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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: The Devil in the Details: What Controls Vertical and Lateral Variation of Hydrocarbon Source and Shale-Gas Reservoir Potential at Millimeter to Kilometer Scales?
ExxonMobil Upstream
Research Company
Houston, Texas
Hydrocarbon source and shale-gas reservoir character varies
at the cm-scale vertically and at the km-scale laterally in
systematic ways that can be deciphered using process-based models
within a sequence
-stratigraphic framework. Even in mudstonedominated
strata deposited hundreds of kilometers from coeval
shorelines, parasequences exhibit systematic variations in physical,
biogenic, and chemical attributes. These variations arise from
changing relations among the key sets of processes that influence
the production, destruction, and dilution of organic matter.
Paleozoic, Mesozoic, and Cenozoic
examples (including the New Albany,
Barnett, and Mowry Shales, Monterey
and Sisquoc Formations) exhibit major
shifts in mudstone properties at
sequence
boundaries, flooding, and
downlap surfaces. These shifts can be
recognized independently on seismic,
log
, core, and thin-section data. They
also show systematic vertical and lateral
variations in those properties at the
parasequence- and parasequence-set
scale. Prospective facies tend to occur in
discrete packages that are diachronous
across a basin, making it essential to
employ the various physical
sequence
-stratigraphic surfaces
to correlate appropriately and to decipher the distribution of
hydrocarbon potential.
Location with respect to sediment sources and shorelines is a
key factor that can be discerned using close examination of
sedimentary structures in thin section and hand specimen,
integrated with detailed well
-
log
correlation and geochemical
analyses. Parasequences in proximal reaches tend to have totalorganic-
carbon (TOC) content inversely related to sandstone
content, maximum grain size, sandstone-bed thickness, level of
bioturbation, and skeletal phosphate content. The maximum
TOC is positively correlated with hydrogen index (HI) at small
TOC values and inversely correlated with HI at large TOC values.
TOC is largest at parasequence bases. All observations in
proximal sections indicate that dilution by non-hydrogen-rich
material is the dominant control on source potential.
In distal areas, maximum TOC content is positively correlated, but only weakly, with maximum grain size and bed thickness. Maximum TOC content is also positively related to phosphate content, HI, and, counter-intuitively, level of bioturbation. TOC is largest near parasequence tops—the opposite of what is seen in proximal areas. The positive correlation of TOC, HI, and phosphorous content suggests that variations in primary organic production or preservation were the key influence on source character (and not dilution). The positive correlation of TOC and HI with bioturbation index further indicates that production was the controlling factor in this setting. Increased bioturbation and slow sedimentation rates should lead to decreased preservation of organic matter through increased consumption and decreased burial efficiency. The observed accumulation of organic matter under these conditions points to production rates of organic matter in excess of the capacity to consume or degrade it.
An appreciation of variations at such small scales should enable the selection of appropriate and representative samples, an understanding of how they correlate away from sample control, and the calculation of net source or reservoir. These relations influence such economically important factors as evaluation and assessment of net volumes of source or gas-in-place, expected hydrocarbon type and quality, and timing of generation.
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