Relationship of P-wave seismic attributes, azimuthal anisotropy, and commercial gas pay in 3-D P-wave multiazimuth data, Rulison Field, Piceance Basin, Colorado

This case history is one of three field projects funded by the US Departmen t of Energy as part of its ongoing research effort aimed to expand current levels of drilling and production efficiency in naturally-fractured tight-g as reservoirs. The original stated goal for the 3-D P-wave seismic survey w as to evaluate and map fracture azimuth and relative fracture density throu ghout a naturally-fractured gas reservoir interval. At Rulison field, this interval is the Cretaceous Mesaverde, approximately 2500 ft (760 m) of lent icular sands, silts, and shales. Three-dimensional full-azimuth P-wave data were acquired for the evaluation of azimuthal anisotropy and the relations hip of the anisotropy to commercial pay in the target interval. The methodo logy is based on the evaluation of two restricted-azimuth orthogonal (sourc e-receiver azimuth) 3-D P-wave volumes aligned with the natural principal a xes of the azimuthal anisotropy, as estimated from velocity analysis of mul tiazimuth prestack gathers. The Dix interval velocity, as well as the inter val amplitude variation with offset (AVO) gradient, was calculated for both azimuths for the gas-saturated Mesaverde interval. The two seismic attribu tes best correlated with commercial gas pay (at a 21-well control set) were (1) values greater than 4% azimuthal variation in the interval velocity ra tio (source-receiver azimuth N60E/N30W) of the target interval (the gas-sat urated Mesaverde), and (2) the sum of the interval AVO gradients (N60E + N3 0W). The sum of the interval AVO gradients is an attribute sensitive to the presence of gas, but not diagnostic of an azimuthal variation in the ampli tude. The two-azimuth interval velocity anisotropy mapped over the survey a rea suggests spatial variations in the orientation of the maximum horizonta l stress field and the open (to flow) fracture system.