Unconventional Reservoirs · Module U3
TOC from logs: Passey Δlog R
The Passey method reads organic richness from the separation between a resistivity log and a porosity log scaled to overlie it in lean shale. The module builds delta log R for the sonic, density and neutron, converts the separation to TOC through the maturity term, picks baselines from the shale points, and shows where the overlay fails: gas, over-mature rock, a baseline in the wrong rock and a lean shale that is not free of organic matter.
- Level
- Advanced
- Position
- Module 3 of 8
- Video
- About 17 minutes, narrated
- Format
- Narrated video lesson with captions and slide reading view; with the certified option, a quiz whose pass unlocks the next module.
What the module covers
- Delta log R on the sonic, density and neutron
- The maturity term and the level of organic metamorphism
- Picking baselines from the shale points
- Reading the rescaled overlay tracks
- Negative separation, no-baseline and over-mature flags
- Why gas raises the overlay reading
Learning outcomes
After the module you should be able to:
- Read an overlay TOC on a stated baseline at a stated maturity, with its failure modes named.
- Compute delta log R and convert it to TOC at a stated maturity
- Name the conditions under which an overlay TOC reads high
A short preview
On the sonic overlay, delta log R adds the resistivity separation, as a base-10 logarithm, to 0.02 times the sonic separation in microseconds per foot. Suppose a source rock reads 40 ohm-m against a baseline of 4, and 95 µs/ft against 80. The resistivity term is 1.0 and the sonic term 0.3, so delta log R is 1.3. At LOM 10.5 the maturity multiplier is about 3.35, which gives close to 4.4 wt% TOC. Read the same rock at LOM 9 and the answer climbs to about 7.8: the maturity input matters as much as the logs.
Who it is for
Petrophysicists, geoscientists and reservoir engineers who evaluate organic shales and source rocks from logs and core, and carry TOC, net pay and gas in place into appraisal and development decisions.
Prerequisites: The Intermediate track, or working experience with shale volume, porosity and water saturation; the Geomechanics course is recommended before U8. The hands-on steps use the Organic Matter workspace in Formation Evaluation, and the brittleness steps in U8 use the Geomechanics workspace, available from the Professional plan.
Take the module in the Learning Hub
Unconventional Reservoirs: Specialty course, sold separately: eight modules that evaluate an organic shale from logs and core, from TOC methods fused and calibrated to core, through kerogen-corrected porosity and net pay, to free and adsorbed gas in place and brittleness. The lesson opens in the Learning Hub once the track is purchased; graded quizzes and the certificate come with the certified option.