Summary

The deep Nugget sandstone of Colorado and Wyoming has long been characterized as a reservoir that is dependent on fracture porosity rather than matrix porosity in order to be economically produced. An integrated characterization using core and petrographic data for a group of Nugget sandstone cores in this area indicate the development of isolated porosity zones capable of matrix contribution to production. These “sweet spots” of reservoir quality porosity have developed in less-compacted grainflow sands immediately adjacent to or intersecting interpreted fault traces. In these zones, there is considerable evidence for a late stage dissolution event that removed soluble cements and unstable detrital grains, creating substantial, yet isolated, secondary porosity systems.

The scenario for the development of these secondary porosity zones has significant implications for hydrocarbon migration, maturation, and timing, as well as for prediction and exploration potential of these features. Reactive fluids, probably related to thermal maturation of source rocks at depth, were introduced to more porous and less compacted Nugget sandstone by means of transport along fault zones. The fluids dissolved carbonate and iron cements, as well as unstable detrital grains, including plagioclase and rock fragments. Either subsequent or coeval with the creation of secondary porosity near the fault zones, gas migrated along fault pathways and entered the reservoir. As the initial fluids moved away from the fault zones, they became saturated with iron, resulting in the precipitation of late-stage ferroan carbonate cement elsewhere in the reservoir. Prediction of the distribution of these secondary features may be aided by detailed understanding and delineation of faulting in the area.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas