Petroleum Geology and Geophysics
Petroleum Geology and Geophysics tests the petroleum system, seismic interpretation, trap and seal analysis, and volumetric upside. Expect source-rock maturity, migration, depositional models, and prospect risking.
Core rules
- A working petroleum system needs source, maturation, migration, reservoir, seal, trap, and correct timing.
- The oil window is roughly Ro 0.6–1.3%; higher maturity favors gas. Kerogen type I/II/III affects the product mix.
- Traps can be structural, stratigraphic, or combination. A seal must have sufficient capillary entry pressure.
- Seismic interpretation requires time-to-depth conversion. AVO and direct hydrocarbon indicators support but do not prove a discovery.
- Volumetrics use gross rock volume, net-to-gross, porosity, water saturation, and formation volume factor.
Worked example
A prospect has 500 acres, gross thickness 80 ft, NTG 0.60, phi 0.18, Sw 0.30, and Bo 1.25. Net pay is 80 × 0.60 = 48 ft. OOIP = 7758 × 500 × 48 × 0.18 × (1 - 0.30)/1.25. The numerator is 7758 × 500 × 48 × 0.18 × 0.70 = 23,460,192; dividing by 1.25 gives 18,768,154 STB, or about 18.8 MMSTB. NTG reduces gross thickness, and Sw and Bo convert to stock-tank oil.
Traps
Using gross thickness rather than net pay, applying NTG twice, or mixing area units will distort volumetrics. Do not treat a seismic amplitude as a guaranteed hydrocarbon fill; seal and timing can still fail. Mapping a time horizon as depth without a velocity model is another common error.
Key points
- A petroleum system requires source, migration, reservoir, seal, trap, and timing.
- Oil window is about Ro 0.6–1.3%.
- Seismic indicators support but do not prove hydrocarbons.
- Volumetrics require NTG, phi, Sw, and Bo.
- Seal capillary pressure is a key risk element.