Identification of Infill and Step-Out Drilling Potential Via Acoustic Inversion, 3D Seismic Attribute Analysis, and 3D Seismic Curvature Analysis: Neuquen and San Jorge Basins, Argentina
Authors: Haberman Scott , Aguiar Mariana , Cavero Jose Antonio , Sancho Costa Victor , Mendez Miguel , Di Benedetto Luciano , Goya Fermin , Verdur Horacio Antonio , Bellosi Eduardo , Crandall Robert 
Published: May 2009

Although the San Jorge and Neuquen Basins contain some of Argentina's oldest and most important oil and gas reservoirs, significant potential for infill and step-out drilling remains. The high degree of lateral and vertical heterogeneity within reservoir strata has traditionally made reservoir prediction difficult, while key structural features resulting from multiple phases of tectonic deformation are often subtle but complex, making detailed compartmentalization and fracture analyses uncertain. Nevertheless, a detailed geological characterization is required for the effective re-development of existing reservoirs, particularly when planning secondary recovery and infill drilling programs. Following the acquisition of large 3D seismic surveys in these basins, YPF sought to improve the predictability of reservoir quality and subtle structural elements through the use of seismic attributes.

In many cases, the reactivation of pre-existing faults causes areduction of offset and structural inversion, presenting a definition problem seismically, particularly in areas of high rock velocity and low seismic frequency. In order to accurately define subtle structural features, we have interpreted 3D seismic curvature volumes covering the El Guadal Norte and Bateria 2 Fields of the San Jorge Basin, and the Bajo Del Piche, and Senal Cerro Bayo Fields of the Neuquen Basin. The seismic processing technique employed in creating the volumes, calculates the maximum correlation between each point in the seismic cube and its neighbors. The dip and direction represented by the maxima are assumed to represent structural orientation - similar to a three-dimensional dipmeter - so the magnitude and style of structural folding can be determined. Positive curvature represents anticlinal folding, and negative curvature represents synclinal folding. Faults are located in zones where there are parallel trends of positive and negative bands of curvature. Volumes of most positive curvature and most negative curvature are used to help define structural axes and likely locations for natural fracture systems. Integrating these attributes with engineering data has greatly improved our understanding of reservoir behavior, helping to explain why specific areas exhibit high or low productivity, high water cut, and compartmentalization. This will allow for more intelligent planning of secondary recovery programs, and targeting of infill or step-out drilling programs.

To quantify the relationship between seismic response and reservoir quality, seismic modeling studies were undertaken in the El Guadal Norte and BaterÍa 2 fields of the San Jorge Basin. Wells containing sonic and density logs were used to create log interpolation seismic models, which were made as realistic as possible by rigorously tying them to actual 3D seismic data. This allowed for a better understanding of seismic amplitudes. More importantly, 3D acoustic inversion studies were performed to accurately define reservoir heterogeneity. The 3D impedance volume exhibits distinctly low acoustic impedance in areas known to contain volcanic tuffs which impede permeability. The impedance volume shows significant depositional detail, displaying fingering of the tuffs, and indicating areas where untapped reservoir sands reside. These undrilled sections are often in up-dip combination structural-stratigraphic trap configurations, opening new drilling opportunities.