Typical latitudes for carbonate bank formation: 24-Pet-B4 Well Testing, December 2015, Q14

Question 14 of 18: Typical latitudes for carbonate bank formation

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2015-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, migration/unconventional reservoirs, carbonate traps, structural traps, Canadian basin geography) – 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. The paper is almost entirely qualitative (draw/describe/define/list), with one true numeric calculation (Q3-2, capillary seal-breach column height).

Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps, carbonate systems); 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 and carbonate lithofacies); Allen, P.A. & Allen, J.R., Basin Analysis: Principles and Applications to Petroleum Play Assessment, 3rd ed., Wiley-Blackwell (structural styles, unconventional systems); Biddle, K.T. & Wielchowsky, C.C., “Trap Types in Petroleum Basins,” AAPG Memoir 60, ch.12 (structural trap classification); James, N.P. & Jones, B., Origin of Carbonate Sedimentary Rocks, Wiley-Blackwell (carbonate platform/ramp/sabkha facies models); Law, B.E. & Curtis, J.B., “Introduction to unconventional petroleum systems,” AAPG Bulletin 86 (basin-centred gas); Mossop, G.D. & Shetsen, I. (eds.), Geological Atlas of the Western Canada Sedimentary Basin, CSPG/Alberta Research Council, 1994 (WCSB stratigraphy and Canadian basin geography).

Section 4, Q4-3: Typical latitudes for carbonate bank formation (2 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.

Carbonate banks form predominantly in the tropical to subtropical belt, roughly between 30°N and 30°S latitude. This range corresponds directly to the warm (generally above 18–20 °C), sunlit surface-water conditions required by the reef-building corals and calcareous algae that construct most carbonate-bank margin material, matching both the modern distribution of the Bahama Banks, the Persian Gulf, and the Great Barrier Reef, and the position of ancient reef-trend deposits such as the Devonian Leduc platform of the WCSB (Q6-1).

Topic. Climatic control on carbonate factory location.

Key relations. Carbonate production requires warm water (generally > 18–20 °C for coral/algal calcification) and sufficient light penetration for photosynthetic symbionts (zooxanthellae in corals, algae) – both conditions are met within roughly ±30° of the equator.

Why this works. Because most carbonate sediment is biologically precipitated (a “carbonate factory”) rather than clastically transported, its geographic distribution tracks the physiological limits of the calcifying organisms directly, unlike siliciclastic deposition, which can occur at any latitude given a sediment source.

Common pitfall. Assuming carbonates cannot form outside this belt at all – cool-water (non-tropical) carbonate factories (bryozoan/foraminiferal, e.g. southern Australia) do exist at higher latitudes, but they are volumetrically and reservoir-quality minor compared with the tropical reef/shoal factory targeted by this question.

Source. James & Jones, ch.1 (carbonate factories); Selley & Sonnenberg, ch.8.