Stratified Carbonate Reservoirs-Parasequences as Barriers to Vertical Fluid Flow: An Example from the Greater Aneth Field, Paradox Basin of Southeastern Utah
WEBER, L. JAMES, and F. M. WRIGHT, Mobil Exploration and Producing U.S., Inc., Midland, TX, J. R. MARKELLO, Mobil Research and Development Corp., Dallas, TX, and J. F. SARG, Consulting Geologist, Midland, TX
Since discovery of the Aneth field (1956), 360 million barrels of oil have been produced from Middle Pennsylvanian (Desmoinesian) carbonates of the lower Ismay and Desert Creek intervals. The carbonate platform spans an area of approximately 100 sq mi and is composed of repetitive shoaling cycles (parasequences). A greater understanding of the depositional environments and diagenesis within a sequence stratigraphic framework can lead to improved operational efficiency through better prediction and identification of bypassed, banked, and/or undeveloped oil.
Parasequence sets (up to 150 ft thick) are recognized only within aggradational and progradational highstand systems tracts and are comprised of 3 to 5 stacked parasequences (10-50 ft thick). Repetitive facies successions include deep water lime mudstone and skeletal wackestone that grade up into shallow water phylloid algal buildups,
peloidal/skeletal packstone, and oolitic grainstone. Cycles are capped by oolitic/pisolitic grainstone, laminitic and evaporitic dolostone, and calcareous siltstone-sandstone that display evidence of subaerial exposure. Deeper water facies are nonporous. Shoal water facies have the best reservoir porosity and reveal preserved primary pore systems that are secondarily enhanced by fresh water leaching of less stable carbonate mineralogies.
Distinctive facies stacking patterns and associated vertical diagenetic trends resulted in the formation of highly stratified Desert Creek and Ismay reservoirs. Porosity and permeability are developed at the top of parasequences. Sequence stratigraphy provides a framework for more insightful correlation and mapping of genetically related facies that comprise these stratified reservoirs. Cursory examination of similar Pennsylvanian carbonate reservoirs in west Texas suggests an analogous approach for understanding reservoir stratigraphy could be applied with success.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)