Modeling of Noncontinuous Fort Union and Mesaverde Sandstone Reservoirs, Piceance Basin, Northwestern Colorado
Authors: Knutson C.F. 
Published: December 2007

A recent Federal Power Commission study indicated up to 600 trillion ft of natural gas in place in the Rocky Mountains in low-permeability place in the Rocky Mountains in low-permeability Cretaceous and Tertiary nonmarine sandstones. This type of reservoir rock bas been studied in the surface and subsurface in the Piceance Basin area of northwestern Colorado. The sandstone geometry and orientation are characterized in this paper. A "random intersection" technique is developed that uses the geological information to generate a spectrum of reservoir models that may be used to simulate the performance of conventional as well as hydraulically and nuclearly fractured wells. Example reservoir models for fractured wells are compared with reservoir models for nonfractured wells.

In general, point-bar sandstones can be simulated by radial models and channel-fill sandstones by linear models.

In a sequence of channel-fill sandstones with Piceance Basin geometry, an average well will be Piceance Basin geometry, an average well will be connected to about 18 percent of the in-place reservoir volume in a 320-acre area. Connected reservoir volumes of greater than 70 percent in a 320-acre area are calculated for fractured wells with fracture wing dimensions of 2,000 ft, with actual values depending on fracture extent and pervasiveness. The percent of connected reservoir pervasiveness. The percent of connected reservoir volume decreases as well spacing increases.

Introduction

Selecting an appropriate model for reservoir simulation can be a difficult task, especially if a minimum amount of subsurface data are available. In many areas, the reservoir rock outcrops within a reasonable distance of the field. Thus, the meager subsurface data can be supplemented by outcrop studies. This paper briefly summarizes the results of outcrop studies on the Mesaverde and Fort Union formations in northwestern Colorado and presents an evaluation of some reservoir models generated by using the outcrop information.

This study was an attempt to use all available information sources to supplement the usual subsurface data. It is recommended that data from studies of rock types and geometries be used to help develop that best reservoir model possible; otherwise, important information is being ignored.

SUMMARY OF GEOLOGY

GEOLOGICAL SETTING

The Piceance Basin is located in northwestern Colorado. The basin is a large structural downwarp with a sedimentary sequence more than 20,000 ft thick. The principal gas-bearing units are a series of fluvial Tertiary and Cretaceous sandstone lenses in the Fort Union and Mesaverde formations. A schematic section of the horizons encountered in the Project Rio Blanco area is presented in Fig. 1. A more comprehensive geological description is presented in Appendix A. presented in Appendix A. SUMMARY OF OUTCROP STUDY

The Fort Union-Mesaverde sandstone bodies studied in outcrops along the periphery of the northern Piceance Basin appear to be predominately fluvial channel-fill deposits with some point-bar sandstones. The point-bar sandstones occur in the lower Fort Union section. The most commonly observed thickness was between 20 and 30 ft (manly channel-fill sandstones) and the log mean length/thickness ratio (L/h) was 35 for all of the observed sandstones lenses, with a minimum of about 19/1 and a maximum of about 140/1.

Length/width (L/W) ratios (shape of body in plan view) average 2-4/1 for the Fort Union point-bar sandstones) and greater than 10/1 for the Mesaverde channel-fill sandstones (Appendix B). Average length/width/thickness (L/W/h) ratios are the following.

Sandstone Formation Type L/W/h

Fort Union Point-bar 190/90/1(Typical Case) - (7,600/3,200/40 ft) Mesaverde Channel-fill 140/14/1(Typical Case) − (3,500/350/25 ft)

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