Frontiers + Innovation – 2009 CSPG CSEG CWLS Convention 501 Geological Microseismic Fracture Mapping – Methodologies for Improved Interpretations Based on Seismology and Geologic Context S. C. Williams-Stroud* MicroSeismic, Inc, Houston, TX USA swmsstroud@microseismic.com and Leo Eisner MicroSeismic, Inc., Houston, TX USA Summary The results of microseismic monitoring of hydraulic fracture treatments are significantly more valuable when a high-confidence interpretation of results can be done. The most common approach is to look for trends, or directions, along which microseismic events line up that would indicate the direction of induced fractures. This type of interpretation is more difficult when the signal is low or when the events appear to define multiple directions. Analysis of geologic structures in the area of the reservoir can provide useful information to support or eliminate interpretations of apparent trends as stimulated fractures, as well as provide an explanation for fracture trends that deviate from the expected orientation from the in-situ stress directions. In some cases, structural geology observed at the surface can be extrapolated to the subsurface to aid the interpretation. When seismic reflection scale horizon and fault interpretations are available, they can be used to infer that smaller scale faults and fractures of similar orientation may have been activated at the wellbore. Inversion of seismic source mechanisms for microseismic events provides the most well- constrained parameters to describe the failure mechanisms of the hydraulic fracture treatment, and by combining this information with the geologic context a better understanding of the reservoir rock behavior during the stimulation treatment can be developed. In this paper, examples will be presented that describe interpretations of microseismic monitoring results based on geologic context and confirmed by source mechanism inversions. Introduction When microseismicity indicates a hydraulic fracturing result that deviates from the expected tensile fracture orientation that would form parallel to the maximum compressive stress directions, pre-existing fractures and faults could be responsible. Existing fracture planes favorably oriented for shear will fail at lower stresses than are required to create new fractures. Geologic mapping and regional to local in-situ stress information will allow informed interpretation of the resulting microseismicity patterns as well as providing predictive capability for fracturing patterns of treatments in subsequent area wells and production planning. The impact of the in-situ stress field on a natural fracture or a fault will be controlled by its orientation relative to that stress field. Examining the relationship of available stress data to existing regional structures can provide clues to explain whether or not natural fractures were activated as opposed to new fractures