Abstract: The Impact of Parasequence Stacking Patterns on Vertical Connectivity Between Wave-Dominated, Shallow Marine Shoreface Parasequences, Book Cliffs, Eastern Utah.
ARNOT, MALCOLM J. and TIMOTHY R. GOOD, Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, EHI4 4AS, Scotland, UK.
Within shallow marine reservoirs, the vertical connectivity between shoreface sandstones in adjacent parasequences may have a significant impact on reservoir flow. The lateral extent of marine flooding surface shales and updip coastal plain coals/lagoonal shales that overlie parasequence boundaries are likely to be the critical factors controlling the degree of vertical connectivity between adjacent parasequences.
The upper Campanian Blackhawk Formation exposed along the Book Cliffs in east-central Utah can be traced for approximately 160 mi (260 km) from the near Helper City to the Utah-Colorado Boarder. The Blackhawk Formation comprises six lithostratigraphic units: the Spring Canyon, Aberdeen, Kenilworth, Sunnyside, Grassy and Desert Member (Young, 1957 and Balsley, 1980), which comprise predominantly marine sandstone bodies that were prograding into the Western Interior Seaway. More recently, eight, high frequency sequences have been defined in the Blackhawk Formation by Van Wagoner et al. (1990) Taylor and Lovell (1991), and O'Byrne and Flint (1995). The lower sequences are characterized by aggradational parasequence stacking patterns. In contrast, higher sequences are characterised by more progradational stacking patterns (O'Byrne and Flint, 1995).
Outcrop studies undertaken on the Grassy Member and the Spring Canyon Member, to collect data on the on the lateral extent of marine flooding surface shales and coals along parasequence boundaries. These studies showed that marine flooding surface shales do not have simple up-dip terminations but may pass laterally into a zone of discontinuous shales overlying shoreface sandstones of the underlying parasequence. It was also found within the Grassy Member which occurs in the upper part of the Blackhawk Formation and has a strongly progradational stacking pattern, the zone of vertical connectivity between parasequences is on the order of 1000's of meters. In contrast, within the Spring Canyon Member, which has a more aggradational parasequence stacking pattern, the zone of vertical connectivity is only on the order of 100's of meters.
The results from this study highlight the importance of applying sequence stratigraphic concepts to shallow marine reservoirs. Perhaps more importantly, it suggests that developing quantitative sequence stratigraphic frameworks at outcrop may enable predictive relationships to be developed between parasequence stacking patterns and the expected vertical connectivity between adjacent parasequences. Application of these relationships in reservoir modelling studies may then improve predictions of fluid flow in subsurface cases.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah