Prediction and Exploitation of Basement-Controlled Production Trends in Piceance Basin Fractured Tight Gas Reservoirs: Results of an Integrated Analysis
Authors: Hoak T. E. , Klawitter A. L. , Squires S. G. , Decker A. D. 
Published: December 2007

The ability to delineate and accurately predict fractured reservoir conditions represents critical information necessary for field development strategies, and development of play concepts in less-developed areas. To demonstrate relationships between fracture-controlled production, stratigraphy and structural geology, the Piceance Basin is being used as the site for an integrated fracture detection and reservoir characterization program utilizing high-resolution aeromagnetics, seismic, and conventional subsurface structural and stratigraphie mapping (see Fig. 1 for project overview). Many of the tools that have been used in this analysis are readily applicable to other basins dominated by tight gas sands, complex tectonics, and basement-involved deformation.

In the Piceance Basin, there are two primary controls on well performance. The first is reservoir thickness and the second is deliverability, a function of fracture permeability. Reservoir thickness is controlled by depositional systems whereas fracture permeability is controlled by tectonic deformation. In Rulison Field, a sidetrack well with a 142 foot difference in bottomhole location shows a SO % difference in net sandstone pay between the two wellbores. This intense variability underscores the difficulty of predicting sand geometries in the basin. In contrast, through the use of integrated analysis, we can significantly decrease drilling risk by predicting zones of enhanced fracture permeability before drilling.

Depositional systems analysis is important as a means of predicting reservoir quality and reservoir thickness, however, in the Piceance Basin, reservoir thickness and quality cannot be accurately predicted because of complex fluvial and paludal stratigraphy, In addition, stratigraphy does not exert the greatest control on production economics. Instead, fracture permeability is the predictable and most important variable for successful development programs. In support of this, the orientation of fracture-controlled production trends lie either orthogonal or oblique to depositional trends in White River Dome, Divide Creek, Shire Gulch, Plateau, Grand Valley, Parachute and Rulison fields.

Production trend analysis of these fields confirms that fracture-controlled production is tied to local and regional structures identified through detailed subsurface mapping. These structures are, in turn, controlled by basement structures recognized on newly-acquired high-resolution aeromagnetic calibrated with 2D seismic data. In most of these fields, especially anticlinal structures such as White River Dome, Divide and Woif Creek Anticlines and Rulison fields, the dominance of fracture systems and related fracture-controlled production parallels the fold axes where axis-parallel fracturing is best-developed. In other fields, the fracture-controlled production trends parallel the zones of maximum local flexure corresponding to structural dip changes. In most of these fields, the fracture and production trends are oriented NW/SE and have aligned themselves along the dominant NW trending regional structural anisotropy.

Remote sensing LANDSAT (TM) imagery analysis has correlated some surficial features with deep regional structures interpreted in both aeromagnetic and seismic data. In other areas, integrated analysis has recognized shallower structures. For example, in the NW-trending anticlinal Divide Creek Field, imagery analysis has documented the presence of NE-trending normal faults that segment the anticline along its structural axis. This interpretation indicates that normal faults subdivide the structure into isolated reservoir compartments that each must be tapped for maximum field potential. In other areas of the basin, especially where alluvial cover is thick, remote sensing has difficulty delineating basement features that control production trends.

An integrated reservoir characterization program represents a powerful tool to recognize and predict fractured production trends. By application of various individual fracture against independent data sources, additional validity is given to the interpretation. Throughout the globe, the enormous challenge and rewards of successful tight gas production underscores the importance of developing successful integrated exploration methodologies.