Abstract

Natural fractures dominate the permeability of tight gas reservoirs in the Mesaverde Formation of the Piceance Creek Basin, northwestern Colorado. Piceance Creek Basin, northwestern Colorado. Roughly 450 natural fractures have been detected in reservoir sandstones and siltstones in 4200 ft of core from the U.S. Department of Energy's Multiwell Experiment (MWX). Sixty percent of the fractures are calcite-mineralized, vertical extension fractures that are probably the greatest contributors to reservoir permeability. However, the contribution of the other mineralized fractures should not be discounted.

Introduction

The Mesaverde Formation is naturally fractured at the Department of Energy's Multiwell Experiment (MWX) site. MWX is a field laboratory consisting of three closely spaced (113 to 215 ft apart) wells in the Piceance Creek Basin. Fracture descriptions presented in this report were the result of detailed presented in this report were the result of detailed scrutiny of 4200 ft of 4-inch diameter core from the three MWX wells. Thirty percent of the core was oriented using standard multishot techniques.

Approximately 450 natural fractures representing at least 4 different types have been identified in Mesaverde sandstones and siltstones at MWX. Well tests indicate that many of these fractures contribute substantially to reservoir permeability. Reservoir permeabilities measured during MWX well tests are one to three orders of magnitude greater than laboratory-measured, restored-state, matrix permeabilities of MWX core. permeabilities of MWX core. Some 1430 additional fractures, believed to have little to no bearing on reservoir permeability, were also identified in MWX core. These fractures are primarily low to intermediate angle dewatering and primarily low to intermediate angle dewatering and compaction features occurring in nonreservoir mudstones. Such fractures may have formed as small-scale compaction and dewatering features within the mudstones as they underwent subsidence and burial. These fractures are not discussed further here as this paper deals primarily with natural fractures occurring in Mesaverde sandstones and siltstones. Finley and Lorenz have provided a detailed characterization of all natural fractures in MWX core.

At the MWX site, the top of the Cretaceous Mesaverde Formation is at a depth of 4000 ft (1220 m). The section is approximately 4300 ft (1310 m) thick and is comprised of rocks from five different depositional environments. In descending stratigraphic order, these environments include the paralic zone (4000 ft to 4400 ft, (1220–1340 m)) paralic zone (4000 ft to 4400 ft, (1220–1340 m)) characterized by distributary and estuary sandstones; the fluvial zone (4400 ft to 6000 ft (1340–1830 m)) characterized by wide meanderbelt sandstones; the coastal zone or upper delta plain (6000 ft to 6600 ft (1830–2010 m)) characterized by lenticular distributary channel sandstones; the paludal zone or lower delta plain (6600 ft to 7450 paludal zone or lower delta plain (6600 ft to 7450 ft (2010–2270 m)) characterized by lenticular distributary channel sandstones and coals; and the marine shoreline zone (7450 ft to 8300 ft (2270–2530 m)) characterized by blanket sandstones. Some changes in fracture distributions are roughly coincident with changes in depositional environment.

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