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24-Pet-B4 Well Testing · December 2015

Question 15 of 18: Thrust fault play with repeated sandstone sequence

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 5, Q5-1: Thrust fault play with repeated sandstone sequence (15 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.

Thrust-fault play with a repeated permeable sandstone Cross section footwall sandstone (reservoir) thrust fault plane hanging-wall sandstone (repeated) seal (shale) spill pt. footwall block hanging-wall block ≈ 1 km Plan view thrust trace closure (fold-and-thrust trend) ≈ 3 km
Cross section (left): a low-angle thrust places a hanging-wall block over the footwall, repeating the same permeable sandstone reservoir sequence twice in the same well; each repeat is sealed above by shale, with a spill point marking the structural closure limit at the up-thrown fold crest. Plan view (right): an elongate, fold-and-thrust-belt-parallel structural closure trend defined by the mapped thrust trace.

Play description. A low-angle thrust fault carries an allochthonous hanging-wall sheet up and over an autochthonous footwall, so that the same permeable sandstone reservoir horizon is repeated twice (or more) in a single vertical well – once in the footwall, once again structurally higher in the hanging wall – each capped by its own overlying shale seal and folded into a fault-related (fault-bend or fault-propagation) anticline. The trap's spill point is the structurally lowest point of closure along the fold crest at which the trap begins to leak; below the spill point, hydrocarbons cannot be retained regardless of seal quality above.

Key risks and economic viability. (1) Fault-seal risk – a thrust plane can juxtapose the reservoir sandstone against itself (self-juxtaposition) across the fault, or against another permeable unit, defeating the seal entirely regardless of the shale cap; alternatively, clay/gouge smeared into the fault zone during slip can create an effective lateral seal – whether the specific fault seals must be assessed case-by-case (e.g. shale-gouge-ratio analysis), not assumed. (2) Charge/timing risk – the trap-forming thrust event must post-date or be contemporaneous with hydrocarbon generation and migration from the basin's source rock(s); an anticline that folded after all charge had already migrated through and been lost holds nothing. (3) Reservoir-quality risk – the same deformation that creates the trap can also destroy or enhance reservoir quality unpredictably: fracturing associated with thrusting can add valuable secondary permeability, but cataclasis (grain crushing) and associated diagenetic cementation along fault zones can equally destroy porosity in the same rock. (4) Structural/seismic-imaging risk – complex, steeply dipping and imbricated thrust geometries are notoriously difficult to image accurately with reflection seismic (velocity pull-up/push-down beneath thrust sheets), so structural closure, depth, and even the presence of a real trap can be significantly mis-mapped. (5) Economic viability – thrust-belt (foothills/mountain) plays are typically in remote, rugged terrain with high drilling and access costs, and complex near-surface velocity structure raises well-placement risk; however, where seal integrity holds across multiple stacked repeats of the reservoir in one well, a single well can access two or more pay zones, materially improving per-well economics relative to a single-target conventional well.