Study Guides
Concise revision guides distilled from past-paper patterns — the definitions, formulas and reading rules that exams keep testing. Skim them before practice, return after marking.
On this page
- Well Logging Study Guide — Read gamma-ray, resistivity and density logs step by step.
- Reservoir Engineering Revision Guide — Key formulas and concepts for rock and fluid properties.
- Prefer testing yourself? Try the practice quizzes.
Well Logging Study Guide
Gamma-ray log
- Measures natural radioactivity — shales read high (60–150+ API), clean sands low (<30 API).
- First log to reach for when picking sand/shale tops; it is insensitive to porosity and fluid type.
- A cleaning-upwards (fining-upwards) gamma motif suggests a point-bar or channel deposit.
Resistivity logs
- Hydrocarbons are resistive, so a high deep-resistivity reading against a porous zone flags potential pay.
- Compare shallow vs deep curves: invasion separates them where mud filtrate has flushed the near-wellbore.
- Archie's equation links formation resistivity factor, water saturation and porosity — know Sw = (a·Rw / (φ^m · Rt))^(1/n) in outline.
Density–neutron combination
- Density log reads bulk density → porosity once matrix (sandstone 2.65, limestone 2.71 g/cc) is assumed.
- In gas zones the neutron porosity collapses while density stays high — the classic 'crossover' signature.
- Always read the lithology the log was calibrated for; a limestone-calibrated pair misreads a sandstone by several porosity units.
Reading order that works: gamma ray for tops → density/neutron for porosity and lithology → deep resistivity for pay. Practise the interpretation itself under exam conditions. Try a practice quiz →
Reservoir Engineering Revision Guide
Porosity and permeability
- Porosity φ = pore volume / bulk volume; effective porosity excludes isolated pores.
- Darcy's law: q = k·A·ΔP / (μ·L). Permeability k is a rock property; it does not depend on the fluid.
- Cut-off conventions matter: 'net-to-gross' changes with the porosity and shale-volume cut-offs you choose — always state them.
Fluid saturation and contacts
- Sw + So + Sg = 1. Above the transition zone, Sw sits near irreducible water saturation.
- The capillary transition zone thickens with lower permeability and lower density contrast between fluids.
- FWL, OWC and free-water level are not synonyms — the OWC is the shallowest level producing water-free oil on test.
Recovery mechanisms
- Primary recovery relies on natural drive (solution gas, gas cap, aquifer, compaction); solution-gas drive typically recovers only ~5–25%.
- Waterflooding sweeps oil by injecting water in a pattern (five-spot is the exam favourite); mobility ratio M ≤ 1 gives stable displacement.
- Volumetrics: STOIIP = 7758 · A · h · φ · (1 − Sw) / Bo (acre-ft units) — practise unit discipline, that is where marks are lost.
Formula sheet first, then past papers under time pressure. When the theory feels solid, check yourself before moving on. Try a practice quiz →
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