Petrophysical (rock and reservoir) properties of Petrophysical (rock and reservoir) properties of Mesaverde strata in northwestern Colorado change significantly with variations in both lithology and depositional environment. The Mesaverde Croup at the U.S. Department of Energy's Multiwell Experiment site contains strata that were deposited in shallow marine, lower and upper delta plain, and fluvial environments. The detrital compositions of the original sands in these environments were similar, but differing depositional and diagenetic modifications produced zones of differing sandstone mineralogy and texture. Different mineralogies in turn cause significant differences in reservoir porosities and permeabilities, and in rock porosities and permeabilities, and in rock properties such as Young's modulus, Poisson's ratio, properties such as Young's modulus, Poisson's ratio, fracture toughness, capillary pressure, and compressive and tensile strength.
Introduction
Despite gross petrologic similarity, reservoir and petrophysical properties in sandstones of the petrophysical properties in sandstones of the Mesaverde Group, northwestern Colorado, can change significantly with changes in the environment in which the sandstones were deposited. The purpose of this paper is to present the petrophysical and reservoir properties that were measured during the course of the US Department of Energy's Multiwell Experiment (MWX) to explore the basic petrological characteristics that underlie the differences in the petrophysical and reservoir properties, and to petrophysical and reservoir properties, and to discuss the sedimentological causes for the observed variability.
Varying depositional and diagenetic conditions modified the initial sand composition in different ways in different zones. These effects can be seen in detailed analyses of 1280 m (4200 ft) of core from the Mesaverde Group, obtained from three Multiwell Experiment wells located in Sec. 34, T6S, R94W, in the Rulison gas field (Figure 1). MWX was a field laboratory consisting of three wells arranged in a triangle with leg lengths of between 34–66 m (110–215 ft) at depth. Its objectives were to characterize low-permeability, natural gas reservoirs, and to use this characterization for both the design of optimal stimulations and the assessment of existing and new stimulation technologies.
Natural fractures in the reservoir strata of the MWX site dominate the reservoir system permeability." However, the petrophysical properties presented in this report are important to gas drainage from matrix blocks into the fracture system, to reservoir volume considerations, and to questions of reservoir damage during drilling and simulation. Therefore, an understanding of the variation in these properties as a function of depositional environment properties as a function of depositional environment provides insight into the production of gas from provides insight into the production of gas from these reservoirs.
STRATICRAPHY AND DEPOSITIONAL ENVIRONMENT
The Mesaverde Group in the Piceance basin consists of the Iles and Williams Fork formations (Figure 2). The Iles Formation is primarily of marine origin, and contains the Corcoran, Cozzette, and Rollins Sandstones members. The Williams Fork Formation has not been stratigraphically subdivided, but is recognized to contain three types of nonmarine deposits. Each type of marine and nonmarine depositional environment produced a unique type of sandstone reservoir. The types of reservoirs encountered in the 1220-m (4000-ft) thick Mesaverde Group at the MWX site are—from the bottom up (Figure 2)—shallow-marine to shoreline blanket sandstones;lenticular distributary-channel sandstones and overbank splays of the lower delta plain (" paludal") environment;reservoirs plain (" paludal") environment;reservoirs similar to those of the paludal (but without interbedded coals) of the upper delta plain.
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