25th International Meeting on Organic Geochemistry IMOG 2011

P-368
Comparison of the geochemical and microstructural coevolution
of two shale lithotypes through artificial maturation
Hendrik Grotheer 1 , Nicholas Drenzek 1 , John Valenza 1 , Michael Herron 1 , Sean Sylva 2 ,
Jeffrey Seewald 2 , Roger Slatt 3
1 Schlumberger-Doll Research, Cambridge, MA, United States of America, 2 Woods Hole Oceanographic
Institution, Woods Hole, MA, United States of America, 3 University of Oklahoma, Norman, OK, United States
of America (corresponding author:ndrenzek@slb.com)
Driven by the recent expansion in unconventional gas
and oil shale resource development, renewed study of
source rock microstructure has underscored the
importance of organic matter maturity and distribution
in regulating fluid flow dynamics. Quantitative models
describing the catagenic co-evolution of shale organic
and mineral composition with matrix microstructure at
the micro- to nanometer scale remain ill-constrained,
however, hampering efforts to predict reservoir quality
in such complex and heterogeneous systems.
In order to more directly relate the effects of thermal
maturation to pore structure changes and
corresponding petroleum generation, migration, and
storage potential, hydrous pyrolysis experiments were
performed on calcareous- and organic rich core
horizons from the Barnett formation (Texas, USA)
using the sealed gold capsule technique. 1 We
monitored kerogen morphology by optical and SEM
microscopy, kerogen and petroleum composition by
elemental analysis, GC-MS, and pyrolysis GC-MS,
co-generated gas composition and isotopic signature
by headspace GC and GC-IRMS, mineralogy by FTIR
spectroscopy, and matrix microstructure by
conventional gas sorption techniques at three
different time points (2, 6, and 18 days) under
constant temperature (350°C) and hydrostatic
pressure (250 bar) conditions.
Hydrocarbon gas yields increased over time, with the
organic-rich interval generating approximately five
times more C1-C4 gases (500 µg/g) of wetter
composition (C1/C2+ ~0.4) after 18 days than the
calcareous interval (Figure 1). Large amounts of CO2
(~15,000 µg/g), likely the result of carbonate thermal
decomposition, were also generated after only 2 days
and remained nearly constant thereafter. Bitumen
extracts, while low in mass typical of many gas-stage
shales, displayed classic early ingrowth of a broad, low
CPI paraffin distribution followed by cracking to lighter
compounds.
Interestingly, specific surface areas of both native state
and pyrolyzed samples with and without their bitumen
extracted increased initially and declined at later stages
(Figure 1), possibly reflecting creation and subsequent
consolidation of kerogen-hosted, partially fluid-filled
micropores. Nanoscale SEM imagery of ion-polished
matrix surfaces reveal a fine pore network that is both
primarily confined to kerogen maceral structures and
more fully developed near organic-mineral interfaces.
Coupled with FTIR mineralogy, these findings likely
reflect the importance of catalysis reactions, moisture-
and heat-sensitive mineral transformations, and the
interplay of differential fluid and interfacial chemistries.
Reference
[1] Seewald et al., 2000. GCA 64 (9), 1577-1591.
Figure 1 – Specific surface areas (left axis) of untreated
(closed symbols) and bitumen-extracted (open symbols)
shale microstructure along with hydrocarbon gas generation
(right axis) as a function of artificial maturation (pyrolysis)
time.
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