--> Modeling Controls on the Architecture of Phylloid Algal and Associated Reservoirs from Outcrop Analogs: Lower Ismay Zone (Pennsylvanian), Utah, USA

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Modeling Controls on the Architecture of Phylloid Algal and Associated Reservoirs from Outcrop Analogs: Lower Ismay Zone (Pennsylvanian), Utah, USA

Abstract

This study of the Pennsylvanian Lower Ismay zone of the Paradox Formation, Paradox Basin, Utah, evaluates controls on facies distribution to improve forward modeling and 3D reservoir geomodels of similar sequences. New outcrop studies show facies are distributed in ten units within 2 sequences. Sequence 1 (Units 1-5), consisting of Black Laminated Mudstone, Spicule Mudstone, Crinoid Packstone, Algal Bafflestone, and Packstone, evidences shallowing from 50-100m depth to subaerial exposure. A minor relative rise in sea level during the overall fall led to a shift of algal mounding updip. Relative sea-level fall resulted in a downdip shift in algal mounding and subaerial exposure (Sequence Boundary 1). Sequence 2 (Units 6-10), consisting of: Fusulinid Packstone, Skeletal Wacke-Packstone, Skeletal Wacke-Packstone-Chaetetes, Peloidal Mudstone, Quartz Sandstone and Siltstone, shows evidence of a relative rise that produced draping beds, followed by a relative fall, or filling of accommodation, where facies filled topographic relief. Shape, distribution, and internal facies variation of phylloid algal facies is a key component in building accurate 3D geomodels useful in subsurface analogs. Previous studies of these outcrops described mounds as large linear waveforms or fully constructional mounds. This study shows that the mounds are isolated complexes, the distribution of which is controlled by the interaction between sea level and paleotopography. The mounds are not linear bedforms or fully constructional. They are made up of generations of near-circular mounds (40m across) that stack asymmetrically on the crests of underlying mounds, creating an undulose geometry with 0.4-2.1m of relief. Overlying subaerial exposure (SB1) created a karst landscape that accentuated the low areas between the mounds through erosion and increased the original relief to 2.9-5.7m. Incorporation of these geometries into subsurface models could lead to improved prediction of reservoir performance. This study shows that the interaction of sea level with paleotopography creates a complex architecture that fits a build-and-fill motif for reservoir and non-reservoir facies; the observations define controls on architectures useful in evolving forward models for such systems. Additionally, new observations on phylloid algal mounds refine understanding of mound growth and geometry; the outcrop analogs are geobodies ready for direct incorporation into subsurface reservoir models.