18-Geol-A3 Sedimentation and Stratigraphy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2015-May. Closed book, 3 hours.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate classification, stratigraphic principles and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, diagenesis, evaporites, phosphorites); Selley & Sonnenberg, Elements of Petroleum Geology (reservoir quality, subsurface wireline-log interpretation).
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The answer is (b) Identification of the nature of fluids present in subsurface. The caliper log is a purely MECHANICAL/GEOMETRIC tool — a set of spring-loaded arms measuring borehole DIAMETER versus depth — and carries no direct information about the chemical or physical nature of the pore fluid (oil, gas or water); fluid identification instead requires resistivity, neutron, density or NMR logs (or direct fluid sampling), none of which the caliper tool provides.
The other four uses ARE legitimate applications of a caliper curve, because hole diameter itself responds systematically to rock character: (a) weak, poorly consolidated or fractured permeable intervals tend to WASH OUT (enlarged hole, over-gauge reading), while a permeable zone that has developed mud-cake instead reads UNDER bit size — both signatures distinguish porous/permeable zones from tight, competent, on-gauge non-porous zones; (c) and (e) abrupt changes in caliper reading mark bed boundaries, letting stratigraphic INTERVAL BOUNDARIES (and hence true bed THICKNESS, once the caliper-derived volume/depth correction is applied) be picked directly from the log; (d) different lithologies respond characteristically to drilling and mud exposure (shale commonly washes out badly; well-cemented sandstone or hard carbonate tends to hold gauge), so the caliper's hole-condition response is itself a useful, if indirect, LITHOLOGIC indicator, and caliper-derived borehole volume is also the standard correction applied to every other log (density, resistivity) before those tools' own lithologic interpretation is trusted.
A well-site geologist therefore treats the caliper trace as a first-pass, purely mechanical screen — flag washed-out and mud-caked intervals before interpreting anything else from the log suite — and reaches for the fluid-sensitive tools (resistivity contrast against a water-saturated baseline, neutron-density crossover, or NMR where available) only once that borehole-geometry correction has already been applied.
