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Abstract: Understanding the Distribution and Architecture of Algal Mound Reservoirs Through Outcrop-Based High-Resolution Sequence Stratigraphy: An Example from the Paradox Basin, USA

G. M. Grammer, and G. P. Eberli, F. S. P. Van Buchem, A. M. Schwab, P. Homewood, G. Stevenson

An outcrop-based study combining detailed lithofacies analysis with high-resolution sequence stratigraphy in the Paradox basin, southwestern United States, has shown that complex lateral and vertical facies variations occur in relatively consistent and predictable patterns. Large-scale facies shifts of several kilometers occur across major sequence boundaries whereas small-scale lateral and vertical variations develop from stacking of individual cycles. Understanding of vertical stacking patterns and lateral distribution of facies in outcrop can be directly applied to both three-dimensional distribution of reservoir facies in the subsurface as well as the evaluation of heterogeneity within individual reservoirs.

The Desert Creek and Ismay intervals of the Paradox Formation (Pennsylvanian) exposed along the San Juan River in southeastern Utah are characterized by high-frequency cyclic repetition of carbonate and siliciclastic facies controlled primarily by 4th and 5th-order changes in relative sea level. These cycles are typically mixed and consist of a basal sandstone unit interpreted to have been deposited during a relative sea level lowstand and subsequently reworked during the early phases of the next marine transgression. Black sapropelic dolomudstones, typically referred to as "shales", record the major part of the transgressive phase. Overlying carbonates, which may contain the phylloid algal mound facies, are characterized by a well-defined shallowing-upward trend deposited during late t transgression, highstand, and the earliest stages of the next fall.

These general cycles may consist of up to 10 separate and discreet facies types that vary considerably in both lateral and vertical dimensions. Thickness variations of 20-60% for 4th order cycles and 25-30% for 5th order cycles occur on a kilometer and hundreds of meter scale respectively. In addition, significant facies changes occur across 4th order sequence boundaries indicating rapid landward and seaward stepping of facies belts. For example, the Lower Ismay 4th order sequence is a major landward stepping unit compared to the Upper Ismay sequence. Within each of these fourth order sequences, cycle stacking patterns document an overall thinning with a continuous regressive trend in the facies. These patterns have been confirmed in core and are recognizable on wireline log data, the efore providing a valuable tool when interpreting wireline log suites with limited rock data. Furthermore, the three-dimensional architecture of algal mounds in outcrop can be compared to 3D seismic data providing guidance in understanding subtle amplitude variations and therefore heterogeneity within the reservoir. High resolution outcrop studies of this type may thus provide an invaluable tool for enhancing the predictability of subsurface reservoir facies at both the exploration and development scale.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France