BOREHOLE BREAKOUT AND HORIZONTAL STRESS FIELD ANALYSIS IN THE NEUQUEN AND MENDOZA RETROARC

The main objective of this research is to analyze the horizontal stress field acting in the retroarc of Mendoza and Neuquen provinces using borehole breakouts data.

The horizontal stress field was deduced from borehole breakouts analysis of 120 wells distributed along the Neuquen and Mendoza retroarc. The studied area is located in the central Andes, among the 34º S and 39º S. At these latitudes the subduction of Nazca oceanic plate below the South American plate is oblique to the continental margin with a convergence angle of 75º. If the maximum horizontal stress direction for this sector of the Andean retroarc is controlled by the convergence vector between the Nazca and South American plates the expected breakouts directions should be perpendicular to the convergence vector near azimuth 165º. The calculated breakouts are generally inside the prospective values. However to the west of the study area there are some breakouts which are not consistent with the regional tendency, it has been interpreted as response to local stress fields controlled by particular structures.

We compared the maximum horizontal stress directions obtained from breakouts data with the expected maximum horizontal stress orientation and obtained a map of the differences between the expected and the calculated values. From this map we could determine three principal anomalies; one of these is located to the east which was interpreted as a response to basement structures control. To the west there are two anomalies, one to the south of the Colorado River and the other to the north. The south anomaly can be explained in terms of stress partitioning, it represents a relative high stress partitioning zone. The north one is not very clear because there are not enough data to support it.

From the breakouts analysis we concluded that the horizontal stress field along the Neuquen and Mendoza retroarc is not uniform; to the east the maximum horizontal stress orientation presents an important rotation interpreted as a response of a basement structures control. There is a difference in the maximum horizontal stress direction to the north and to the south of the Colorado River. To the north the horizontal stress presents an azimuth sub-parallel to the convergence vector and to the south if we discharge the eastern values controlled by basement structures has an azimuth of 90º.