Quantifying Slope Intra-channel Facies Architecture from Outcrop
Daniels, Benjamin G.; Macauley, Ryan V.; Fletcher, Sean; Jackson, Allison A.; Stright, Lisa; Romans, Brian; Hubbard, Stephen M.
Deep-water slope channel deposits represent significant reservoir targets on numerous continental margins. Channel systems are imaged in high-resolution 3-D seismic data, which yield insightful perspectives at the scale of 10s to 100s of meters or more. However, sub-seismic scale stratigraphic architecture, which comprises critical information about continuity of potential barriers to fluid flow, is not resolvable in sparse or low-resolution subsurface data. We use an outcrop analog study of two channel complex sets, within approximately 300 m of strata over a 2.5 km-long outcrop in the Tres Pasos Formation of southern Chile, to extract measurements of slope channel facies relationships. Quantitative metrics that capture intra-channel facies connectivity at the reservoir scale are presented, considering the impact of features such as channel-base mudstone drapes on production.
Channel strata were documented with 58 measured sections (3300 m), which were captured at the cm-scale. Channel element and complex boundaries were surveyed with differential GPS; the digital model provides the framework for data compilation and quantification. The deposits can be subdivided into three main facies associations, representing channel axis, off-axis, and margin deposition. Quantitative metric analysis of the channel elements was used to differentiate the intra-channel architectural zones. Parameters analysed included net-to-gross ratios (NTG), amalgamation ratios (AR), and amalgamated sandstone thicknesses (AST). NTG and AR values are highest in the axes (mean values = 0.95; 0.68) and decrease towards the margins (mean values = 0.39; 0.17). AST values follow this trend, with mean measurements of 6.91 m in the axes and 0.93 m in the margins. Individual channel elements commonly contain basal mudstone units, which drape the channel incision surface and largely represent deposition from bypassing turbidity currents. These drapes are often extensive, spanning over 250 m along the length of the outcropping channels. Drapes are present in approximately 42% of channel axis, 69% of off-axis, and 84% of margin deposits. More than 55% of these drapes exceed 1 m thick. Standard deviation values for drape thicknesses are highly variable, ranging from 40% of the mean, to exceeding the mean in rare circumstances. The metrics generated can be used directly in building reservoir models that can more accurately predict flow connectivity in analogous slope channel deposits.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013