Geologic constraints

It is not possible in general to constrain rock property distributions to honor the physical constraints that govern the erosion, transport, deposition, sorting, and diagenesis of sedimentary materials and the observations of properties at well locations. In many cases, however, the spatial covariance can be used to quantify the geological plausibility of a distribution of rock properties. Although there are many cases where this is not a good measure, it is important to understand what the covariance means in terms of the spatial relationship of the properties. We can begin by examining well-log traces from a well in Alaska.

Figure 5.1 shows two scaled well-logs from the same interval in the N. Kapilik 1 well (NK-1). One log is a spontaneous potential or SP log. The potentials are primarily created by diffusion of ions between the formation and the wellbore caused by differences in salinity in the fluids. Variation in the potential along the well path is due to variations in permeability as well as variations in salinity. Because clay particles have such a large effect on permeability, the SP log is often used as a shale indicator. On the other hand, the SP has relatively poor vertical resolution, and can not resolve small shales.