Chapter 68 Reservoir Properties Logs and Correlation

Year 4 · Petroleum Geology & Petroleum Engineering · G441-L2

Calculate porosity and saturation, interpret well logs cautiously and distinguish true thickness from vertical thickness.

Illustrative poorly and well sorted grain-size populations. Original Geology Hub teaching diagram.

This independent lesson follows the topic framework of the supplied University of Peshawar BS Geology curriculum, session 2004–05 onward. It is newly written study material, not an official university course or qualification.

Learning objectives

Calculate porosity and fluid saturation consistently.
Explain the information and ambiguity in common logs.
Correlate wells without confusing thickness and depth.

Storage and flow are different properties

Porosity is pore volume divided by bulk volume. Effective porosity refers to a connected pore system under the definition used, so the method must be stated. Permeability depends strongly on pore throats, connectivity and fractures rather than simply total pore percentage. A clay-rich rock can have substantial porosity but very limited permeability. Grain packing, cementation, dissolution and compaction help explain reservoir quality through the rock history.

Fluid saturation is the fraction of pore volume occupied by a particular fluid. The saturations of the fluids considered sum to one if the accounting is complete. Resistivity, density, acoustic velocity and other responses depend on rock and fluid properties in different ways. Temperature, pressure, salinity and clay effects complicate interpretation. Surface forces and capillary behavior help determine fluid distribution, while compressibility influences changes in stored fluid and pore volume as pressure changes.

Logs are measurements with interpretation models

Wireline logs provide measurements along a borehole. Gamma ray can help identify radioactive components and often supports shale-sand comparisons, but not every high value is shale. Resistivity reflects the combined rock-fluid electrical system; a high value alone does not establish hydrocarbons. Density, neutron and sonic logs help constrain lithology and porosity through stated assumptions. Use multiple logs together with cores and cuttings rather than selecting one curve that matches the desired interpretation.

A lithological log describes observed material and intervals. Cuttings have sampling lag and may mix across depths; caving can introduce material from above. Cores have more direct depth context but may be incomplete. Correlating wells requires a consistent depth datum and attention to faulting and facies changes. An isochore shows vertical thickness, whereas an isopach shows true stratigraphic thickness. They coincide for horizontal beds but diverge as beds dip. A panel diagram must make these choices explicit.

Why pore volume and flow respond differently

Total porosity includes all pore space counted by a stated measurement method. Effective porosity emphasizes connected space relevant to the chosen definition. Two rocks with identical total porosity can behave differently if one contains large connected throats and the other mainly isolated or very small pores. Cement may close throats while leaving some pore volume, and fractures can provide flow paths through otherwise tight rock. A thin section gives useful texture but may not capture the connectivity of an entire reservoir volume.

For a simple single-fluid Darcy relationship with other factors fixed, flow rate increases with permeability, cross-sectional area and pressure difference, and decreases with viscosity and flow-path length. Compare two conceptual plugs with equal permeability and geometry: doubling pressure difference doubles predicted flow in this idealized relation; doubling viscosity halves it. These proportional comparisons do not establish field production rates because multiphase flow, heterogeneity and boundary conditions introduce additional controls.

Integrating logs instead of reading one curve as a diagnosis

A gamma-ray increase can support a shale interpretation when the mineralogy and local calibration agree, but radioactive minerals can occur outside shale. Resistivity also responds to salinity, saturation, clay and pore structure. Density and neutron responses depend on rock and fluid properties and require suitable calibration. Comparing several logs can identify an inconsistent simple interpretation: a proposed clean porous sandstone should fit the wider measurement pattern, not only one favorable resistivity peak.

Tie logs to directly described core or cuttings where available. A sharp curve change may mark a lithological boundary, a fluid change or a measurement effect, so test its geological explanation. Record borehole conditions and gaps that could affect quality. Depth matching is especially important when measurements come from separate runs or sample systems. A thin reservoir interval can be misrepresented if a lithological sample is matched to a log at the wrong depth.

A transparent storage calculation

Use a fictional reservoir footprint of 1 square kilometre, net thickness 10 metres, porosity 0.20 and hydrocarbon saturation 0.75. Bulk net reservoir volume is 10 million cubic metres; pore volume is 2 million; hydrocarbon pore volume is 1.5 million cubic metres at the conditions represented. This is not a recoverable or surface-volume estimate. Fluid expansion, pressure conditions, disconnected volumes and recovery require separate treatment. Show each multiplication with units to prevent a storage calculation becoming an unsupported production claim.

