reservoir rocks deposited within incised valley systems of the Lower Carboniferous Marar Formation in western Libya. Outcrop data were collected in the Tinedhan Anticline, located at the southern margin of the Ghadames Basin. Four discrete intervals with channelized sandstones were identified in a section dominated by alternating offshore mudstones and shallow-marine clastics. The incised channels were cut during major sea-level lowstands, and filled by fluvial sandstone packages up to 50 m thick. Fifty-eight samples from four different localities, representing three lowstand systems tracts, were analysed to obtain a statistically meaningful mineralogical and compositional dataset. 

In addition to burial compaction, three main diagenetic events influenced the reservoir quality of the sandstones. Firstly, early eodiagenesis involved kaolinitization of plagioclase grains. This  began before subsequent calcite cementation, probably as a result of flushing by meteoric pore-waters. The deformation of kaolinite during later compaction resulted in the formation of pseudomatrix which further reduced porosity and permeability. Kaolinite is commonly transformed to illite at temperatures above 140°C in the presence of K-feldspar.  Although K-feldspar was recorded in the samples, no illite was observed, suggesting that the Lower Carboniferous strata in the study area were not buried in excess of approximately 3.5 km.

The second diagenetic phase was the precipitation of calcite cement, present either dispersed throughout the sandbodies or as concretions up to 2 m across, in both cases reducing reservoir quality. The high intergranular volumes (IGV) of calcite-cemented sandstones (ranging between 35% and 40%) suggest that cementation occurred at burial depths of <500 m. Sandstones without calcite cement have lower IGV of between 17% and 25% as a result of mechanical and chemical compaction.  Stable C and O isotope analysis of the calcite cement also supports precipitation at shallow burial depths, indicating a meteoric pore-water source for the calcite.  The third and final diagenetic stage was partial  chloritisation of  kaolinite during meso-diagenesis. The elevated temperatures required for this transformation indicate burial to a minimum depth of approximately 2.5 km, which is consistent with the compaction data.

Despite these diagenetic effects, the fluvial sandstones have an average porosity of 12%, with a range from 0.5% up to 25%. Permeability measurements on four sandstone samples indicate that the development of pseudomatrix did not reduce permeability significantly.

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