Laterally Discontinuous Reservoir Facies Controlled by Multiple Orders of Sea Level Change, Paradox Basin, Utah

G. M. Grammer, G. P. Eberli, F. S. P. Van Buchem, R. Eschard, P. Homewood, G. M. Stevenson

Hydrocarbon production from Pennsylvanian phylloid algal mound reservoirs in the Paradox Basin has exceeded 400 MMBO and estimates suggest similar amounts of recoverable reserves still in-place. The exploration and development history of the basin indicate that cycles containing prolific algal mound reservoirs are characterized by extreme lateral variability. An outcrop-based study combining detailed facies analysis and cyclostratigraphy from equivalent facies on the exposed Paradox shelf has provided insight into the three-dimensional architecture and distribution of both the algal mound reservoirs and their associated facies, as well as the timing of mound growth and porosity development relative to sea level change.

The Desert Creek and Ismay intervals of the Paradox Formation (Desmoinesian) 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 are characterized by a basal sandstone unit, transgressive black "shales", and a well-defined shallowing-upward trend in carbonates deposited during the relative highstand and early stages of the next fall.

Many of these high-frequency cycles contain source rocks, reservoir facies, and seals. The black "shales", which are typically sapropelic dolomites, act as source rocks and may contain up to 5% TOC in outcrop. The algal mound reservoirs, deposited during latest transgressive phase through highstand, typically contain primary shelter and secondary moldic porosity, with minor amounts of fracture-enhanced porosity. Moldic porosity in the algal mounds, as well as in cycle-capping ooid grainstones, developed during subsequent exposure of the platform during sea level lowstands. Reservoir seals in the basin are provided by onlapping and overlying evaporites.

Thickness of 4th-order cycles in outcrop may vary from 20-60% on a kilometer scale as a result of both regional exposure (erosional) events as well as proximal versus distal positioning of facies belts on the shelf. Because of the low gradient of the Paradox shelf, landward and seaward shifts in 4th order depositional packages may result in a displacement of algal mound facies along the shelf by tens of kilometers.

Lateral variations in 5th-order cycle thickness are primarily facies-controlled. High frequency cycles containing phylloid algal buildups may vary in thickness by 25-30% within tens of meters depending upon whether the algal facies accumulates as a relatively thin biostrome, or whether well-developed mounds form. This in turn is at least partially controlled by the amplitude of relative sea level change, and the creation of accommodation space. Aggradational growth of some mounds, either due to more favorable environmental conditions, or perhaps initiation on an antecedent high, resulted in the near-filling of available accommodation space, whereas adjacent platform carbonates were unable to keep up with the relative sea level rise.

Understanding of the lateral variability in phylloid algal reservoir facies caused by 4th and 5th-order relative sea level oscillations can provide a valuable exploration and development tool. The realization that 4th order sea level changes may result in the seaward or landward shifting of facies belts by tens of kilometers can help direct and refine exploration efforts. Likewise, anticipating the rapid lateral variability of algal mound reservoirs in 5th-order depositional cycles should help to maximize initial development as well as enhanced recovery operations.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995