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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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