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24-Pet-A1 Principles of Stratigraphy and Sedimentation · Undated paper

Question 17 of 19: Wireline Logs for Subsurface Stratigraphic Correlation

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Petroleum Engineering, 17-Pet-A1 Principles of Stratigraphy & Sedimentation, 2019-May. 3 hours duration; closed book, approved Sharp/Casio calculator permitted. The paper has two parts: Part A (Questions 1–10, Sedimentology and Sedimentary Processes) – Questions 1 and 2 are mandatory (10 marks each, 20 marks), plus any five of the remaining eight (3–10) at 6 marks each (30 marks), for a Part A total of 50 marks; and Part B (Questions 11–19, Stratigraphy and Sedimentary Basin Analysis) – answer any six of the nine at 6 marks each, for a Part B total of 36 marks – an 86-mark maximum (50 for Part A + 36 for Part B).

Check: the mark values and question count used throughout this solution (6 marks each for Questions 3–19, Part A = 50, Part B = 36, maximum = 86) are as printed on the paper.

Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (grain texture, sediment transport, bedforms, carbonate/evaporite systems, sequence stratigraphy, unconformities, stratigraphic principles); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (sandstone/carbonate classification, diagenesis, porosity); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial/deltaic/deep-marine systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, alluvial fans, deltas, deep-marine systems); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (basin analysis, well-log correlation, seismic/acoustic impedance); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).

Question 17: Wireline Logs for Subsurface Stratigraphic Correlation (6 marks)

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.

Wireline (electric) logs record a continuous, depth-registered physical property of the rock penetrated by a borehole; because the same regionally-persistent marker beds (commonly shales, coals or thin limestones) produce a recognizable, repeatable log signature, matching that signature "bed-for-bed" between wells is one of the most reliable subsurface stratigraphic correlation tools available, especially where core or cuttings control is sparse or absent entirely.

Gamma-ray (GR) log – records natural formation radioactivity (mainly from clay-hosted K, Th, U); shale/mudstone reads high GR, clean sandstone/carbonate/coal reads low GR, so a GR curve traces the shale-versus-clean stratigraphic pattern directly and is the standard correlation curve because it works in both open and cased holes and in any borehole fluid. Spontaneous potential (SP) log – records the natural electrochemical/electrokinetic potential generated where permeable, porous beds contact the (fresher) drilling-mud filtrate against saline formation water; permeable sand deflects the SP curve away from a stable "shale baseline," giving a similarly reliable, if fluid-dependent, correlation marker.

Hypothetical GR wireline-log correlation, two wells Well A Well B marker shale (correlated) low GR (clean sand) ←→ high GR (shale)
Hypothetical GR wireline-log correlation between two wells through horizontally stratified rocks of differing lithology: two regionally-persistent shale peaks (high GR, red and green marker lines) are matched bed-for-bed between Well A and Well B, correlating the intervening clean-sand package across the field even though its thickness and internal detail differ slightly between wells.