Abstract: Potential Reservoir Identification in the Rio Blanco Formation (Argentina), Using New Techniques of Log Analysis

Mancilla, Oscar; Daniel Crivaro; Claudio Haring - YPF

Cuyana Basin, located in Mendoza Province, Argentina, has a rift origin related to a simple shear model filled by Triassic continental sediments (Fig.1). The Rio Blanco Fm. was analyzed in order to be able to identify and classify its reservoirs. The sequence stratigraphy analysis criteria was used to divide the column along with a complete set of well logs, cores and cuttings. As such, distribution and relationship of contemporaneous sedimentary bodies could be established.

A method of objectively determining the best levels of the reservoir was implemented using new well log analysis techniques that allowed to develop a predictive tool to identify potential reservoirs and their distribution, in order to search for subtle traps.The first step consisted in dividing the stratigraphic column according to the electric response, lithologic composition and vertical bed design. In addition to the conventional use of the cross-plots, the use of the Briggs Cube Technique allowed to match several physical parameters and see their variations, continuity or cyclicity in depth, in the analyzed interval, most of them in coincidence with the previously known.The Briggs Cube is the representation of three well log-related physical parameters, by mean of the color cube. Color variations with depth represent measured changes in rock composition, fluid type, petrophysical characteristics, etc. Adequate choice of measured properties results in a powerful tool for petroleum geology analysis.

The Rio Blanco Fm. is built up by 4 sequences, being the lower two fluviodeltaic and the upper two fluvial in origin. This array can be clearly recognized on the Briggs Cube and the Collin-Doveton log. It was determined to separate each unit with objective criteria and treat each interval individually, taking into account lithologic types and fluid salinity (Rw) to determine Vsh, Total and Effective Porosity and Sw and to analyze well logs from each sequence. Pikett's plots were used to determine Formation Factor and a.m. parameters. Therefore it was possible to compare Porosities, Vsh, etc. between different sequences.

The results were checked later with the outcome of the production tests. This procedure allowed the calibration of the results, the standardization and the definition of two important Critical Parameters in the reservoir's behavior: the Minimum Effective Porosity (MEP) and the Maximum Clay Volume (MCV). Based on cross-plots analysis and tests results Potential Reservoirs were defined as units with more than 9% of MEP and less than 40% of MCV (Fig. 2). After this classification, maps for the potential reservoirs from each sequence were drawn (Medium Effective Porosity, Isopach, Number of Beds, Sand-Clay Percentage) which allowed analysis of the evolution of different properties through time. The results obtained from this study can contribute to several analyses. For the reservoir characterization, sequence stratigraphic division was of fundamental importance. In addition, individual treatment of each sequence simplified the well logs analysis, as critical parameters do not show significative changes within them. The relationship between well and seismic information allows the extension of observations from the holes to undrilled areas.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil