Bayo Field, located in the San Jorge Basin of southern Argentina, produces oil from sandstone reservoirs in the Cretaceous Comodoro Rivadavia Formation. Petrophysical evaluation of these reservoirs is complicated by the presence of heterogeneous volcaniclastic facies, variable diagenetic mineralogies, highly variable pore system geometries, low-salinity formation water, variable, but generally high Rw, and severely limited wireline logging tools. As a result, existing evaluation procedures for selecting individual sandstone intervals to swab test are based on a combination of the following observations: (1) the presence or absence of microlog separation and mudcake (a permeability indicator), (2) correlation with the type of fluids tested from individual zones in nearby wells, and (3) visual estimates of the amount of porosity and the degree of oil staining observed in recovered percussion sidewall cores. This procedure is only partially effective at predicting reservoir quality and pay. A potentially more effective method of formation evaluation would involve the use of a more complete logging suite designed to evaluate heterogeneous, low-salinity reservoirs (e.g., downhole NMR). Until recently, however, the addition of new logging tools has not been a viable option because of time and cost constraints.

As an alternative, this study describes a proposed rock-based method for enhancing the ability to predict reservoir zones for swab testing and fracture stimulation in Bayo Field (and in other fields in the Cerro Dragon contract area) without adding additional wireline logging tools or substantially increasing evaluation cycle time. The results of this study may be used to enhance reservoir management in two ways: (1) High-quality reservoir zones that are suitable for swab testing and/or fracture stimulation may be more accurately identified, creating new opportunities for added production in zones that were previously either not tested at all, or had experienced poor test results and were not fracture stimulated. (2) Poor quality reservoir zones may be more accurately identified, thereby minimizing the time and money spent testing and/or stimulating non-productive or marginally-productive sands.

Over 400 sidewall cores from 56 wells in Bayo Field were studied petrographically. Eight basic petrophysically significant rock types were observed from thin-section analysis. Each of these were compared with existing production data. This comparison reveals that some apparent poor-quality (untested, dry or low swab rate) sands can actually be porous, high-quality reservoirs. The distinction between high-quality sands and truly non-prospective sands cannot be made reliably with the currently-used evaluation procedures. However, high-quality sands can be identified, and their probable swab rates and response to fracture stimulation can be predicted from petrographic analysis prior to well testing. By including inexpensive petrographic observations to the current evaluation system, production can be added from zones that were previously considered to be either unsuitable for testing altogether, or were of such poor quality that fracture stimulation was not attempted (especially “dry” zones). In addition, truly non-prospective, poor-quality sands can also be identified and eliminated from further testing, thereby saving money and rig time spent previously on testing and/or stimulating non-productive intervals.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah