Section 7: Structural Traps: 04-Geol-B6, December 2014, Q7

Section 7: 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 — Anticline trap.

Simple anticline (four-way dip) trap: gas cap at the crest above an oil leg above a water leg, sealed by a folded shale caprock over a folded sandstone reservoir. GOC = gas-oil contact, OWC = oil-water contact. Classic Canadian example: Turner Valley field, Alberta Foothills.

A simple anticline (four-way dip closure) traps hydrocarbons where a porous sandstone reservoir is folded upward into a domal closure and sealed above by a folded, impermeable shale caprock. Density segregation within the trap stacks the fluids by buoyancy: gas (lowest density) occupies the crest above the gas-oil contact (GOC), oil forms the middle leg down to the oil-water contact (OWC), and formation water fills the reservoir below the OWC and saturates the flanks. A classic Canadian example of an anticlinal oil/gas field is Turner Valley, Alberta (discovered 1914), a four-way dip closure in the disturbed belt of the Alberta Foothills.

Q7-2 — Salt dome traps.

A piercement salt dome creates at least five distinct trap types around one structure: (1) crestal trap in porous caprock, (2) radial/flank fault traps, (3) truncation traps where beds abut the salt, (4) rim-syncline (turtle-structure) traps in the withdrawal basin, and (5) sub-salt traps beneath an overhang.

A rising, low-density, ductile salt diapir deforms and punctures the overlying sedimentary section, creating at least five distinct trap types around one structure: (1) crestal/caprock trap — porous dissolution caprock (anhydrite/gypsum/carbonate residue) capping the salt stock; (2) radial/flank fault traps — extensional faults radiating from the dome as strata drape and stretch over it; (3) truncation (salt-flank) traps — reservoir beds abutting directly against the salt stock, sealed by the salt itself; (4) rim-syncline ("turtle-structure") traps — anticlinal closures in the withdrawal basin flanking the dome, formed as salt is evacuated from beneath the adjacent minibasin; and (5) sub-salt/overhang traps — reservoir sealed beneath an overhanging salt canopy. The classic salt-dome oil-producing region is the U.S. Gulf Coast Basin (Texas-Louisiana), exemplified by the Spindletop field (1901), the discovery that launched the modern Gulf Coast petroleum industry.

Q7-3 — Compressional thrust-belt traps.

Three stacked imbricate thrust sheets (fault-bend/fault-propagation folds) each carrying a fault-bounded structural trap in its hanging-wall anticline, verging toward the undeformed foreland.

Benefits of exploring in thrust sheets (versus undeformed foreland rocks): (i) fault-bend and fault-propagation folds in the hanging wall of each thrust create large, well-defined structural closures, often larger than typical foreland anticlines; (ii) imbricate stacking repeats the same reservoir-seal pair in each successive sheet, so a single well can test multiple stacked pay zones; (iii) the thick allochthonous shale packages emplaced by thrusting can provide very effective top and lateral seals.

Risks: (i) structure beneath and within steeply-dipping thrust sheets is difficult to image seismically (velocity pull-up/push-down, poor illumination beneath the thrust plane), so trap geometry and depth are poorly constrained before drilling; (ii) fault seal along the thrust itself is uncertain — the thrust plane can act as a migration conduit rather than a seal, and juxtaposition/clay-smear behaviour is hard to predict; (iii) steep, variable dip creates significant drilling hazards (directional control difficulty, lost circulation, wellbore instability); and (iv) later erosion, back-thrusting or out-of-sequence faulting can breach an otherwise well-formed trap. Undeformed foreland rocks are structurally simpler and lower-risk to image and drill, but typically offer smaller, single-reservoir closures with correspondingly lower reserves potential per well — the classic higher-risk/higher-reward versus lower-risk/lower-reward trade-off between fold-and-thrust-belt and foreland exploration.

Topic. Structural trap families (fold, salt-related, fault/thrust) and the risk-reward trade-off of thrust-belt exploration.

Key relations. GOC/OWC fluid contacts set by buoyancy segregation; salt-dome trap family (crestal, flank-fault, truncation, rim-syncline, sub-salt); imbricate thrust stacking repeats reservoir-seal pairs.

Why this works. All three trap families are fundamentally about creating and preserving a closure faster than it can leak: an anticline does it by simple folding, a salt dome does it by actively deforming the section around a rising, impermeable body (multiplying the number of closures from one structure), and a thrust sheet does it by repeating the whole reservoir-seal package several times over — which is exactly why thrust belts can host more total reserves per structure than an equivalent single undeformed anticline, at the cost of much greater structural and seal uncertainty.

Common pitfall. Assuming every fault around a salt dome or within a thrust sheet is sealing by default (both settings are notorious for faults that instead leak or juxtapose reservoir-on-reservoir); and picking only the crestal salt-dome trap while ignoring the four other trap types the same structure generates.

Source. Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed., ch.10 (fold traps), ch.12 (salt structures) & ch.13 (thrust-belt structures); Selley & Sonnenberg, Elements of Petroleum Geology, 3rd ed., ch.8 (structural traps).

Question 7 of 7: Section 7: Structural Traps

Section 7: Structural Traps (20 Marks)