Flow unit determination of the upper shallow marine sandstone resevoirs in the Bredasdorp Basin, offshore South Africa
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Date
2024
Authors
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Publisher
University of Fort Hare
Abstract
This study comprehensively analyses reservoir flow units and distribution in four wells (E-E1, E-S3, E-S5, and E-W1) of the lower Cretaceous upper shallow marine sandstone reservoirs in the Bredasdorp Basin, offshore South Africa. The study integrated multiple techniques, including the Stratigraphic Modified Lorenz Plot, Improved Stratigraphic Modified Lorenz Plot, Winland pore throat radius, Stratigraphic Flow Profile, and Flow Zone Indicator, to establish the spatial flow unit distribution of the upper shallow marine reservoirs. Analyses of 400 core plugs, 14 thin section slides, and 15 Scanning Electron Microscopy samples and core logging provided detailed insights into petrophysical properties and diagenetic minerals affecting the flow units within the reservoir. A comprehensive examination of gamma-ray logs and core samples identified a prominent glauconitic sandstone reservoir in the upper shallow marine formations. The conventional core analysis revealed substantial variability in porosity (0.3 % - 18 %, average 6.4 %) and permeability (0.008 - 170 mD, average 24.5 mD). Stratigraphic Modified Lorenz Plot Identified three flow units: barrier, baffle, and normal flow units, with storage capacity and flow capacity, ranging from 1.6 – 12.9 and 0.08 – 32.6, respectively. Improved Stratigraphic Lorenz Plot (ISMLP) results identified six flow unit types: barrier, semi-barrier, semi-conductor, conductor, superconductor, and fractured. Flow unit ranking indicated reservoir quality ranges from barrier/seal (0°- 15°) to fractured (75° - 85°). Pore throat radius (R35) results revealed diverse rock-type distributions from nanoporous to macroporous, ranging from 0.3 µm to 6.6 µm. These rock types indicated fair reservoir quality. Flow Zone Indicator (FZI) identified five distinct Hydraulic Flow Units (HFUs): HFU 1, HFU 2, HFU 3, HFU 4, and HFU 5, with values ranging from 0.19 µm to 28.7 µm, with an average value of 2.9 µm. Flow zone correlation revealed widespread low-flow zones and localized moderate-flow zones. Core logging analysis revealed distinct lithological characteristics among the reservoirs. Wells E-E1 and E-S3 comprise medium- to coarse-grained sandstones with rip-up clasts of claystone and siltstone, predominantly pyritic and glauconitic. In contrast, well E-S5 exhibits cross-bedded sandstones, fining upward with claystone clasts, pebble shaped lithic grains, and conglomerates formed from rounded gravel. E-W1 consists of fine- to medium-grained sandstones and conglomerates with scattered pebbles. Multiple facies were identified in each well's cored intervals, with well E-E1 containing five facies (A-E), wells E-S3 and E-S5 having four facies (A-D), and well E-W1 having three facies (A-C). Diagenetic minerals such as glauconite, pore-filling illite, smectite, and cementations (quartz overgrowth and calcite) significantly affected the baffle and barrier flow units. The results from this study will improve the understanding of flow unit distribution and reservoir heterogeneity in Bredasdorp Basin. It can also be used as input parameters for the 3D dynamic flow unit modeling of the upper shallow marine sandstones in this Basin.
Description
Masters dissertation
Keywords
Sandstone -- South Africa, Petroleum -- Geology -- South Africa, Porosity, Permeability
Citation
Mudau, N. (2024) Flow unit determination of the upper shallow marine sandstone reservoirs in the Bredasdorp Basin, offshore South Africa. MSc (Geology) dissertation. Alice, South Africa: University of Fort Hare.