Abstract

Accompanying the resource potential of gas shales is a new interest in understanding the physical and petrophysical properties of shales. Shale by geochemical or stratigraphic measures is arguably the most common lithology encountered in sedimentary basins. Despite this, shales remain little studied while engineers and explorationists focused on conventional reservoirs. Geophysicists did this knowing full well that often a reflection coefficient from a reservoir was controlled by the shale properties of the cap rock. We compensated for this ignorance by arguing that shales are deposited in deepwater environments in which lateral and vertical changes are slow and therefore inconsequential. We further compounded this ignorance by assuming that the shales were isotropic. An example of the consequences of this ignorance was clearly documented by Margesson and Sondergeld (1999). Engineers share culpability for this ignorance too, since most of the drilling problems occur in shales and most of the lithologies drilled through to reach the target reservoirs are shales; however, shales were rarely sampled unless a problem was encountered. Shales are now universally recognized as being anisotropic. Laboratory measurements are key to defining symmetries and magnitudes of anisotropy and indicate that “weak anisotropy” (defined here to be less than 10% in VP and VS) is the exception and not the rule.

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