ABSTRACT: Geologic Model of San Andres Reservoir, Roberts Unit CO2 Phase III Area, Wasson Field, Yoakum County, Texas

BENT, JAMES V., JR., Texaco Inc., Midland, TX

Roberts unit is a mature San Andres waterflood project located in Wasson field, Yoakum County, Texas. Texaco, as operator, has evaluated the reservoir for CO2 flooding, and a four-phased CO2 project has been designed for the unit. A critical aspect of CO2 flood design is the development of geologic reservoir management, such as flood monitoring and evaluation of infill drilling. The geologic reservoir model established for the southeastern part of the unit (the CO2 Phase III area) is an example of this design.

The reservoir consists of stacked carbonate depositional sequences. The cyclic nature of these depositional sequences is reflected in both core-defined lithofacies and porosity log character. Sequences consist of basal mudstones, restricted-shelf skeletal wackestones, open-shelf skeletal wackestones and packstones, solution and brecciated zones, and peloidal packstone caps. Intertidal mudstones and wackestones occur at the top of the reservoir and in the overlying reservoir seal. Porosity distribution is controlled by diagenetic events, but these events are closely related to depositional facies.

Reservoir geometry and reservoir quality are interpreted from study of carbonate lithofacies, porosity and permeability relationships, and injection characteristics. Depositional sequences are subdivided into layers (flow units) for use in reservoir simulation. Log normalization, core description, porosity interpretation, reservoir mapping, three-dimensional modeling, and joint effort between project geologists and engineers contributed to development of the reservoir model.

AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)