Using Elemental Geochemisty to Improve Sandstone Reservoir Characterization: a Case Study From the Unayzah A Interval of Saudi Arabia
- Douglas Hardman (Saudi Aramco) | Robin MacDonald (Saudi Aramco) | Ronald Sprague (Saudi Aramco) | Yacine Meridji (Saudi Aramco) | Witjaksono Mudjiono (Saudi Aramco) | James Galford (Halliburton) | Marvin Rourke (Halliburton) | Michael Dix (Weatherford) | Michael Kelton (Core Laboratories)
- Document ID
- Society of Petrophysicists and Well-Log Analysts
- Publication Date
- October 2011
- Document Type
- Journal Paper
- 344 - 356
- 2011. Society of Petrophysics and Well Log Analysts
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- 246 since 2007
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The sandstones of the Permian Unayzah A reservoir in Saudi Arabia are an active target for natural gas exploration and development. These sandstones were deposited mainly in eolian dune complexes and sand sheets, as well as in playas and ephemeral streams in interdune areas. The detrital mineralogy is, in most cases, relatively simple. Grains are dominated by quartz, with minor feldspars, lesser lithic fragments, and little primary clay. Diagenetic components, however, can make the bulk-rock mineralogy and reservoir quality much more variable; pore-filling cements can include quartz, carbonates, anhydrite, and illite. Anhydrite nodules can impart significant heterogeneity in mineralogy and porosity. Because of these factors, determination of fundamental reservoir-quality parameters using the typical triple- or quad-combo wireline log suites can be challenging. Measurements of rock matrix elemental concentrations performed on drill cuttings or obtained from wireline logs provide additional information related to the mineralogical composition of the formation that can improve log-derived determinations of reservoir parameters.
Whole-rock elemental concentrations are a powerful data set for characterizing and correlating sandstone reservoirs, especially fluvial-olian sequences such as the Unayzah A that are devoid of fossils and have few shaly zones. For this study, a detailed mineralogical analysis was conducted on a 60-ft section of core, composited from 1-ft intervals of slabbed material. X ray diffraction analysis for mineralogy, inductively-coupled plasma spectroscopy and X ray fluorescence spectroscopy were used to determine elemental composition for 55 elements. This data validated elemental concentration logs obtained by a new-generation wireline neutron-induced capture gamma-ray spectroscopy tool, (GEM™), capable of measuring ten formation elements run over the same interval.
Log examples and core results are used to illustrate how Unayzah A sandstone mineralogy can be predicted from elemental data using error-minimization modeling and theoretical forward modeling. In addition to sophisticated predictive models, we show how a series of simple binary and ternary plots based on major-element concentrations or ratios can be constructed to give precise information about mineralogy, lithology, reservoir quality, and even depositional facies. These techniques are a valuable aid to selection of the mineral assemblage that will be used to perform petrophysical interpretations of logs. Although the data and examples presented in the paper are unique to the Unayzah A reservoir interval, the concepts demonstrated in the paper can be applied to other complex reservoirs.
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