Question 5 of 10: Factors Affecting Log Resistivity
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Examinations, 17-Pet-B1, Well Logging and Formation Evaluation — May 2019, 3 hours, closed book (Sharp or Casio approved calculators permitted), 10 questions, all marked. Every question on this paper is Well Logging & Formation Evaluation content, solved to the paper as printed. Every datum here was read from the paper: the Question 9 SP track prints "SSP −80 mV" and "PSP 47 mV", its gamma-ray track prints 28, 92 and 44 API against Zones B, C and A, and the attachments supply the gas-sand chart and SP departure chart used in Questions 7 and 10.
Reference texts: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (SPE Textbook Series Vol. 4); Asquith & Krygowski, Basic Well Log Analysis, 2nd ed. (AAPG); Ellis & Singer, Well Logging for Earth Scientists, 2nd ed.; Schlumberger, Log Interpretation Charts / Log Interpretation Principles and Applications.
Formation-water resistivity and salinity, Rw. Resistivity is fundamentally an inverse function of the connate-water ion concentration (and its temperature); a saltier or hotter formation water reads a much lower Rt at the SAME porosity and saturation, per Archie's law.
Porosity and pore geometry, φ. More pore space (or more pore-throat tortuosity/cementation, the Archie exponent m) means more current-carrying water volume per unit rock, so Rt falls as φ rises for a fixed Sw and Rw.
Water saturation, Sw (hydrocarbon content). Hydrocarbon is essentially non-conductive, so displacing water with oil or gas (lowering Sw) sharply raises Rt — the entire basis of using resistivity as a hydrocarbon indicator.
Shale/clay content, Vsh. Clay minerals carry additional (cation-exchange, "excess") surface conductivity independent of the pore-water path, so a shaly formation reads a LOWER apparent resistivity than a clean formation of the same true water saturation, requiring a shale-corrected saturation model.
These four factors are not independent in practice: at a fixed Rt, a log analyst cannot immediately tell whether a reading is low because the zone is wet, because it is shaly, because it is tighter (lower φ) than a neighbouring sand, or simply because the local formation water is more saline than assumed elsewhere in the well. Untangling them is exactly why resistivity is never interpreted as a single curve in isolation — it is always read jointly with an independent porosity measurement (Questions 7–8) and a shale-volume indicator (Question 9) so that Rw, φ, Vsh, and Sw can be solved for as four separate unknowns rather than guessed from one number. A produced-water sample or an SP-derived Rw (Question 9's sibling method) is the usual way of pinning down the first factor before the other three are attributed any remaining resistivity variation.
