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24-Pet-B4 Well Testing · December 2014

Question 7 of 18: Primary migration mechanisms

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2014-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, generation, migration, 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); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, oil/gas windows, primary migration); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock lithofacies); Allen, P.A. & Allen, J.R., Basin Analysis: Principles and Applications to Petroleum Play Assessment, 3rd ed., Wiley-Blackwell (extensional basins, structural styles); Biddle, K.T. & Wielchowsky, C.C., “Trap Types in Petroleum Basins,” AAPG Memoir 60, ch.12 (stratigraphic/structural/salt trap classification); Mossop, G.D. & Shetsen, I. (eds.), Geological Atlas of the Western Canada Sedimentary Basin, CSPG/Alberta Research Council, 1994 (WCSB stratigraphy and play types).

Section 3, Q3-1: Primary migration mechanisms (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.

Primary migration mechanisms out of source rock source shale, compacting 1. Compaction / pore-water expulsion overpressured source rock 2. Microfracturing (overpressure) continuous kerogen/oil network 3. Continuous oil-phase / diffusion
Three primary-migration mechanisms: (1) compaction-driven expulsion, in which pore water (carrying dissolved/dispersed hydrocarbons) is squeezed out as burial increases effective stress; (2) microfracturing, where kerogen-to-hydrocarbon conversion raises internal fluid pressure above the rock's tensile strength, opening transient escape pathways; (3) migration along a continuous oil-wet phase (kerogen/bitumen network) or by molecular diffusion of dissolved hydrocarbons through pore water.

1. Compaction-driven expulsion. As a source rock is progressively buried, increasing effective (grain-to-grain) stress compacts the sediment and expels pore water. Early in burial this water carries dissolved and colloidally dispersed organic compounds out of the shale and into adjacent, more permeable carrier beds. This mechanism is most efficient at shallow-to-moderate burial, before the shale's permeability drops to near zero.

2. Microfracturing from generation-induced overpressure. Converting solid kerogen to liquid/gaseous hydrocarbon increases the volume of the pore-filling fluid phase substantially (hydrocarbons are less dense and occupy more volume than the kerogen they replace). In a low-permeability, compacted shale that cannot bleed this fluid off fast enough, pore pressure rises until it exceeds the rock's minimum principal stress plus its tensile strength, opening horizontal or sub-horizontal microfractures. Expulsion occurs as discrete pulses through these transient, self-sealing fracture networks, which is why primary migration is thought to be episodic rather than continuous.

3. Migration along a continuous hydrocarbon (kerogen/bitumen) network, and molecular diffusion. At sufficient thermal maturity and organic richness, generated bitumen/oil can form an interconnected, oil-wet film along kerogen macerals and mineral grain boundaries throughout the rock; once this network is continuous, the hydrocarbon phase itself becomes mobile under its own buoyancy/pressure gradient without needing a separate carrier fluid. Separately, for gas and light hydrocarbons, simple molecular diffusion through the water-filled pore network (down a concentration gradient from the generating source rock into surrounding, less-charged rock) is also a recognized, if slow, primary-migration pathway, particularly important over geological time for gas.