Sequence stratigraphy, alluvial architecture and potential reservoir heterogeneities of fluvial deposits: evidence from outcrop studies in Price Canyon, Utah(Upper Cretaceous and Lower Tertiary)

Abstract

An alluvial reservoir analogue study has been carried out in the Price Canyon area in east-central Utah. The area is 12 x 6 km and is characterised by 3-10 km long, straight canyons with 300-400 m high cliff exposures. A 1300 m thick succession of fluvial deposits is found in the area, and has previously been divided into the Blackhawk, Castlegate, and Price River Formations (Campanian age) and the North Horn Formation (Maastrichtian to Paleocene age). The canyon sides were mapped in detail using multi-model photogrammetry, and accurate vertical sections covering up to 6 km long exposures were produced in a scale of 1:2000. These sections, more detailed 2-D panels and vertical sedimentological logs were then used to divide the succession into a series of unconformity-bounded alluvial sequences. The sequences can be subdivided further into alluvial zones, defined on the basis of the sandstone-shale ratio, bedding style and alluvial architecture. The zones are laterally extensive and can usually be recognised in all parts of the 50-70 km 2 large study area. The five sequences all show the same internal architecture, related to a general base-level fall-rise-fall cycle of long duration (several million years). Thick and laterally very persistent sand-dominated zones are found immediately above the sequence boundaries. Zones in the intermediate and upper part of the sequences are, on the contrary, characterised by lenticular, single-storey sandstone bodies encased in a matrix of fine-grained overbank material. On the basis of the outcrop examples from Price Canyon, an idealised alluvial stratigraphic sequence, comprising four levels, has been constructed. Application of this model through recognition of comparable subsurface sequences may provide increased prediction in the exploitation of alluvial reservoirs. The basal sandstone sheet would be the most interesting production interval. Sandstone bodies in the intermediate and upper parts of sequences are generally always lenticular and isolated, and would therefore be more difficult to correlate and produce.