Journal
MARINE AND PETROLEUM GEOLOGY
Volume 28, Issue 3, Pages 675-697Publisher
ELSEVIER SCI LTD
DOI: 10.1016/j.marpetgeo.2010.07.006
Keywords
Slope equilibrium; Stratigraphic grade; Morphology; Lewis Shale; Ross Sandstone Formation; Trajectory of shelf edge; Trajectory of depocenter
Categories
Funding
- Chevron
- Bureau of Economic Geology, University of Texas
- Petroleum Research Fund [40277-AC8]
- GCSSEPM Ed Picou Award
- AAPG
- SIPES Foundation
- Gulf Coast Association of Geological Society
- University of Colorado
- Lewis Shale Consortium at the Colorado School of Mines
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Stratigraphic grade is the similarity of the morphology of successive slope-to-basin profiles in a genetically related depositional system. In this article we use data collected from regional cross-sections of six depositional systems, stratal architecture derived from outcrops of the Lewis Shale (Wyoming, USA), and the Ross Sandstone (Ireland), and supplementary outcrop and subsurface data from other depositional systems to determine how stratigraphic grade relates to stratal (reservoir) architecture in deepwater systems. Four methods are developed that collectively define stratigraphic grade: (1) regional stacking patterns of fourth-order stratigraphic surfaces, (2) the relationship between the trajectory of the shelf edge (T-se) and the trajectory of the depocenter (T-d) for fourth-order stratigraphic units, (3) morphology of the slope-to-basin profiles of fourth-order stratigraphic surfaces, and (4) the similarity of the morphologies of slope-to-basin profiles of fourth-order surfaces in a system (sigma(s), sigma(r)). Several characteristics of stratigraphic (reservoir) architecture of fourth-order stratigraphic cycles are related to stratigraphic grade: (1) longitudinal distribution of sandstone in fourth-order cycles, (2) location of maximum sandstone relative to the depocenter of fourth-order cycles, (3) lengths of fourth-order submarine fans, and (4) longitudinal and vertical distribution of architectural elements. Stratigraphic grade is thus a predictor of reservoir architecture and can thereby be used to reduce the uncertainty in the interpretation of subsurface data. The concept of stratigraphic grade is useful in understanding the stratigraphic evolution of deepwater systems. Most deepwater systems analyzed in this study initiated as out-of-grade and temporally evolved to graded systems over a time span of millions of years. Systems rarely evolve from graded to out-of-grade. First-order controls on stratigraphic grade are determined to be angle of slope, tectonically forced changes in angle of slope during deposition, and sediment supply. (C) 2010 Elsevier Ltd. All rights reserved.
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