24-Pet-B4 Well Testing · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Petroleum Engineering, 2016-Dec. 3 hours duration; closed book. This sitting's own cover page reads “98-Pet-B4, Petroleum Geology” and every question is descriptive/interpretive petroleum geology (source rocks, hydrocarbon chemistry, oil sands and tight-oil development, shale gas and coalbed methane, carbonate traps, siliciclastic/deltaic traps) – no well-test pressure-transient content anywhere. Five (5) of the paper's six 20-mark sections are marked (NOTES item 5); all six are solved in full below so this set also serves as a complete study reference. This sitting's Section 6 is a Siliciclastic Traps section (wave-dominated delta, grain-size–permeability–porosity relations, sandstone diagenesis). The paper is entirely qualitative (draw/describe/define/list), with no numeric given data anywhere.
Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps, carbonate systems, oil sands); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, maceral groups, catagenesis); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock, deltaic and carbonate lithofacies; sandstone diagenesis); James, N.P. & Jones, B., Origin of Carbonate Sedimentary Rocks, Wiley-Blackwell (carbonate platform/ramp/sabkha facies models); Butler, R.M., Thermal Recovery of Oil and Gas, Prentice Hall, 1991 (SAGD, CSS, oil sands thermal recovery); Green, D.W. & Willhite, G.P., Enhanced Oil Recovery, SPE Textbook Series Vol. 6 (thermal EOR mechanisms); Law, B.E. & Curtis, J.B., “Introduction to Unconventional Petroleum Systems,” AAPG Bulletin 86, 2002 (shale gas, tight gas, coalbed methane); Lee, W.J. & Wattenbarger, R.A., Gas Reservoir Engineering, SPE Textbook Vol. 5 (unconventional gas reservoir characterization).
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.
As a sandstone is progressively buried it undergoes several diagenetic processes, most of which reduce porosity and permeability, though some (dissolution) can locally restore it. (1) Mechanical compaction – increasing overburden stress causes grain rotation, slippage and repacking into a tighter fabric, reducing porosity from a typical depositional ≈40–45% toward ≈25–30% over the first ≈1–2 km of burial; ductile grains (mica, rock fragments) can also be plastically deformed, further occluding pores. (2) Chemical (pressure) compaction – at higher effective stress and temperature, quartz grains dissolve preferentially at their stressed contacts (pressure dissolution, producing sutured/stylolitic grain contacts), further reducing porosity and providing a local source of silica for cementation. (3) Cementation – precipitation of new mineral cement (most commonly quartz overgrowths, but also calcite, dolomite, or authigenic clay such as illite or chlorite) into remaining pore space, progressively occluding porosity and, because cement preferentially blocks the narrowest pore throats first, reducing permeability disproportionately faster than porosity. (4) Dissolution (secondary porosity generation) – acidic pore fluids (e.g. organic acids and CO2 generated during hydrocarbon maturation, or meteoric water at an unconformity) can dissolve unstable grains (feldspar, lithic fragments) or earlier carbonate cement, creating secondary (oversized, moldic or vuggy) porosity that can locally reverse the burial-porosity trend and produce anomalously good reservoir quality at depth. (5) Authigenic clay growth/illitization – smectite converting to illite or direct authigenic illite/chlorite growth lines pore walls and throats, which can severely reduce permeability (by pore-throat lining/bridging) with only a modest effect on total porosity, since the clay itself occupies little volume but blocks flow paths.
Overall, porosity and permeability both decline systematically with burial depth in the absence of dissolution, following a roughly exponential porosity–depth trend; permeability declines faster and less predictably than porosity because it is controlled by the (increasingly cement- or clay-occluded) pore-throat geometry rather than by pore volume alone – the same throat-size sensitivity already established for grain size in Q6-2.
