Application Of Cretaceous Interior Seaway Outcrop Investigations To Fluvial-Deltaic Reservoir Characterization: Part I, Predicting Reservoir Heterogeneity In Delta Front Sandstones, Ferron Gas Field, Central Utah

Abstract

Data collected from outcrops of the Cretaceous Ferron Sandstone, central Utah, support the theory that stratigraphic heterogeneities are predictable and can be related to position within a stratigraphic cycle (Gardner 1993; Barton 1994; Barton 1995). Specifically, strongly progradational parts of the fluvial-deltaic Ferron system are characterized by mud-rich, internally heterogeneous delta front deposits interpreted to have been rapidly deposited along a river-dominated coastline under conditions of low accommodation and high sediment supply. In contrast, aggradational to retrogradational parts of the system are characterized by sand-rich, internally homogeneous delta front deposits interpreted to have been deposited along a wave-dominated coastline under conditions of relatively high accommodation and low sediment supply. Data collected from Ferron outcrops was integrated with borehole and production data from the nearby Ferron gas field. The Ferron gas field is located just 5 km to the northwest of outcrops studied and produces largely from the Ferron Sandstone. The description of reservoir architecture within the Ferron gas field was aided by calibrating key surfaces and facies successions identified in borehole data with nearby outcrop equivalents. The gas field is positioned near the seaward extent of the Ferron system and consists of four upward shoaling, shallow marine successions. The basal succession steps strongly basinward, the overlying succession is vertically stacked, and the upper two successions are arranged in a backstepping pattern. A comparison of sandstone body geometry and cumulative production on a well-by-well basis for each succession illustrates areas of unrecovered gas and styles of reservoir compartmentation. Aggradational and backstepping shallow marine successions contain relatively large reservoir volumes that are effectively contacted and drained by conventional well patterns. Favorable sites for stratigraphic entrapment, however, exist near the landward pinchouts of these successions where they interfinger with lagoonal mudstones. In contrast, offlapping and downstepping shallow-marine successions are highly compartmentalized by marginal marine and marine mudstones. Past completions have contacted only small reservoir volumes. Optimal reservoir drainage within these units may require additional infill wells outside the standard field limits or at intervals less then conventional well spacing. All factors being equal, these types of shallow-marine deposits possess the greatest amount of reserve growth potential.