AAPG editor Rasoul Sorkhabi wrote an article for AAPG Explorer about source rocks, specifically “generative” source rocks, since not all source rocks generate producible hydrocarbons. He defines generative source rocks as follows:
“A generative (or an effective) petroleum source rock is
a clay-rich or carbonate sedimentary rock that has generated, or is generating,
and has expelled, or is expelling, oil and/or gas.”
Recently, I learned about
(and blogged about)
an exception, where a very good oil & gas basin offshore Indonesia has
sandstone source rocks that carried leaf and coal fragments via turbiditic
currents. That, however, was an unusual situation, quite rare.
He reviews what makes a
petroleum system, and below gives seven “grades: of source rock.
“Source rock with trap, seal, reservoir, and migration
pathways is what constitutes the petroleum system.”
He notes that source rock
investigations are very important for shale plays, which are “self-sourced” in
that the source rocks and the reservoir rocks are one and the same. Even some
shale plays are not viable due to hydrocarbons migrating out of the source
rocks. I remember when American shale plays were being explored after the
advent of successful high-volume hydraulic fracturing and horizontal wells.
There was one potential play in Alabama’s Black Warrior Basin, the Floyd Shale,
in which were found what are known as “dead oil stains,” indicative of oil that
was once in the reservoir but no longer present. It was considered highly
prospective since it met most of the requirements for a good source rock. The
problem with the Floyd Shale is thought to be a loss of reservoir seal, which
allowed much of the hydrocarbons to move out of the shale reservoir. Other
evidence of this includes the lack of overpressure in the shale.
Sorkhabi notes that carbon, or TOC,
alone is insufficient. TOC changes with thermal maturity. Hydrogen index (HI),
determined by pyrolysis, combined with adequate TOC, is a better overall
indicator of hydrocarbon yield.
He mentions two methodologies
found in the literature that can evaluate source rock potential:
“Researchers Gerard Demaison and Bradley Huizinga
suggest using the Source Potential Index (SPI). This index combines rock
thickness and density with potential yield (a sum of S1, free hydrocarbon, and
S2, generated hydrocarbon in pyrolysis) and then calculates metric tons of
hydrocarbon per square meter.”
“Another research team, Andrew Pepper and Elizabeth
Roller, suggest using the Ultimate Expellable Potential (UEP) metric, which
integrates rock thickness and density with TOC, HI, transformation ratio, and
oil versus gas components to estimate generated and expelled oil volume.”
“Basin modeler Zhiyong He of ZetaWare considers both
metrics useful in their own ways. SPI gives mass, and UEB gives the volume of
source hydrocarbons. Both SPI and UEP help create generative maps laid over the
depositional span of source rock formations.”
Other methodologies include
determining oil mobility and expulsion efficiency, and determining original TOC
and HI values through maturity and statistical analysis.
Sorkhabi explores the
importance of kerogen typification. Kerogen is the insoluble organic matter
that is cracked via burial heat and pressure. Its type is based on thermal
maturity, which indicates what hydrocarbons the rocks will produce, whether
oil, condensate, or natural gas. He discusses the Van Krevelen diagram or
cross-plot of atomic hydrogen-to-carbon and oxygen-to-carbon ratios, originally
developed to determine coal ranks, and the limitations of this diagram.
“To address these issues, geochemists suggest
organo-facies (visual kerogen) analysis. Another technique is to analyze S2
peak materials from pyrolysis by gas chromatography.”
Below is his concise analysis
of thermal maturity:
Source rock analysis is in
the realm of petroleum geochemistry, and he says there is still much more to
learn about it. He gives two current challenges to understanding via source
rock analysis and basin modeling:
1) “Well
data are location-specific. The deep hydrocarbon kitchen would be the last
space to drill, or it may remain undrilled, even after a long production
history from reservoirs.”
2) “Validating
the results of source rock evaluation applied to a basin is tricky. Reservoir
oil to source rock fingerprinting using age-diagnostic or facies-diagnostic
biomarkers obtained from gas chromatography-mass spectrometry can be helpful to
some extent, but such data are usually scarce.”
References:
How to
Spot Generative Source Rocks: A review of proposed evaluation methods and
criteria. Rasoul Sorkhabi. AAPG Explorer. April 1, 2026. How to Spot Generative Source Rocks
Lessons
learned from the Floyd shale play. Harry Dembicki Jr. and Jonathan D. Madren. Journal
of Unconventional Oil and Gas Resources. Volume 7, September 2014, Pages 1-10. Lessons learned from the Floyd shale
play - ScienceDirect
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