Controls on Stratigraphic Architecture in Fluvial Overbank Successions: Implications for Hydrocarbon Recovery in Low Net-To-Gross Fluvial Systems
Outcrop-based studies of fluvial successions often focus on sand-prone channel complexes. Less attention has been directed towards finer-grained fluvial overbank successions, although these accumulations can constitute volumetrically significant parts of fluvial reservoirs. In addition, they contain thin but extensive sand bodies, which can enhance reservoir connectivity. This work presents a depositional model to account for the origin of observed stratigraphic variability and complexity in fluvial overbank successions through a quantitative facies and architectural-element analysis of fine-grained intervals of the Cretaceous Castlegate and Neslen formations, Utah. Results establish criteria for the recognition of architectural elements, examples of common facies associations, geometric relationships between elements and a generic classification scheme for overbank elements. Overbank elements represent the preserved products of crevasse channels and splays, levees, abandoned channel fills, floodplain lakes and mires. Crevasse splay and channel elements thin and fine away from major trunk channels from which they emanate; these bodies exhibit diverse but generally radial patterns of paleocurrents and have facies dominated by upper-stage plane beds and gutter casts in proximal areas; these pass laterally into climbing-ripple cross-laminated and small-scale deformed beds in more distal areas. Such trends in element thickness and constituent facies associations serve as a useful tool for prediction of both distance and direction to primary trunk-channel elements. Multiple splay elements commonly stack vertically in a trend whereby successive sand bodies thicken and coarsen upwards, culminating in an erosional trunk channel element at the top of a preserved cycle; such trends record progressive migration of a master channel towards the site of deposition. Elsewhere, splay elements can be mapped out via lateral tracing around a series of canyons; multiple stacked splays are commonly offset from one another in a style indicative of compensational stacking. The distal parts of overlapping splay elements commonly merge into one another, and this demonstrates the significance of these thin sandstone bodies as potentially important connectors that link larger sand bodies within generally low net-to-gross reservoir successions.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015