DIAGENETIC EVOLUTION OF INCISED CHANNEL SANDSTONES: IMPLICATIONS FOR RESERVOIR
CHARACTERISATION OF THE LOWER CARBONIFEROUS MARAR FORMATION,
S. Fröhlicha*, J. Redferna, L. Petitpierrea, J.D. Marshallb, M. Powerc and P.V. Grechd
a North Africa Research Group, School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL.
b Earth and
c formerly Intellection
UK Ltd. Present address: SGS UK Ltd,
d formerly Woodside Energy (N.A.) Ltd.
Present address: PETROM
* Corresponding author. email:
email@example.com Present address: Statoil
ASA, Mølnholtet 42,
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
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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