Seismic Guided Mapping of Effective Porosity - A Case History
Authors: Reeves James J. , Smith W. Hoxie 
Published: October 2008

Abstract
A reservoir characterization study of the lower Grayburg and upper San Andres reservoir is provided as a case history that integrates petrophysical and seismic attribute analyses for seismic guided mapping of effective porosity. The study involves a petrophysical evaluation of the reservoir at approximately thirty well locations and the interpretation of a nine square mile area of 3-D seismic data. The project is conducted using a network of state-of-the-art geological, geophysical, and engineering analysis software running on high performance UNIX computer workstations. Specific pore types are defined within the reservoir interval based on pore connectivity and lithology. The petrophysical results are loaded into reservoir modeling software for integration of results with seismic attributes to produce seismic guided reservoir maps. Later these maps are available to export reservoir properties to populate a finite difference reservoir simulation grid The main objective of this study is to determine new drill locations corresponding to favorable net effective porosity and to develop a completion methodology to delineate productive zones. It is equally important to identify water producing zones so as to avoid those intervals. The project is organized into the following subtasks to evaluate the reservoir architecture of the Grayburg and San Andres reservoir:Development of computer algorithms to classify pore types from modem wireline log suites.Map distribution of porosity types and hydrocarbon saturations using well control horizontally and vertically through the reservoir.Determine 3-D seismic attributes that can be correlated with the mapped reservoir properties.Seismic guided mapping of average effective porosity and other reservoir properties in the interwell region.Evaluate current completion techniques and identify workover potential.Recommend new drill locations and additional oil productive zones in the reservoir.

Introduction
A comprehensive reservoir development study is out-sourced by a major oil company to meet critical time-lines in the development of a Grayburg and San Andres new field discovery. A seven member integrated team (consisting of one geologist, one petrophysicist, two geophysicists, two reservoir engineers, and one systems analyst/technician) is organized to perform the project. A dedicated team approach devised by Smith and Reeves in a U. S. Department of Energy study (Reeves et al., 1996) is a strong factor in completion of this project on schedule (within 80 days). These studies show that out-sourcing of labor intensive reservoir development projects to a focused technical team is a cost effective alternative for management to obtain timely results.

Geological Setting
The study area is on a structural closure, located behind a carbonate shelf-edge platform and extending eastward into a carbonate lagoonal platform setting The upper San Andres reservoir is characterized by a series of upward shallowing high frequency cycles, each being followed by a maximum flooding surf". The Grayburg was deposited in a similar paleo-environment. Reduction of porosity from anhydritic cement in the upper Grayburg forms the reservoir top seal. Maximum flooding surfaces are typically indicated by carbonaceous siltstones, and exhibit a hot gamma-ray log signature. In many cases, the siltstones seem to be of simcant thickness such that they form barriers to vertical flow, and seem to inhibit frac treatment energy.