Question 3 of 7: Section 3: Migration

Section 3: Migration (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.

Q3-1 — Driving forces of secondary migration. Secondary migration (movement through carrier beds after expulsion from the source rock) is driven mainly by buoyancy: hydrocarbons are less dense than formation water, so the density contrast produces an upward/up-dip force once hydrocarbons form a continuous phase in a water-wet carrier bed. Superimposed on buoyancy is hydrodynamic flow of the formation water itself (regional groundwater/basin fluid flow can tilt hydrocarbon-water contacts and redirect migration paths off the pure structural up-dip direction), and residual overpressure from compaction/generation in the source and adjacent beds can add an additional expulsion-related push, particularly close to the kitchen.

Q3-2 — Effective porosity and its relation to permeability. Effective porosity is the fraction of bulk rock volume occupied by interconnected pore space that is accessible to fluid flow, as opposed to total porosity, which also counts isolated, dead-end pores that trap fluid but cannot transmit it. Because permeability is a measure of a rock's ability to transmit fluid through connected pathways, it correlates with effective porosity (not total porosity) — a rock can have high total porosity (e.g. vuggy carbonate with abundant isolated moldic pores) yet near-zero permeability if those pores are not connected. Even among rocks of similar effective porosity, permeability further depends on pore-throat size, sorting and tortuosity (Kozeny-Carman-type relationships), so effective porosity is a necessary but not sufficient predictor of permeability.

Q3-3 — Migration pathways and top three exploration targets.

In both maps, hydrocarbons generated in the shaded "mature source rock" kitchen migrate up-dip along the most permeable connected carrier path, focused toward the nearest, best-connected structural closure. In Map A, the normal fault juxtaposes the closed dome against the kitchen and can act as either a conduit (if permeable sand is juxtaposed across it) or a partial barrier; migration is ranked (1) directly up-dip into the crest of the closed contour dome — the largest, most proximal closure; (2) along the fault plane itself where it may channel flow toward the shallower fault block; and (3) the outer flank of the dome, a smaller/less-charged target reached only after the primary closure has filled or leaked. In Map B, the reverse fault and three-fold train (anticline-syncline-anticline) provide two independent structural highs; ranking is (1) the nearer anticline crest immediately up-dip and fault-adjacent to the kitchen (shortest migration distance, best charge access); (2) the second, more distal anticline along the same up-dip fold trend (still well charged but a longer migration path with more opportunity for the charge to be lost to leakage or a shallower trap along the way); and (3) the reverse-fault zone itself, ranked last because reverse faults in a compressional regime are more likely to juxtapose reservoir against reservoir (poor lateral seal) than the fold closures are.

Q3-4 — Irreducible water saturation. Irreducible (connate) water saturation, $S_{wi}$, is the water saturation remaining in the reservoir after hydrocarbon has migrated in and displaced water down to the point where capillary and adsorptive forces hold the remaining water so tightly (in the smallest pore throats and as a thin wetting film on grain surfaces) that no additional buoyancy pressure, however large, can displace it further — it represents a physical floor on water saturation, not a producible quantity. In a water-wet, medium-grained, well-sorted sandstone reservoir, pore throats are relatively large and uniform, so capillary retention is comparatively low; a typical value is $S_{wi}\approx 15\text{-}25\%$ (versus >40-50% in a poorly-sorted or fine-grained/silty sandstone, where much smaller pore throats hold far more irreducible water).

Q3-5 — Pressure-depth profile, normally pressured region.

In a normally pressured region, pore pressure follows the hydrostatic gradient (≈10.2 kPa/m for typical formation water, SG≈1.04) because the pore-fluid column is in open, connected communication all the way to a free water table/surface outlet. The lithostatic (overburden) gradient (≈22.6 kPa/m, using an average rock+fluid bulk density of ≈2.3 g/cm³) is much steeper because it is carried by the full weight of overlying rock, not just the pore fluid. An impermeable boundary such as a shale bed is identified on the plot where the measured pressure trend breaks away from the hydrostatic line and steps toward (though rarely all the way to) the lithostatic line: below the seal, compaction-expelled pore fluid cannot escape upward, so it partially supports the overburden load itself and pressure rises faster than hydrostatic — this abrupt change in gradient (rather than a smooth continuation of the hydrostatic trend) is the diagnostic signature of a top-seal/pressure-compartment boundary on a pressure-depth (or "pressure vs. depth" mud-weight) plot.