- Copyright © 2017, SEPM (Society for Sedimentary Geology)
The regressive and subsequent transgressive transit of a shoreline across a clastic shelf generates the standard reservoir flow unit in most marginal to shallow marine hydrocarbon reservoirs. The stratigraphic unit produced by the shoreline transits is commonly referred to as the high-frequency (104 to 105 years), regressive–transgressive sequence (RT sequence). The unit is usually bounded top and base by marine shales marking flooding surfaces, which can act as barriers to fluid flow. Stratigraphic architecture in the RT-sequence is also known to control the internal flow behavior of many reservoirs. Hence, an ability to consistently characterize reservoirs at a sub RT-sequence scale is critical to enabling prediction of reservoir performance and optimization of resource extraction strategies.
This study describes the internal architecture of one shallow-water (< 10 m), low-accommodation regressive shoreline succession from the Campanian of the Alberta Basin, Canada, based on an extensive outcrop and subsurface dataset that has been convolved into a 3D geocellular computer model. The architecture and evolution of the ancient mixed-process (waves, tides, and fluvial processes) regressive deltaic shoreline system is compared and contrasted with a partial Holocene analog from northeastern Australia. The same stratigraphic surfaces and units are identified in both the modern and the ancient regressive systems. The key architectural unit is the element complex set (ECS), which is a multi-kilometer-scale, discontinuity-bounded unit that is the product of the reorganization of the coastline, often caused by autogenic backwater-driven avulsions. Multiple avulsions during a regressive shelf transit episode lead to lateral offsets of ECS units in low-accommodation systems. These systems are termed “avulsion-driven systems.” An increasing component of vertical stacking of ECS units is observed in higher-accommodation regressive systems. The mechanisms for generating accommodation on a regressive-shoreline shelf-transit time frame may be allogenic (tectonic subsidence or eustatic sea-level change), autogenic, or a combination of the two mechanisms. A key, localized, autogenic mechanism is related to the distance of progradation during the transit of a deposystem across the shelf. In proximal shelf positions, ECS units tend to offset laterally due to limited available accommodation, whilst in more distal positions, early differential, load-induced, compactional subsidence of underlying prodelta and shelf muds can promote vertical stacking of ECS units. The critical down depositional-dip distance from the transgressive turnaround point at which ECS units become preferentially vertically stacked is a function of shelf gradient, shoreline trajectory, sandstone fraction, and prodelta and shelf mud rheology and is termed the “critical autogenic ECS stacking distance.” Vertical stacking of ECS units may also occur when ECS units overstep underlying shelf topography, such as the distal termination of an older RT sequence.
Recognition criteria and nomenclature for intra-regressive-shoreline surfaces and stratigraphic units, as well as predictive models for the ancient record are detailed across a spectrum of types of deltaic systems.