Read the Well: Petrophysics
From a raw log display to barrels with error bars. Porosity, saturation, permeability, and the judgment calls between, worked on one honest well from first look to final volume.
You can read a standard log suite, compute porosity and water saturation with the right model and defend the exponents, estimate permeability, place the contacts, and carry Ogbon-1 from curves to an oil-in-place number you would sign.
The rock and the fluid
Four questions run this whole discipline: how much pore space, what fills it, will it flow, and how much is there.
Porosity is the space and saturation is the split; every log you will ever read is trying to measure one of these two.
Flow and the free-water level decide whether the pore space is a reservoir or a curiosity.
The borehole environment
Every log reads a rock that drilling already disturbed; invasion and depth of investigation are the corrections between what you measured and what is there.
Gamma ray and SP
The humble gamma ray is the first curve every interpreter reads, and Vsh from it quietly enters every equation downstream.
The spontaneous potential is the oldest log still run, and it hands you Rw, the number Archie cannot live without.
Porosity logs
Two porosity tools that err in opposite directions; crossplot them and lithology and gas both confess.
Crossover is the log suite's loudest fluid signal, and shale corrections decide what part of your porosity can actually hold hydrocarbon.
Wyllie's time average is half physics and half habit; know its corrections and when Raymer-Hunt-Gardner reads truer.
Resistivity
Resistivity is the only log that sees fluid type directly, and the formation factor is the bridge from rock texture to saturation.
The profile through flushed and virgin zones must be unwound to a true Rt; the tornado chart is where that correction lives.
Saturation
One equation carries the industry; the Pickett plot turns it into a graphical argument you can defend in front of anyone.
Constant BVW is the quiet test of a reservoir at irreducible water, and Rw uncertainty is the biggest lever on every Sw you will ever compute.
Clean Archie fails the moment clay conducts; Simandoux, Waxman-Smits, and their kin are how saturation survives real rocks.
Permeability
Permeability is the property everyone wants and no log measures; estimators and the poro-perm crossplot are the honest workarounds.
RQI and FZI cut a reservoir into rocks that flow alike, which is what the simulation engineers actually need from you.
NMR
NMR reads pore size where other logs read bulk properties; the T2 cutoff splits bound from free fluid and pays directly into permeability.
Capillarity and height
Capillary pressure is why contacts are zones, not lines; the physics here explains every saturation profile you will ever map.
J-functions turn core capillary data into a field-wide saturation model, which is how petrophysics escapes the wellbore.
Lithology
When the rock is not one mineral, the photoelectric factor and the crossplot family are how the matrix confesses its mixture.
Pay and volume
Every cutoff is a business decision wearing a petrophysics costume; sensitivity analysis is how you keep it honest.
STOIIP is where every curve you computed becomes barrels; each symbol in the equation is a chapter of this path.
Pressures and contacts
Pressure gradients are fluid density in disguise; their intersections place contacts more honestly than any single log.
Two wells with different gradients are two reservoirs no matter what the map says; pressure is the compartment detector.
The Ogbon-1 capstone
One well, every curve, your call: the full evaluation from raw logs to barrels with uncertainty, proven by doing it.
The whole discipline compressed onto one card you can pin next to the log viewer.