Facies architecture and reservoir quality of Unit B, Permian Laingsburg Formation, southwestern Karoo Basin, South Africa
Date
2013-03
Authors
Lombard, Donovan Joseph
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
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.
Description
Thesis (MSc)--Stellenbosch University, 2013.
Keywords
Karoo Basin, Slope channel deposits, Diagenesis, Dissertations -- Earth sciences, Theses -- Earth sciences, Facies (Geology) -- South Africa -- Karoo, Geology, Structural -- South Africa -- Karoo, Laingsburg Formation (South Africa)