Prepare for the CPG by practicing integrated petroleum-systems decisions on paper: build a prospect risk register, check whether each risk factor is truly independent, defend a column-height call from closure versus seal capacity, and verify a log interpretation with at least one independent cross-check. Work each concept through a scenario before memorizing its definition.
Prospect Risking: Why Independent Versus Dependent Factors Decide the Answer
A prospect chance estimate multiplies probabilities only for independent factors. Charge, reservoir, trap, and seal are the usual groups, but migration timing, trap fill, and seal capacity are frequently linked, so treating them as separate coin flips overstates confidence.
A geologic risk factor is independent when knowing its outcome tells you nothing about another. Reservoir presence at the crest and a mature source kitchen in the basin center can plausibly be independent. By contrast, trap fill depends jointly on structural closure and seal capacity: either one alone limits the column, so 'trap volume' and 'seal quality' risked separately are really one coupled judgment split in two.
Worked Scenario A: A training-style prospect lists source presence, source maturity, migration, trap presence, and seal presence, each assigned a favorable probability, and multiplies all five. The mistake is double counting: maturity and migration are components of charge, not separate draws, and the trap-and-seal pair controls fill together. The better structure is three judgments — charge (source plus maturity plus migration treated as one system), reservoir, and trap-seal coupled — then multiply those. This matters because the inflated product would rank this prospect above a rival whose risk register was built correctly, distorting which prospect gets drilled first in any exercise or real portfolio.
- Charge: source presence, kerogen quantity and quality, maturity, migration pathway, and timing judged together unless the data genuinely separate them
- Reservoir: presence of porosity and permeability at the target, judged from facies and diagenesis evidence
- Trap and seal: structural closure, stratigraphic pinch-out, and seal capacity assessed jointly because either can limit the hydrocarbon column
Source Rock Numbers: What TOC, Rock-Eval, and Vitrinite Reflectance Each Tell You
TOC measures organic quantity, Rock-Eval hydrogen index measures kerogen quality, and vitrinite reflectance or Tmax measures maturity. A single number cannot carry all three judgments; each answers a different charge question.
TOC is a quantity measure — how much organic carbon the rock holds — but a high TOC from inert, dead carbon is barren of generative potential. Rock-Eval S2 and the hydrogen index (S2 normalized to TOC) speak to quality: oil-prone kerogens plot high, gas-prone lower. Plotting S2 against TOC is the standard way to separate indigenous generation from migrated bitumen contamination, because reworked or contaminated samples fall off the kerogen-type trend lines.
Maturity needs an independent measure. Vitrinite reflectance (Ro) rises irreversibly with thermal exposure and anchors a burial-history judgment; Tmax shifts higher as maturity increases but can be suppressed by organic sulfur or caving contamination, so reading Tmax alone invites error. In a decision, pair them: an oil-prone HI means nothing if Ro shows the interval is post-mature, and a suppressed Tmax that conflicts with Ro should push you toward the reflectance. For CPG-style reasoning, the exercise is stating, for any source rock dataset, which question — quantity, quality, or maturity — each parameter answers before drawing a charge conclusion.
Traps and Seals: The Column Height Decision That Closes Both Ways
A trap stores hydrocarbons; a seal limits the column. The achievable column height is the smaller of structural closure and seal capacity, so trap geometry and seal quality must be assessed together, never as separate pass-fail boxes.
Structural traps rely on folded, faulted, or tilted geometry; stratigraphic traps rely on facies change, pinch-out, or unconformity; many real prospects are combinations. The practical difference shows in your evidence: structural traps are demonstrated by depth-converted mapped closure, stratigraphic traps by well-to-well facies correlation and seismic character change. Fault-dependent traps add a further question — juxtaposition — whether the reservoir interval is faulted against a sealing lithology or against itself across the fault plane.
Worked Scenario C (compact): A prospect sketch shows 80 meters of closure above a mapped spill point, and a training exercise gives a shale seal whose capillary properties support roughly 50 meters of column. The plausible mistake is calling the full closure the trapped volume. The better decision is to size the prospect on the seal-limited 50-meter column and recheck whether the mapped contact still sits within the reservoir interval. This matters because reserve estimates, contact placement, and spill-point arguments all flow from column height, and the seal, not the structure, is the limiting element in this example.
| Element | Structural trap | Stratigraphic trap |
|---|---|---|
| Basis of trapping | Deformation: folds, faults, tilts | Deposition or erosion: pinch-out, facies change, unconformity |
| Primary evidence | Depth-structure map showing closed contours in two directions | Well correlation, seismic facies change, depositional model |
| Leading technical question | Depth conversion accuracy and fault juxtaposition | Lateral seal effectiveness and updip pinch-out position |
| Common verification | Does closure survive a second velocity model? | Does the facies change hold between every well pair? |
Reservoir Quality: Why Porosity and Permeability Are Separate Verdicts
Porosity measures storage volume; permeability measures flow capacity through pore throats. They correlate in clean sands but diverge in chalks, clay-rich lithologies, and fractured or diagenetically altered rocks, so net pay needs both plus a saturation judgment.
