Question 6 of 6: Section 6: Structural Traps

Section 6: Structural Traps (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.

Q7-1 — Simple anticline trap.

In a simple four-way-dip anticline, buoyancy segregates fluids strictly by density beneath an impermeable cap rock (shale): gas (lowest density) occupies the crest above the gas-oil contact (GOC), oil occupies the flanks between the GOC and the oil-water contact (OWC), and formation water fills the reservoir below the OWC and saturates the surrounding syncline. The reservoir rock is a porous, permeable unit (e.g. sandstone) folded into the closure, sealed above by an impermeable, ductile cap rock (shale). A well-known giant anticlinal trap is the Kirkuk field, Iraq, one of the largest oil fields in the world, held in a long, doubly-plunging anticline in Cretaceous-Tertiary carbonate reservoir rock.

Q7-2 — Salt dome trap types.

A rising salt diapir deforms and punctures the overlying strata, creating several structurally and stratigraphically distinct trap styles around a single dome: (1) turtle-back (crestal) anticline traps in strata draped and arched over the dome crest before withdrawal synclines form flanking rim synclines; (2) up-dip pinch-out / truncation traps where beds thin and are truncated against the steep salt flank; (3) fault traps in the radial and peripheral normal faults that form over the dome's shoulders as strata stretch and collapse around the rising salt; (4) caprock traps in the porous, karsted gypsum-anhydrite-calcite caprock that forms at the top of the salt stock by dissolution/alteration; and (5) flank (drape) traps in permeable beds arched and pinched directly against the salt flank at depth, sealed laterally by the salt itself (an excellent, ductile, self-healing seal). The classic salt-dome province is the Gulf Coast of Texas and Louisiana, where the discovery of the Spindletop dome in 1901 launched the modern petroleum industry.

Q7-3 — Thrust-sheet structural traps.

In a compressional, thin-skinned fold-and-thrust belt (e.g. the Alberta foothills), an imbricate stack of low-angle thrust faults repeats the same stratigraphic package — each sheet carries older rock up and over younger, undeformed foreland strata, so a well drilled through the belt can pass through the same reservoir-source-seal package multiple times, once per thrust sheet.

Benefits of exploring in thrust sheets: (i) very large structural closures (fault-bend and fault-propagation folds, footwall/hanging-wall anticlines) can trap giant hydrocarbon volumes; (ii) the repeated, stacked reservoir-source-seal package multiplies the potential pay intersected per well; (iii) deep tectonic burial beneath the overriding thrust sheet can push an otherwise immature source rock into or through the oil/gas windows; and (iv) surface outcrop of the same folded/faulted stratigraphy in the foothills provides direct geological control for subsurface mapping.

Risks: (i) the complex, multiply-repeated and often steeply dipping or overturned geometry is very difficult to image with seismic beneath the shallow thrust sheets (velocity pull-up, poor illumination of sub-thrust structure); (ii) fault-related fracturing and juxtaposition can compromise seal integrity and increase the risk of hydrocarbon leakage along the thrust or associated splays; (iii) drilling hazards increase — steep bedding dips, unpredictable fault intersections, and localized overpressure raise well costs and non-productive time relative to gently dipping foreland strata; and (iv) structural uncertainty in trap definition and reserve estimation is inherently higher than in simple, undeformed anticlinal or stratigraphic traps.