Net thickness excludes intervals that do not meet the stated reservoir definition, whereas gross thickness includes the whole considered interval. Both depend on explicit criteria. In dipping parallel beds, a vertical well interval and true stratigraphic thickness differ, so isochore and isopach inputs should not be mixed unknowingly. A correlation panel should show where a net interval is observed and where it is inferred to continue; uncertainty in continuity can dominate arithmetic precision.

Worked activity

A fictional core plug has bulk volume 100 cubic centimetres and connected pore volume 18 cubic centimetres, giving 18 percent effective porosity under the stated measurement. If water occupies 30 percent of that pore volume, hydrocarbon saturation in a two-fluid example is 70 percent. Hydrocarbon pore volume is 18 × 0.70 = 12.6 cubic centimetres. This is storage, not an estimate of how much will be recovered. Now compare two rocks with the same porosity but coarse connected pores versus tiny isolated pores and predict which is likely to transmit fluids more readily.

Practice and interpretation

Create a simple well-correlation panel with two lithological logs and one explicitly uncertain connection. Place raw observations beside interpreted reservoir intervals. List the assumptions needed for a porosity estimate from any selected log and explain how a core measurement could test them. Keep sample descriptions separate from inferred fluid content. A useful subsurface interpretation reveals where measurement ends and a petrophysical or geological model begins.

Self-check

Does equal porosity imply equal permeability?
Can resistivity identify hydrocarbons alone?
How do isopach and isochore differ?

Check your answers

No. Pore geometry and connectivity can differ greatly.
No. Fluid salinity, lithology and other factors must be assessed with supporting data.
Isopach is true stratigraphic thickness; isochore is vertical thickness.

Sources and further study

The explanations and teaching examples are original. Use these institutional resources to check terminology and explore the subject further.

Chapter 68 multiple choice questions

Choose one best answer for each question. Open each answer panel after making your choice.

A 100-cubic-centimetre core has 18 cubic centimetres of connected pore volume. What is its effective porosity under this definition?
1. 5.6 percent
2. 18 percent
3. 82 percent
4. 180 percent
Answer and explanation
B. 18 percent Porosity is pore volume divided by bulk volume: 18/100 = 0.18, or 18 percent.
A sample has 18 cubic centimetres of pore volume and 30 percent water saturation in a two-fluid system. What is its hydrocarbon pore volume?
1. 5.4 cubic centimetres
2. 18 cubic centimetres
3. 12.6 cubic centimetres
4. 30 cubic centimetres
Answer and explanation
C. 12.6 cubic centimetres Hydrocarbon saturation is 1 − 0.30 = 0.70. Multiplying 18 by 0.70 gives 12.6 cubic centimetres.
Two rocks have equal porosity, but one has larger connected pore throats. What difference is reasonable to investigate?
1. The two rocks must have equal permeability
2. The connected rock must contain only hydrocarbons
3. The porosity measurement determines rock age
4. The connected rock may have greater permeability
Answer and explanation
D. The connected rock may have greater permeability Flow depends on pore geometry and connectivity as well as pore volume. Equal porosity does not imply equal permeability.
A well log shows high resistivity. Which conclusion is justified by this measurement alone?
1. The interval has an electrical response that needs integrated interpretation
2. The interval is certainly a productive hydrocarbon reservoir
3. The formation contains no water under any conditions
4. The rock is necessarily sandstone of a known age
Answer and explanation
A. The interval has an electrical response that needs integrated interpretation Resistivity depends on fluids, lithology and other conditions. It must be interpreted with other logs and geological evidence.
In dipping parallel beds, which pair correctly distinguishes thickness maps?
1. Isochore shows age; isopach shows porosity
2. Isochore shows vertical thickness; isopach shows true stratigraphic thickness
3. Isochore shows true thickness; isopach shows only burial depth
4. Both necessarily show the same thickness at every dip
Answer and explanation
B. Isochore shows vertical thickness; isopach shows true stratigraphic thickness Vertical and true stratigraphic thickness differ as beds dip. The lesson uses isochore for vertical thickness and isopach for true thickness.

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