Variability in Slope Sandstone Bodies: Linkage to Slope Morphology and Evolution
Romans, Brian; Hubbard, Stephen M.; Stright, Lisa; Auchter, Neal C.
Prediction and characterization of sandstone reservoirs deposited in deep-water slope environments has significantly improved over the past two decades, largely as a result of 3-D seismic data acquisition for appraisal and development. However, consistently accurate prediction of sub-seismic reservoir heterogeneity remains elusive. High-relief slope systems are particularly challenging because of local gradient changes caused by mass wasting and/or syn-depositional substrate deformation, in addition to turbidity current processes. Comparison of sandstone bodies from two outcropping slope systems in the Magallanes Basin, Chile, provide insights into patterns of sandstone body geometry and facies distribution, as well as linkages between sub-seismic- and seismic-scale stratigraphic packaging. The slope systems analyzed are both from the Tres Pasos Formation; high relief slopes prograded axially along the foredeep. The northern Tres Pasos (NTP) slope system is dominated by evidence for frequent mass wasting, and the southern Tres Pasos (STP) consists of mapped clinoforms with > 1 km of relief. The older NTP is ~50 km north of the STP.
Sandstone bodies of the NTP are attributed to intraslope fans and channelized lobes. Thick (20-100 m) mudstone-dominated intervals interpreted as mass transport deposits in between the sandstone bodies have a noticeable effect on overall sand body architecture. The sandstone bodies of the STP represent channel-fills with systematic internal facies distributions, including sandstone-rich axes transitioning to sandstone-poor facies in off-axis and margin positions. High-resolution 3-D mapping demonstrates that STP channel-fill bodies of similar size and geometry cluster to form larger-scale channel complexes with dimensions similar to those imaged in seismic data. Although many of the NTP sandstone bodies have favorable reservoir characteristics internally, their overall geometry is more variable and their stacking less systematic compared to the channelized STP architecture. We interpret these fundamental differences to be a function of the overall slope gradient and aggradation history. In the north, slope readjustment occurred frequently enough to significantly influence the substrate topography over which sand-laden turbidity currents traversed. In contrast, the systematic channel-fill deposition and stacking of the STP system suggests a long-lived phase of turbidity current delivery to the base of slope.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013