A Statistical Approach to Pay Identification in the Tight, Fractured, Heterogeneous Reservoirs of the Piceance Basin
Authors: Esphahanian C. , Storhaug D.G. 
Published: December 2007

Abstract
In the Mamm Creek Field, post stimulation production is highly unpredictable. Reservoirs with similar open hole log or core properties produce very differently. A statistical analysis of 151 individual reservoir tests showed a good correlation to stratigraphic interval and associated reservoir area. Within these intervals performance was more predictable, particularly in thicker reservoirs.

Introduction
The main producing reservoirs in the SE Piceance Basin are sandstone reservoirs within the 3,500-ft thick Williams Fork Formation of the Mesaverde Group. Commercial production requires large hydraulic fracture treatments of multiple reservoirs. Production from each reservoir is highly erratic. Ultimate reserves average more than 1 BCF (1E+09 scf) per well.
Studies of these sandstone reservoirs describe them as heterogeneous and discontinuous with an over print of natural fractures. Neither conventional log analysis or core data predict reservoir production rates due to rapid changes in reservoir geometry, reservoir quality, and natural fracture intensity.
A statistical analysis compared production rates from 151 completed reservoirs in 13 scattered wells to various open hole log, drilling, and stratigraphic data. The analysis showed that reservoirs with similar log properties from different stratigraphic intervals perform very differently and that thickness, resistivity, porosity and sub-sea depth can improve the reservoir performance prediction within each stratigraphic interval. The analysis revealed that the shallowest interval with the highest permeability and most fractures was often marginally productive. Improved predictability provides a model to optimize completion and stimulation designs for new wells. The techniques used here can be applied to interpret other highly erratic data sets.

Regional Setting
Mamm Creek Field lies in the east-central Piceance basin of northwest Colorado and represents the eastern, updip extension of the greater Rulison-Parachute field as shown in Figure 1. Production from the greater field in mid-1996 totaled 70,000 Mscf/d from 350 wells and, with several rigs presently drilling, both rate and well count are increasing. The area produces from the shallow Wasatch (Paleocene), the Williams Fork, and the Corcoran (both Upper Cretaceous Mesaverde). Most of the production (55,000 Mscf/d from 250 wells) comes from the Williams Fork. Snyder Oil Corp. is the largest operator in Mamm Creek Field with 50 wells.

Geologic Setting
The 3,500-ft thick Williams Fork Formation, described by Lorenz at the Multiwell Experiment Site (MWX), consists of a progradational sequence from lower delta plain (paludal), to upper delta plain (coastal), to fluvial point bars and meander belts, capped with a marine transgression (paralic). The sandstone reservoirs within these facies have variable and typically small areal extent. Reservoir widths within the paludal section approach 500-ft, the coastal widths are less than 200-ft, and fluvial widths can exceed 1,000-ft. Figure 2 shows the KRK 7-15 log from Mamm Creek field, the MWX-2 log, and Lorenz's schematic profile of sandstones at MWX. A 300-ft shaley interval in the center of the fluvial section, locally called the J1 Shale, is an apparent vertical seal to the gas column possibly eliminating vertical fracture communication. The 2,000-ft Williams Fork section below the J1 Shale is the principle producing target at Mamm Creek. The gross thickness of this interval is fairly constant in the area. The section below the J1 Shale contains many sandstone reservoirs, totaling about 15% of the section, surrounded by mudstones siltstones, and coals. A second shaley interval, the 450-ft coastal sub-zone (upper delta plain) can be correlated across the field.

Completion Practices
Completion designs vary, but typically the Williams Fork reservoirs below the J1 Shale are fracture stimulated in one to three stages.
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