CPG Case Analysis & Practical Application 3 — Questions and Answers
Question 1: A stratigraphic trap is identified in a deltaic sequence where reservoir pinches out up-dip into tight shale. Which seal risk element is most important to evaluate?
- Lateral seal integrity along the updip pinch-out and absence of faults cutting the shale (Correct answer)
- Thickness of the reservoir sand
- Porosity of the reservoir at the crest
- Depth to the top of overpressure
Correct answer: Lateral seal integrity along the updip pinch-out and absence of faults cutting the shale
The updip shale seal must be laterally continuous and unfaulted for the stratigraphic trap to hold hydrocarbons.
Question 2: A field's primary recovery factor from a water-drive reservoir is estimated at 35%. The operator is evaluating waterflooding to increase recovery. The most important reservoir property to assess first is:
- Permeability distribution and vertical communication between layers (Correct answer)
- Depth of the reservoir
- Color of crude oil
- Surface temperature of injected water
Correct answer: Permeability distribution and vertical communication between layers
Waterflood efficiency depends critically on reservoir heterogeneity and vertical permeability communication, which control sweep efficiency.
Question 3: A geologist observes that two adjacent fault blocks have different reservoir pressures measured at the same datum. This most directly implies:
- The fault between the blocks is sealing and prevents pressure communication (Correct answer)
- Both blocks share the same fluid column
- The reservoir is in hydraulic continuity across the fault
- The pressure gauges were miscalibrated
Correct answer: The fault between the blocks is sealing and prevents pressure communication
Pressure differences across a fault at the same datum confirm that the fault is acting as a barrier to fluid flow and pressure communication.
Question 4: When constructing a prospect's resource distribution for probabilistic volumetrics, which parameter typically contributes the greatest uncertainty range?
- Recovery factor, because it integrates reservoir quality, drive mechanism, and development design (Correct answer)
- Gross rock volume, which is tightly constrained by seismic
- Porosity, which rarely varies more than ±2 p.u.
- Formation volume factor, which changes negligibly
Correct answer: Recovery factor, because it integrates reservoir quality, drive mechanism, and development design
Recovery factor spans the widest range because it depends on multiple interacting subsurface and engineering uncertainties.
Question 5: A CPG is tasked with risking a deepwater prospect and assigns a 15% POS. This means:
- There is a 15% probability that all required elements (trap, reservoir, seal, charge, timing) are simultaneously satisfied (Correct answer)
- The prospect has a 15% chance of being drilled
- 15% of the reservoir volume will be producible
- The prospect has failed 15% of the geological checks
Correct answer: There is a 15% probability that all required elements (trap, reservoir, seal, charge, timing) are simultaneously satisfied
Probability of Success (POS) is the geologic chance that all risk elements are present and functional, yielding a commercial accumulation.
Question 6: A well log shows a sharp resistivity spike in a thin sand bounded by shales, but no gas shows were observed while drilling. The most cautious interpretation is:
- The interval may contain tight carbonate cement or pyrite causing false resistivity, and further evaluation is needed (Correct answer)
- The resistivity confirms commercial gas
- The zone is non-prospective because of absent gas shows
- The shales above and below are the reservoir
Correct answer: The interval may contain tight carbonate cement or pyrite causing false resistivity, and further evaluation is needed
High resistivity without gas shows can result from tight cementation or conductive minerals like pyrite, requiring core or additional log evaluation.
Question 7: During a field development study, the geologist recommends a horizontal well to target a thin, laterally extensive carbonate reservoir. The primary geological justification is:
- Horizontal wells maximize drainage area in low-permeability, sheet-like reservoirs with high kh/kv ratios (Correct answer)
- Horizontal wells are cheaper than vertical wells
- Carbonate reservoirs always require horizontal drilling
- Horizontal wells avoid all water coning issues
Correct answer: Horizontal wells maximize drainage area in low-permeability, sheet-like reservoirs with high kh/kv ratios
Horizontal wells maximize contact with thin, laterally continuous reservoirs and are most effective where horizontal permeability far exceeds vertical permeability.
CPG Case Analysis & Practical Application 3
A stratigraphic trap is identified in a deltaic sequence where reservoir pinches out up-dip into tight shale.
Which seal risk element is most important to evaluate?