1. Department of Geology and Geological Engineering
Van Tuyl Lecture Series- Spring 2016
4:00-5:00 p.m. in Berthoud Hall Room 241   
Thursday, April 21, 2016
Jesse Melick
“Proximal Basin-Floor Sedimentary Architecture of the Permian Upper Member of the Brushy Canyon Formation, Delaware Mountains, West Texas”
Jesse J. Melick (BP Americas) and Michael H. Gardner (Get Smart Geoscience)
Abstract: Well-exposed outcrops of basin-floor submarine fan systems are important analogs for deepwater oil and gas reservoirs. This study examines a 6 km-long by 100 m-thick segment of the Middle Permian (Guadalupian), informal upper member of the Brushy Canyon Formation in the Delaware Mountains of West Texas. The upper Brushy Canyon Formation is the youngest of three informal chronostratigraphic members mapped in outcrop and across the subsurface of the Delaware Basin. They each record temporal and spatial changes in submarine channel architecture and depocenter distributions within a deepwater depositional episode corresponding to the Lowstand Systems Tract of one third-order sequence. The well-preserved fourth-order basin-floor fans, sourced by canyon feeder systems and deposited below an exposed shelf margin, consist of sandstones bounded by organic-rich siltstones that record cyclicity at multiple scales. Compared to lower and middle Brushy Canyon episodes, the upper Brushy Canyon records a decrease in frequency and magnitude of failure-initiated, sandstone sedimentation events and the onset of marine deepening as evidenced by multiple criteria. These include: 1) increased proportion in shelfal allochems, carbonate mud and silty sandstone; 2) significant increase in concentrations of organic matter of marine algal origin; 3) the most continuous mudstone drapes at the base of aggradational channel-fills; 4) equilibrated channelform to lobeform body ratio and: 5) landward shift in depocenter, recording the onset of depositional outbuilding of the slope during back-filling of submarine canyons.
Detailed field mapping of upper Brushy Canyon facies, event beds and sedimentary body architecture entailed describing centimeter-scale measured sections with scintillometer profiles and correlated to continuous siltstone markers walked out with GPS waypoints collected in mapping the sedimentary architecture on high-resolution photo panels. Across the upper Brushy study area, sand percent varies from 90% in channel fairways, defined by a high density of channels, to 60% in interfairway regions. Three channel complexes, 500m-wide by 15-40m-thick cluster to define two 1-2 km-wide fairways. Elementary channel fills consist of fine-grained structureless and dewatered sedimentation units. Channel-base siltstone drapes increase in continuity and thickness upward and in interfairway regions. Lobe complexes form km-wide and up to 12m-thick amalgamated sandstone benches that flank and cap channel bodies at multiple scales. Elementary lobe bodies contain scour-and-fill cross-stratified fine-grained sandstone sedimentation units. Laterally continuous and organic-rich siltstone intervals, mapped across the study area, and across the outcrop and basin, represent stratigraphic cycle boundaries interpreted to record marine deepening. The possible origins for marine deepening in the upper Brushy basin-floor strata include: (1) eustatic sea-level change, (2) increase in frequency and magnitude of flood-initiated hyperpycnal flows recording secular middle-late Permian climate change, (3) regional sea-level change associated with along-strike shift in shelf sediment source, (4) decrease in siliciclastic sand influx due to changes in the source terrain.
This high-resolution stratigraphic framework permitted documenting systematic changes in sedimentary energy at multiple scales. These trends in the upper Brushy Canyon are associated with one cycle of basinward to landward step during the 3rd-order backstep. Unidirectional lithological trends (increasing TOC in siltstones, carbonate allochems, silty sandstone) track the longer-term base level rise and smoother slope, while bidirectional trends in sedimentary body attributes (type, shape, size and fill style) follow higher-frequency cyclicity. Generating stratigraphic frameworks based on systematic changes in sedimentary energy can help de-risk reservoir characterization and improve prediction of well performance in deep-water drilling programs.