Facies architecture and reservoir quality of Unit B, Permian Laingsburg Formation, southwestern Karoo Basin, South Africa
Thesis (MSc)--Stellenbosch University, 2013.
This study presents a facies outcrop characterization and petrographical analysis of Unit B of the Permian Laingsburg Formation. Unit B is interpreted as a base-ofslope system, which represents a strikingly sand-rich succession. The base-of-slope system is defined by a channel-levee complex. The study provides systematically a clear understanding and description on reservoir heterogeneities, in terms of facies distribution, physical processes and architectural elements. The dataset included detailed sedimentary logs, photomosaic interpretations, supplemented by a petrographical study to determine the textural and compositional attributes of the studied sandstones. Seven lithofacies was recognised within Unit B, based on detail observation and description on grain size and sedimentary structures. They mainly consist of 1) thick to massive bedded ‘structureless’ sandstone, 2) horizontal and ripple cross-laminated thin-medium bedded sandstone, 3) silty sandstone, 4) structureless siltstone, 5) hemipelagic mudstone, 6) muddy slump, and 7) sandy slump. Palaeocurrent analysis indicates that the mean sediment transport direction of Unit B was to the E and NE. Lithofacies 1 comprises thickly to massive bedded, frequently amalgamated, mostly very-fined grained sand, mixed grading, irregular to sharp upper contacts, structured upper bedding planes, large floating mudstone clasts and granules, rare groove and flute casts. Also, scour and fill features have been documented. Lithofacies 1 has been interpreted to result from channelized sandy debris flow currents. Lithofacies 2 composes of thin-medium bedded, very fine-grained sand, ungraded, sharp upper contacts, discrete units with traction bed forms, horizontal and cross-lamination, mud-draped ripples, internal erosional surfaces and preserved crests. Lithofacies 2 shows diagnostic sedimentary features for a deep-water bottom reworking current. Lithofacies 5 composes of very fine–grained mud, ‘structureless’ to finely horizontally laminated, fissile mudstone. Deposition resulted from suspension settling of mud fractions out of a low-energy buoyant plume. Lithofacies 6 composes of contorted and convoluted bedding, steeply dipping layers and irregular upper contacts. Deposition occurred via slumping on an unstable slope. Lithofacies 7 composes of fine–grained ‘structureless’ sandstone, amalgamated units, with dark floating mudstone granules. Lithofacies 7 has been interpreted to form from channelized flows evolving into slump deposition on an unstable slope. The petrographic data reveals that the reservoir quality of the sandstones is strongly controlled by depositional processes and diagenetic products. The sediments of the Karoo Basin appear to be diagenetically controlled as a function of burial depth. The major diagenetic products controlling the reservoir quality of the sandstones, includes compaction (mechanical and chemical), and authigenic porefilling constituents (quartz cement, feldspar dissolution and partial to complete replacement, calcite cement, chlorite and illite). Compaction played a major role in the evolution of the sediment, as compared to the effect of quartz cementation, and is considered here to have caused irreversible destruction of depositional porosity and permeability. The sediment has undergone intense mechanical compaction during early-stage diagenesis, low temperature and shallow depth of burial (probably the first 2 km). The high burial palaeotemperature (250 ± 500C) or more specifically the high geothermal gradient of the Karoo Basin consequently increased the number of diagenetic reactions. The high burial temperatures may have increased pressure dissolution and quartz cementation. With compaction been limited, quartz cementation and the authigenesis of chlorite and illite at deeper depths may have had a profound effect on the permeability distribution of the studied sandstones. After the completion of diagenesis, the pore systems of these sandstones were completely destroyed by low-grade regional burial metamorphism.