Abstract
Deposits of wave-dominated shorelines are typically considered to act as relatively simple hydrocarbon reservoirs and are commonly modeled as “tanks of sand.” However, important heterogeneities that can act as barriers to fluid flow occur at the parasequence, bedset, and bed scales, especially in viscous oil or low-permeability oil fields. Heterogeneities at the parasequence and bedset scales have been well studied, but discontinuous mudstone beds occurring within the shoreface have received little attention. The Book Cliffs and Wasatch Plateau are among the best-exposed and best-studied deposits of wave-dominated shallow-marine systems in the world. Two parasequences within these outcrops have been studied in detail to investigate the distributions of intrashoreface shales and to propose models for the controls on their distribution. A data set consisting of 30 km (18.6 mi) of virtual outcrops derived from oblique helicopter-mounted light detection and ranging (LIDAR) scanning with supporting stratigraphic sections makes it possible to collect a large quantity of accurate geometric data of depositional elements from inaccessible cliffs. Nine-hundred and twenty-one discontinuous mudstone beds were measured. These occur as ellipses with long axes oriented normal to the paleoshoreline. Lengths and widths of these mudstone beds exhibit a lognormal distribution, with means of 21.9 and 13.8 m (71.9 and 45.3 ft), respectively. Within the shoreface succession, the number of mudstone beds increases downward whereas size does not vary significantly with stratigraphic height. An average of 100 m (328 ft) cumulative length of shale exists per 100 m (328 ft) of horizontal outcrop; this increases threefold near both wave-dominated deltas and bedset boundaries that reflect minor sea-level fluctuations during progradation.
Highlights
Sediments deposited in wave-dominated shoreline systems provide numerous, prolific hydrocarbon reservoirs, such as the Brent and Humber group deposits of the North Sea (e.g. Richards and Brown, 1986; Howell et al, 1996; Kadolsky et al, 1999; Husmo et al, 2003), the Wilcox Formation onshore Gulf of Mexico (e.g. Galloway et al, 2000), and numerous Tertiary delta systems of South East Asia (e.g. Hodgetts et al, 2001; Ainsworth et al, 2005)
Interbedded sand- and mudstone beds occur in weathered cliff faces as resistant, beige sandstone ledges and darker, recessed mudstone intervals
This is more applicable when using virtual outcrop data, because properties such as grain size, sedimentary structures and texture cannot be measured in the virtual outcrops
Summary
Sediments deposited in wave-dominated shoreline systems (shorefaces and wave-dominated deltas) provide numerous, prolific hydrocarbon reservoirs, such as the Brent and Humber group deposits of the North Sea (e.g. Richards and Brown, 1986; Howell et al, 1996; Kadolsky et al, 1999; Husmo et al, 2003), the Wilcox Formation onshore Gulf of Mexico (e.g. Galloway et al, 2000), and numerous Tertiary delta systems of South East Asia (e.g. Hodgetts et al, 2001; Ainsworth et al, 2005). Wave-dominated shallow marine systems are commonly treated as “tanks of sand”, in which the shoreface is modeled as a shore-parallel belt of homogeneous sandstone (MacDonald and Aasen 1994; Howell et al, 2008a). Wave-dominated, and wave-dominated fluvial influenced shallow-marine systems comprise a series of facies that pass seaward from sand-dominated foreshore, upper- and lower shoreface deposits into a heterolithic offshore transition zone. This in turn passes seaward into a muddominated offshore shelf deposit (Elliot, 1978; Clifton, 2006; Howell et al, 2008a). Distributary channels are commonly heterolithic or mud-filled, sand-filled channels may occur (Olariu and Bhattacharya, 2006)
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