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24-Pet-B4 Well Testing · Undated paper

Question 7 of 13: Vitrinite and vitrinite reflectance as a geothermometer

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 17-Pet-B4 (May 2019, per its page footers). 3 hours duration; closed book. This paper's own cover page reads “17-Pet-B4, Petroleum Geology” and all five sections are descriptive/interpretive petroleum geology (terminology, source rocks, thermal maturation, stratigraphic traps, structural traps), no well-test pressure-transient content anywhere. The section marks are Section 1 = 30, Section 2 = 18, Section 3 = 22, Section 4 = 15, Section 5 = 15 (sum 100), used throughout below. The exam is entirely qualitative (explain/define/sketch).

Reference texts: Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, maceral groups, maturation stages, geothermometers); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (petroleum terminology, source rocks, traps); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock petrology); Allen, P.A. & Allen, J.R., Basin Analysis: Principles and Applications to Petroleum Play Assessment, 3rd ed., Wiley-Blackwell (structural trap styles); Biddle, K.T. & Wielchowsky, C.C., “Trap Types in Petroleum Basins,” AAPG Memoir 60, ch.12 (stratigraphic and structural trap classification); Mossop, G.D. & Shetsen, I. (eds.), Geological Atlas of the Western Canada Sedimentary Basin, CSPG/Alberta Research Council, 1994 (Canadian trap examples).

Section 3, Q3-2: Vitrinite and vitrinite reflectance as a geothermometer (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.

Vitrinite is a maceral group formed by the humification and gelification (coalification) of lignin- and cellulose-bearing woody plant tissue – it is the dominant maceral of coals and is present, in lesser amount, disseminated through most marine and non-marine source rocks that receive any terrestrial plant input (part of Type III kerogen, Q2-2). Vitrinite reflectance (Ro) is the percentage of incident light reflected from a polished vitrinite particle under oil-immersion microscopy.

As a geothermometer: Ro increases progressively and IRREVERSIBLY with increasing cumulative time–temperature exposure (it never decreases, even if the rock later cools), which makes it a true integrated record of a rock's peak thermal history rather than a snapshot of present-day temperature. Because vitrinite is present in nearly every source rock regardless of that rock's own dominant kerogen TYPE, a single Ro measurement directly calibrates where a given sample sits within the three maturation stages of Q3-1: Ro < 0.5% – immature (diagenesis); Ro ≈ 0.5–1.3% – oil window (catagenesis); Ro ≈ 1.3–2.0% – wet gas/condensate (late catagenesis); Ro > 2.0% – dry gas window (metagenesis); Ro > 4% – overmature/graphitized, no remaining potential. A measured Ro-vs-depth profile through a well therefore locates the actual top of the oil window (and the other stage boundaries) in that specific basin, without needing to know or assume its geothermal gradient or burial history independently.