Question 4 of 6: Section 4: Siliciclastic Reservoirs

Section 4: Siliciclastic Reservoirs (20 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.

(The source paper's own printed heading reads "Section 4" while its sub-questions are numbered in the Q5 series, continuing the Section 3/Q3 numbering with Q4 skipped — a numbering discrepancy in the original exam booklet, not an omission here; every printed sub-question below is answered in full.)

Q5-1 — Estuary. An estuary is a coastal, semi-enclosed body of water with a free (tidal) connection to the open sea, within which seawater is measurably diluted by fresh water draining from land — typically the drowned lower reach of a river valley formed as relative sea level rises (transgression) faster than the river can fill the valley with sediment. It is a mixed-energy system governed by the interplay of fluvial, tidal and wave processes, and is bounded landward by fluvial (bay-head delta) deposits and seaward by a marine-influenced mouth (often with flanking barriers or tidal inlets).

Q5-2 — Vertical log and gamma-ray profile, 25 m prograding shoreface-beach succession.

Progradation moves progressively shallower, higher-energy subenvironments out over deeper, lower-energy ones, so the succession coarsens and shallows upward continuously: offshore mud at the base (below storm wave base) passes up into lower shoreface (interbedded hummocky-cross-stratified sandstone and bioturbated mudstone, deposited between fair-weather and storm wave base), then middle shoreface (cleaner, swaley/hummocky to low-angle planar-stratified sandstone, wave- and current-reworked above fair-weather wave base), then upper shoreface (well-sorted trough cross-bedded sand, surf-zone reworked), and finally foreshore/beach at the top (planar-laminated, well-sorted clean sand deposited in the swash zone). Because grain size, sorting and clay content all improve upward with increasing energy, the gamma-ray log is a classic funnel (upward-decreasing GR, coarsening-upward) shape — high GR in the muddy offshore/lower-shoreface interval, decreasing steadily to a low, blocky-clean GR minimum in the foreshore sand.

Q5-3 — Porosity limits for spherical grains.

Given. Uniform spherical grains, packed with no cement or matrix.

Find. The maximum porosity (loosest stable packing) and minimum porosity attainable purely by repacking (no grain-size change, no cement).

1. Maximum porosity — simple cubic packing (loosest stable arrangement, each sphere touching 6 neighbours along orthogonal axes). Solid fraction of a cube of side $2r$ containing one sphere of radius $r$ is $\frac{\frac{4}{3}\pi r^3}{(2r)^3}=\frac{\pi}{6}$, so $$\phi_{max} = 1-\frac{\pi}{6} \approx \boxed{47.6\%}$$
2. Minimum porosity — rhombohedral (closest) packing (each sphere touching 12 neighbours). Solid fraction is $\frac{\pi}{3\sqrt2}$, so $$\phi_{min} = 1-\frac{\pi}{3\sqrt2} \approx \boxed{26.0\%}$$

Q5-4 — Effect of grain size and sorting on porosity/permeability. For a single population of uniform, well-sorted spheres, porosity is theoretically independent of grain size — it is a purely geometric packing property (a box of large marbles and an identical box of tiny marbles, packed the same way, have the same fraction of void space). Permeability, however, increases strongly with grain size, because larger grains produce proportionally larger pore throats (Kozeny-Carman-type relations scale permeability roughly with the square of a characteristic grain/throat diameter), so a coarse well-sorted sand can be orders of magnitude more permeable than a fine well-sorted sand of identical porosity. Going from well sorted to moderately sorted introduces a range of smaller grains that partially fill the pore spaces between the larger framework grains: this reduces both porosity (void space is occupied by the smaller infiltrating grains) and permeability (pore throats are choked and made more tortuous by the finer material) — poorer sorting degrades reservoir quality on both counts simultaneously.

Q5-5 — Shale gas as an unconventional resource. Shale gas is unconventional because the gas is generated and stored within the same low-permeability, organic-rich shale that is also its own source rock — source, reservoir and (top/bottom) seal are effectively one unit, rather than gas having undergone primary and secondary migration into a separate, higher-permeability conventional reservoir trapped beneath a distinct seal. Matrix permeability in gas shale is typically in the nanodarcy range, far too low for gas to flow to a wellbore at economic rates under natural pressure gradients and buoyancy trapping alone; production instead requires artificial stimulation — horizontal drilling combined with multi-stage hydraulic fracturing — to create the fracture network needed to connect the gas-filled matrix to the well.