Abstract

A cooperative study of the Grayburg/San Andres reservoir is being conducted in response to the United States Department of Energy’s Class II Oil Program. The purpose of the study is to preserve access to existing well bores by identifying additional reserves. Production problems associated with a shallow-shelf carbonate reservoir have been evaluated by a technical team integrating subsurface geological and engineering data with 3D seismic data. The 3.5 square mile 3D seismic survey and study cover five leases in a 1.5 square-mile area of the Foster and South Cowden Fields. Results of the study are being used to design a 3D reservoir simulation model and subsequent waterflood.

Several 3D seismic surveys have been acquired over Grayburg/San Andres fields by major oil companies (Chevron - McElroy Field, Amoco - North Cowden, Phillips - South Cowden Field, and Shell - Wasson Field). However, this type of survey has not been undertaken by independent oil operators due to the perceived high cost of imaging shallow reservoirs. This project specifically addresses the application of 3D seismic technology by independents who operate smaller leaseholds of Grayburg/San Andres productions on the Central Basin Platform.

The 3.5 square mile 3D seismic survey was designed to resolve changes in the subsurface on the megascopic scale, allowing the geologist to better correlate stratigraphic information and potential flow units between boreholes thereby resulting in the project reservoir engineer better modeling the flow units. Pre-planning for the survey also included: seismic modeling, use of aerial photos to optimize fold in an area with cultural obstructions and acquisition design for bin fractionization of the data.

While most of the large 3D surveys cover those portions of the San Andres/Grayburg shelf which are relatively uncomplicated by tectonism, this study covers a portion of the shelf which overlies an Ellenburger structure. Structure on the Grayburg and thinning relationships in the post-Grayburg interval indicates the possibility of periodic rejuvenation of deep seated structures.

The spatial distribution of flow parameters important in reservoir characterization are controlled by the sequences and parasequences forming the reservoir. Within the project area the Grayburg can be divided into at least two and possibly as many as four shallowing upward sequences (each 100-150 feet thick). Each sequence consists of a series of stacked, shallowing upward parasequences (5-20 feet thick) that define the flow units. Analysis of the one available core in the study area indicates that the upper Grayburg is composed of an overall shallow shelf shallowing sequence punctuated by sea level drops with minor sand deposition. The main reservoir facies are pelloid oncoid grainstones to packstones with intergranular porosity, partially occluded by late anhydrite. A lagoonal mudstone with large primary anhydrite “snowflakes” forms both the vertical and updip seal. The San Andres is composed of pelloidal fossil ooid grainstones to packstones with filled fenestral intervals. The porosity in the San Andres is also intergranular.

Seismic indicates that the location and extent of facies belts in the Grayburg is determined by subtle variations in pre-existing structure, as grainstone shoals appear to wrap around the windward noses of highs. The San Andres production, immediately below the San Andres/Grayburg boundary, is updip of a slightly older (Judkins) shelf break. Most of the upper San Andres clinoforms develop basinward of this shelf break. Historically, correlations in the San Andres have been assumed to be planar, however, the productive interval in the San Andres appears on the seismic to be composed of a series of small scale clinoforms within a shelf sequence. This has influenced the correlation of the flow units

This study has demonstrated a methodology for reservoir characterization and subsequent development of the Grayburg/San Andres reservoir that is feasible for independent operators. Foremost, it stressed the importance of utilizing an integrated multi-disciplinary approach for reservoir development. This method of reservoir evaluation should be relevant to shallow-shelf carbonate reservoirs throughout the United States.