Geology and petrophysics: measurement to inference

Porosity, saturation and a consistent volume basis

Porosity is pore volume divided by bulk rock volume. Establish whether the reported value is total or effective porosity, how it was measured and which sample conditions apply. Water saturation must refer to the same pore-space definition. Multiplying bulk volume, porosity and hydrocarbon saturation gives an educational estimate of hydrocarbon pore volume at the stated conditions, not recoverable reserves.

Check geometry before multiplying. Area must describe the studied body and thickness must use a compatible net-interval definition. Applying a net-to-gross factor to thickness that is already net can count the reduction twice. Convert percentages to fractions. State the phase assumption: one minus water saturation represents all hydrocarbons together only when the saturation definitions support that interpretation.

The answer retains the reference conditions of the pore volume. It cannot be labelled a saleable oil volume directly. A surface-volume conversion requires consistent PVT information, while recovery requires a separate development assessment. Record missing information rather than supplying an invented conversion.

Walkthrough

Fictional uniform model: A=1000 m², net h=10 m, φ=0.20, Sw=0.30 on a consistent basis. Bulk volume=10,000 m³; pore volume=2000 m³; HCPV=1400 m³. This is hydrocarbon pore volume, not reserves.

Independent task

For A=2000 m², net h=8 m, φ=0.15 and Sw=0.40 find bulk volume, pore volume and HCPV. Explain why the answer cannot be declared recoverable reserves.

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Averaging a heterogeneous interval

An average depends on the quantity it must preserve. Porosity across equal-area intervals is weighted by bulk rock volume, or by thickness when area is constant. An arithmetic mean is appropriate only for equal weights. It can otherwise hide a thin porous bed or overstate the contribution of a thick tight interval. State whether thickness is true thickness or measured along the borehole; do not mix them without geometric justification.

For hydrocarbon volume, calculate each interval contribution before summing. Multiplying separately averaged porosity and saturation generally differs from averaging their product. Correlation between properties can create a material error even when every arithmetic operation is correct.

Do not transfer the porosity averaging rule to permeability. Equivalent flow capacity depends on flow direction, connectivity, boundaries and phases. A layered-flow approximation needs additional assumptions. Average porosity represents storage in the adopted geometry; it does not independently predict production rate, productivity or completion performance.

Walkthrough

At common area 100 m²: 2 m at φ=0.30 and 8 m at φ=0.10. Pore volume=60+80=140 m³; average φ=0.14. Arithmetic mean 0.20 would incorrectly give 200 m³.

Independent task

For the same layers use Sw=0.20 and 0.60 respectively. Compute interval HCPV and compare with the result using thickness-weighted φ and Sw separately.

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From sample to model: uncertainty and scale

A laboratory measurement belongs to a particular specimen, method and set of conditions. Core recovery can favour better preserved intervals. Fractures, clay content and stress changes may affect transfer to reservoir conditions. Logs investigate a different volume and have different vertical resolution. Disagreement first requires checking depth alignment, borehole condition and quantity definitions rather than selecting the more convenient number.

Separate sensitivity from probability. Substituting lower and upper parameter bounds describes selected scenarios; it does not create a confidence interval or assign P10 and P90 probabilities. Probabilistic assessment requires distributions, dependencies and evidence that inputs represent the target. Combining every extreme may produce an unlikely or incompatible case.

Retain the nominal calculation, alternative scenarios and unknowns in the learning report. Identify the additional measurement most likely to change the conclusion. A narrow numerical range does not remove systematic error in the geological concept or an incorrectly mapped boundary.

Walkthrough

For bulk volume 10,000 m³ and Sw=0.30, changing φ from 0.18 to 0.22 changes HCPV from 1260 to 1540 m³. These are scenarios with fixed geometry and saturation, not a probability interval.

Independent task

For bulk volume 10,000 m³ consider φ=0.18…0.22 and Sw=0.25…0.35. Find extreme HCPV and nominal at φ=0.20, Sw=0.30. Can the limits be labelled P90/P10?

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Capstone: audit a reservoir-volume estimate

A fictional area is 1000 m². Interval A: net thickness 4 m, φ=0.25, Sw=0.20. Interval B: 6 m, φ=0.10, Sw=0.50. A report uses arithmetic mean porosity 0.175 and Sw=0.35 and calls the result recoverable reserves. Attached core lacks depth matching; PVT is absent.

Walkthrough

Audit structure: geometry → property basis → interval calculations → comparison → limitations.

Independent task

Compute interval pore volumes and HCPV, quantify the proposed method error, request missing evidence and write a corrected conclusion without declaring reserves.

Attempt in the interactive worksheet →