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Abstract: Spatial Distribution of Acoustic Impedance in a Permian Shelf-Margin: Upper San Andres Formation, Last Chance Canyon, New Mexico

Jeroen A. M. Kenter, Jan Stafleu, Mark D. Sonnenfeld, Wolfgang Schlager

Seismic reflection profiling is an important geophysical method for investigating the presence and architecture of hydrocarbon reservoirs in the subsurface. However, the seismic Section is not the exact equivalent of a geological cross Section. Understanding seismic reflections and the complex angular relationships often visible in seismic sections, e.g. reflection terminations, requires simulation of two-dimensional cross sections.

We have measured P-wave velocity, porosity, density, and insoluble residue of 48 rock samples from outcrops of the Permian upper San Andres Formation, Last Chance Canyon, New Mexico in an effort to construct a realistic model of the spatial distribution of acoustic impedance. P-wave velocity, saturated, ranges from 3.4 to 6.6 km/s; bulk density ranges from 2.4 to 2.8 g/cc; porosity ranges from 1 to 23%; and insoluble residue (non-carbonate fraction, most of which is clay minerals) varies between 0 and 93%. Cross plots show that most of the variation in compressional wave velocity is explained by changes in porosity and insoluble residue.

Sample subsets span the spatial, downdip and vertical, distribution of several individual hemicycles and show gradients of impedance, porosity and insoluble residue. Within these carbonate hemicycles, acoustic impedance gradually decreases downdip as a function of increasing porosity and insoluble residue. As a consequence, differences in impedance between hemicycles are not constant and change laterally along their contact.

These observations suggest a significantly more complex spatial impedance distribution in mixed carbonate - siliciclastic shelf-margins than traditional models where genetically important stratal boundaries are assigned constant impedance contrasts and diachronous facies boundaries are represented by gradual changes in impedance.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France