The Use of Chemostratigraphy to Document Mudrock Variability and Construct Sequence Stratigraphic Correlations in Deep Marine Shales: with Analogs from the Woodford Shale, Oklahoma and the Graneros Shale, Colorado
With the increased viability of unconventional reservoirs and deepwater systems, there is a greater need to document the internal variability present within mudrock dominated systems. Mudrocks have traditionally been viewed as homogenous successions, lacking characteristic facies variability found in coarser conventional sandstone units. Quantitative chemostratigraphic analyses of mudrock successions reveal these units record significant facies shifts resulting from changing environmental conditions. Variations in these chemostratigraphic profiles can be used to identify key stratigraphic surfaces and construct sequence stratigraphic frameworks within apparently homogenous mudrock successions even when located in distal settings in the basin. Finally, ancient deep marine systems, such as the Woodford Shale and the Graneros Shale, can provide insight into the variability present in other deepwater plays. Detrital sediment input is associated with Ti, Zr, Al, and K. The degree of basin restriction correlates with Mo and V concentrations, barring certain mineralogical affinities. Silicon is found in biogenic quartz, detrital quartz, feldspars, and clays. Chemically determining the quartz fraction is typically done by plotting the Si/Al ratio. Evaluation of the Si/Al ratio, in conjunction with the Ti and Zr concentrations, provides a rough approximation of the amount of biogenic quartz present within a sample. At several horizons in the Woodford Shale the Si/Al value spikes and the Ti and Zr value drops; these spikes are interpreted as planktonic blooms. A similar pattern does not appear within the Graneros Shale, suggesting these planktonic blooms were not prevalent in the Graneros’ measured section. Sequence-stratigraphic frameworks can be developed using the following general criteria. Progradational packages record increasing concentrations of Ti, Zr, Al, and K. Retrogradational packages record a declining trend in these elements, indicating the Transgressive Systems Tract (TST). Lowstands Systems Tracts (LST) and Highstands Systems Tracts (HST) can be distinguished by the degree of bottom-water restriction. Low base level generally correlates with a greater degree of basin restriction. The Woodford Shale, in the Arkoma Basin, was initially deposited in incised river valleys in the underlying Hunton Limestone, but evolved to a deep marine setting. A majority of the Woodford Shale is a silicic mudrock unit deposited during a long term transgression (TST) of the Paleotethys during the end of the Devonian Period. The uppermost portion of the Woodford Shale was deposited during a subsequent progradational event (HST) of the Paleotethys. Three cores from Lincoln, Pottawatomie and Pontotoc Counties and two outcrops at the Hunton Anticline Quarry in Murray County cover proximal and distal regions of the Arkoma Basin within southern Oklahoma. The Graneros Shale, in the Denver Basin, was deposited during the incursion of the Western Interior Cretaceous Seaway (WIKS). A majority of the Graneros Shale is an argillaceous mudrock unit deposited during the onset of the WIKS during the Late Cretaceous (Cenomanian Stage). There are multiple transgressions and regressions recorded within the Graneros Shale, but an overall transgressive signal is preserved. The base of the Graneros Shale in this study is interpreted as LST, with a majority of the unit deposited during the subsequent TST.
AAPG Datapages/Search and Discovery Article #90340 ©2018 AAPG Geoscience Technology Workshop, Deep and Ultra Deep Petroleum Systems, Beijing, China, October 26-28, 2018