18-Geol-A3 Sedimentation and Stratigraphy · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2017-Dec. Closed book, 3 hours, no calculator permitted.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, stratigraphic principles, correlation, sequence stratigraphy); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (weathering, sandstone and carbonate classification, diagenesis); Allen & Allen, Basin Analysis, 3rd ed. (basin classification, subsurface mapping); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structure-contour, isopach and lithofacies mapping, syndepositional structures).
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.
| Map type | What is contoured | What it reveals |
|---|---|---|
| Structure-contour map | Elevation (depth) of a chosen stratigraphic marker/surface | Present-day structural geometry — folds, faults, dip direction, closure; identifies structural traps |
| Isopach map | True stratigraphic thickness between two marker horizons | Depositional/subsidence thickness patterns; identifies depocenters, syndepositional structural growth (thinning over highs, thickening into lows), and sediment source direction |
| Lithofacies map | Areal distribution or ratio of lithology within a unit (e.g. sand:shale ratio) | Depositional environment and sediment dispersal/paleocurrent trends; reservoir-quality trends |
Each map isolates a different variable, and their combined importance in basin analysis is that together they reconstruct the basin's full history: the structure-contour map records present-day (post-depositional) geometry, the isopach map records the depositional/subsidence history that built that geometry over time (including syndepositional growth on faults or folds), and the lithofacies map records the depositional environment and sediment-source pattern responsible for the rock types actually present. Used together they are the primary tools for generating and evaluating exploration prospects (mapping structural and stratigraphic trap geometry), for reconstructing subsidence and burial history, and for predicting reservoir-quality trends away from well control.
