Significant Variations in Hydrocarbon Source and Mudstone-Reservoir Character from the Parasequence Scale: Counter-Intuitive Trends, Systematic Relations, and Economic Implications
Source character (TOC, OMT, etc.) varies significantly vertically and laterally (meter to kilometer scale) in systematic ways that can be deciphered using detailed sequence stratigraphic analysis. Even in mudstone-dominated 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. We illustrate these relations with examples from the Cretaceous Mowry Shale and related formations, western USA.
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 TOC content inversely related to sandstone content, maximum grain size, sandstone-bed thickness, level of bioturbation, and skeletal phosphate content; maximum TOC is positively correlated with 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; it is also positively related to phosphate content, HI, and, counter-intuitively, level of bioturbation. TOC is largest near parasequence tops. The positive correlation of TOC, HI, P, and bioturbation indices suggests that variations in primary organic production were the main influence on source character. The positive correlation of TOC with bioturbation index also indicates that production influenced source enrichment more than preservational conditions in this setting-- increased nutrient flux drove primary organic-matter production that overwhelmed consuming organisms.
An appreciation of variations at such small scales should inform how one selects appropriate and representative samples, correlates away from sample control, and calculates 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.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009