Primary porosity is inherited from deposition — intergranular space in a well-sorted sand — while secondary porosity arises from dissolution, fracturing, or dolomitization. Permeability, however, is governed by throat size and connectivity, not by total void volume. A fine-grained chalk or a micritic carbonate can show substantial porosity with negligible permeability because its throats are tiny, whereas a fracture network can deliver high permeability from modest matrix porosity.
This distinction drives the net-pay decision. A pay cutoff set on porosity alone would admit a high-porosity, low-permeability zone that cannot flow; a defensible definition requires porosity above a cutoff, water saturation consistent with hydrocarbons, and a permeability or flow-capacity expectation. For study purposes, practice narrating the chain for a named lithology: depositional environment sets grain size and sorting, diagenesis modifies porosity, and throat geometry determines whether the stored fluid can move. Each link can fail independently, which is exactly why reservoir risk stands as its own factor in a risk register.
Log Interpretation: The High-Resistivity Sand That Was Not Pay
High resistivity indicates low conductivity water or hydrocarbons — it does not prove hydrocarbons. Fresh formation water, thin-bed effects, and tight lithology all raise resistivity, so a pay call needs a resistivity-independent cross-check.
The interpretation chain usually starts with the archie logic: resistivity rises when conductive brine is partly replaced by nonconductive hydrocarbon. But the same rise occurs when the water itself is fresh, because salinity controls water resistivity. Comparing the apparent water resistivity computed from the target zone (Rwa, derived from the zone's resistivity and porosity) against the known or estimated formation water resistivity is the standard discrimination: if Rwa matches Rw, the zone is water-bearing regardless of how impressive the resistivity looks.
Worked Scenario B: A sand on a paper well composite reads high on the induction log and a first-pass interpretation books it as pay. The mistake is stopping at the resistivity reading without a salinity check. The better decision is to compute Rwa from the log's porosity, compare it with the basin's expected Rw, and cross-check the mud log for hydrocarbon shows and the density-neutron pairing for gas effect. In this scenario Rwa matches fresh formation water, and the zone is reclassified as water-bearing. This matters because an unverified resistivity call inflates net pay, which propagates directly into any reserve or prospect-ranking calculation built on that well.
Mapping and Structure: Testing Whether Closure Survives Scrutiny
Closure on a time-structure map is not yet a prospect. Depth conversion, velocity anomalies, and fault geometry can create or destroy closure, so verify with a two-direction closure test and at least one alternate velocity scenario.
A prospect requires closed contours in both map directions at reservoir level after depth conversion. The classic geometric error is reading pull-up beneath a shallow high-velocity layer as structural closure: the reflector appears elevated only because the seismic wave traveled faster through the overlying body. Comparing the time-structure map with regional depth trends and re-running depth conversion with a plausible alternate velocity model exposes whether the closure is real or an artifact.
Fault-dependent closures add a second verification: draw or consult the fault-plane map and check the along-fault seal window. If the reservoir on the upthrown side is juxtaposed against the same sand on the downthrown side across the zone of interest, the fault is not a lateral seal there regardless of how good the mapped closure is. For exam-style practice, the habit to build is a two-question checklist on every mapped structure: does closure survive depth conversion, and does the sealing relationship hold at reservoir level in both map directions? Structure that passes both is ready for trap-and-seal risking.
A Preparation Sequence, Paper Exercise, and Readiness Rubric
Sequence study as concept pairs, then a full paper prospect. Run one capstone exercise — a prospect sketch turned into a risk register with an independence check — and score yourself against a rubric rather than a pass-fail guess.
Adaptable sequence: (1) Week one, petroleum systems — source, migration, maturation parameters, and practice stating which question each parameter answers. (2) Week two, traps and seals — build the closure-versus-seal-capacity comparison for three paper prospects, including one fault-dependent case with a juxtaposition check. (3) Week three, reservoir and logs — compute Rwa on two practice log composites, one pay and one fresh-water case, and write the net-pay rationale for each. (4) Week four, integration — the capstone exercise below, repeated on a second prospect sketch until the rubric is met without notes.
Capstone exercise and expected observations: take any prospect sketch or published play description and produce a one-page risk register. Observations to check: each risk factor is labeled independent or dependent with a stated reason; charge components are grouped, not multiplied separately; the column height is set by the smaller of closure and seal capacity with arithmetic shown; one log call includes an Rwa or cross-check computation. Self-check rubric (learning milestones, not pass predictions): score 1 if a factor is listed but unjustified, 2 if justified with a dependency error, 3 if fully justified and correctly grouped. Aim for 3 on all four elements before treating the topic as ready.
- Readiness check 1: you can define and contrast two related terms (e.g., closure vs seal capacity) without notes in two sentences
- Readiness check 2: you can compute a simple Rwa discrimination and state what a match or mismatch implies
- Readiness check 3: your risk register groups charge, reservoir, and trap-seal with a written independence rationale for each
- Readiness check 4: you can re-derive your column-height call from the limiting constraint when one input changes
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.