Print this pageAssessing the Importance of Pyrobitumen in 
Unconventional
Reservoirs
Tim Ruble
Weatherford
When oil thermally decomposes into gas via cracking of longer
chain hydrocarbons, it leaves behind a residual insoluble
“pyrobitumen”. The role of pyrobitumen in unconventional
shale systems is a topic that is currently receiving considerable
attention. Like kerogen, pyrobitumens may provide a residual
organic matrix capable of significant sorbed gas storage
potential. The development of mesopores within pyrobitumens
may also provide porosity for free gas storage. Because
pyrobitumens are organic residue associated with the primary
expulsion network in the source rock, they represent a continuous
interconnected network. Further, the brittle nature of
pyrobitumens may provide a fracture network for gas recovery
that would extend beyond induced mineral fractures.
New analytical tools allow us to better characterize recalcitrant
organic matter. Traditional organic petrology is integrated
with high-temperature programmed pyrolysis to identify the
newly defined Spy™ peak associated with the occurrence of
pyrobitumen. In samples that have entered into early-stage
metamorphism, x-ray diffraction is used to quantify the abundance
of graphitic pyrobitumen. Illite crystallinity and the determination
of the Kübler index provide useful supplements to
Ro for maturity assessment. Microscopic and SEM examination
along with 3D models derived from FIB‑SEM analysis
show that pyrobitumen can comprise a significant volume of
the rocks. Organic porosity often appears to be interconnected
and gas storage capacity continues to evolve within the graphitic
pyrobitumen matrix during late catagenesis and metagenesis.
The occurrence of pyrobitumen also appears to influence
well log characteristics due to its conductive nature. These
observations, in conjunction with recent work on deep natural
gas stable isotopes, improve our understanding of the potential
for thermogenic shale gas production in early metamorphic
frontiers. Pyrobitumens may also be significant in shale
oil systems due to asphaltene precipitation processes that
may block micropores and inhibit shale oil recovery.
AAPG Search and Discovery Article #90185 © AAPG Geoscience Technology Workshop, Revisiting Reservoir Quality Issues in Unconventional and Conventional Resources
, Austin, Texas, November 12-13, 2013