Characteristics of Permian Tanqua Karoo Deepwater Mudstones
Bruce M. Samuel, Arnold H. Bouma, 2003. "Characteristics of Permian Tanqua Karoo Deepwater Mudstones", Siltstones, Mudstones and Shales: Depositional Processes and Characteristics, Erik D. Scott, Arnold H. Bouma, William R. Bryant
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Turbidite-related mudstones are an integral part of any submarine fan complex. The Permian Tanqua Karoo in southwestern South Africa is an excellent example. Thick-bedded mudstones, 10–50 m thick, subdivide the individual sand-rich fans within a fan complex. The 30–50 cm thick medium-bedded mudstones and 1–30 cm thick thin-bedded mudstones occur between the sandstone layers of a fan system and may act as baffles to fluid flow.
Characteristics of thick-bedded shales are that the middle fan areas (leveed channels) show predominantly alternating fining-upward sequences of parallel laminated mudstones and mud-shales (∼50–60% clay) and clay-stones and clay-shales (∼80–95% clay). A sharp contact typifies the location of the contact between the mud and the overlying clay.
Medium-bedded shales are typically more siliceous. They are comprised of cyclic fining-upward sequences of quartz-rich siltstones, mudstones and sometimes a clay-shale lamination. However, clay-shale preservation is rare. A thin-bedded siltstone (∼10–20% clay), up to 0.5 cm thick, predominates at the base of each sequence and is capped by laminations of mud-shale (∼35–60% clay), which are generally less than 5 mm thick.
Thin-bedded shales are characterized by fining-upward sequences comprised of siltstone (∼20–30% clay), mud-shale (∼35–55% clay), and clay-shale lamina. The sequence is typically mud-rich and the mud-shale often expresses an amalgamated contact with the clay-shale material above.
The limited thickness of thin-bedded shales and the siliceous nature of medium-bedded shales suggests that fluid and gas flow may be possible. Even minor tectonics result in fractures. The combination of the clays with some micas and organics suggest that many of these mudstones can be source rocks and gas reservoir at the same time.
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Siltstones, Mudstones and Shales: Depositional Processes and Characteristics
Siltstones, mudstones and shales have been studied mainly with regard to clay mineralogy and general transport-deposition. Recent studies on deepwater deposits from cores and outcrops have shown that fluid flow properties of deepwater reservoirs are greatly affected by the presence of finer-grained deposits in the reservoir. Initial analysis indicates that the majority of these finer grained deposits have a large silt component and are closer to siltstones rather than mudstones, commonly called shales To date, little attention has been given to their characteristics resulting from different depositional processes.