CPG Formation Evaluation & Petrophysics Flashcards

What does a high spontaneous potential (SP) deflection toward the shale baseline in a sandstone interval typically indicate?

Answer: High formation water salinity or shaly sand

Reduced SP deflection in a sandstone indicates either high formation water salinity (reducing electrochemical contrast) or the presence of clay reducing permeability.

The Klinkenberg effect in gas permeability measurements refers to which phenomenon?

Answer: Gas slippage at pore walls causes apparent permeability to exceed liquid permeability

The Klinkenberg effect describes how gas molecules slip along pore walls at low pressures, causing measured gas permeability to exceed true (liquid) permeability.

Which method is used to estimate formation permeability from wireline logs when no core data is available?

Answer: Timur-Coates (NMR-based) or Wyllie-Rose empirical equations

Empirical permeability transforms such as Timur-Coates (using NMR porosity and free-fluid index) or Wyllie-Rose equations relate log-derived porosity to permeability.

In well log interpretation, what is the primary use of the dual induction log suite (ILd, ILm, SFL)?

Answer: Determining invasion profile to distinguish flushed zone from virgin formation resistivity

The dual induction suite reads at three depths of investigation, allowing the interpreter to model the mud filtrate invasion profile and determine true formation resistivity (Rt).

What is the significance of the 'irreducible water saturation' (Swirr) concept in reservoir evaluation?

Answer: It represents the minimum water saturation that cannot be produced, indicating maximum potential hydrocarbon saturation

Irreducible water saturation is the water held by capillary forces on grain surfaces and small pores that cannot be displaced, setting the upper bound for moveable hydrocarbon volume.

Which log combination is most reliable for identifying dolomitization in a carbonate sequence?

Answer: Neutron-density crossplot combined with the Pe log

Dolomite has a characteristic Pe of ~3.1, density of ~2.87 g/cm³, and crossplot position that distinguishes it from limestone and anhydrite when Pe is combined with neutron-density.