Facies architecture and reservoir quality of Unit B, Permian Laingsburg Formation, southwestern Karoo Basin, South Africa

[摘要] This study presents a facies outcrop characterization and petrographical analysisof Unit B of the Permian Laingsburg Formation. Unit B is interpreted as a base-ofslopesystem, which represents a strikingly sand-rich succession. The base-of-slopesystem is defined by a channel-levee complex. The study provides systematically aclear understanding and description on reservoir heterogeneities, in terms of faciesdistribution, physical processes and architectural elements. The dataset includeddetailed sedimentary logs, photomosaic interpretations, supplemented by apetrographical study to determine the textural and compositional attributes of thestudied sandstones.Seven lithofacies was recognised within Unit B, based on detail observation anddescription on grain size and sedimentary structures. They mainly consist of 1) thickto massive bedded 'structureless' sandstone, 2) horizontal and ripple cross-laminatedthin-medium bedded sandstone, 3) silty sandstone, 4) structureless siltstone, 5)hemipelagic mudstone, 6) muddy slump, and 7) sandy slump. Palaeocurrent analysisindicates 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, raregroove and flute casts. Also, scour and fill features have been documented.Lithofacies 1 has been interpreted to result from channelized sandy debris flowcurrents. Lithofacies 2 composes of thin-medium bedded, very fine-grained sand,ungraded, sharp upper contacts, discrete units with traction bed forms, horizontal andcross-lamination, mud-draped ripples, internal erosional surfaces and preserved crests. Lithofacies 2 shows diagnostic sedimentary features for a deep-water bottomreworking current. Lithofacies 5 composes of very fine–grained mud, 'structureless'to finely horizontally laminated, fissile mudstone. Deposition resulted fromsuspension settling of mud fractions out of a low-energy buoyant plume. Lithofacies6 composes of contorted and convoluted bedding, steeply dipping layers and irregularupper contacts. Deposition occurred via slumping on an unstable slope. Lithofacies 7composes of fine–grained 'structureless' sandstone, amalgamated units, with darkfloating mudstone granules. Lithofacies 7 has been interpreted to form fromchannelized flows evolving into slump deposition on an unstable slope.The petrographic data reveals that the reservoir quality of the sandstones isstrongly controlled by depositional processes and diagenetic products. The sedimentsof the Karoo Basin appear to be diagenetically controlled as a function of burialdepth. The major diagenetic products controlling the reservoir quality of thesandstones, includes compaction (mechanical and chemical), and authigenic porefillingconstituents (quartz cement, feldspar dissolution and partial to completereplacement, calcite cement, chlorite and illite). Compaction played a major role inthe evolution of the sediment, as compared to the effect of quartz cementation, and isconsidered here to have caused irreversible destruction of depositional porosity andpermeability. The sediment has undergone intense mechanical compaction duringearly-stage diagenesis, low temperature and shallow depth of burial (probably thefirst 2 km). The high burial palaeotemperature (250 ± 500C) or more specifically thehigh geothermal gradient of the Karoo Basin consequently increased the number ofdiagenetic reactions. The high burial temperatures may have increased pressuredissolution and quartz cementation. With compaction been limited, quartzcementation and the authigenesis of chlorite and illite at deeper depths may have hada profound effect on the permeability distribution of the studied sandstones. After thecompletion of diagenesis, the pore systems of these sandstones were completelydestroyed by low-grade regional burial metamorphism.

[发布机构] Stellenbosch University