This addendum is in response to the U.S.G.S. report on the Corcoran and Cozzette Intervals, MWX-l and 2, dated January, 1983. The report implied that the clay mineral which was identified by the Bendix Field Engineering Corporation Petrology Laboratory as kaolinite may be a special variety of high iron chlorite. This variety of chlorite is not well documented in published literature and is almost always identified as kaolinite by those unfamiliar with its occurrence. It is essential to differentiate between these two minerals because of their difference in reaction during well stimulation. Kaolinite is relatively nonreactive with acids while a chlorite-acid reaction is potentially damaging to the reservoir. After much discussion with C.W. Spencer, J.K. Pittman, and R.M. Pollastro of the U.S.G.S. it was concurred that it is unlikely that kaolinite occurs in the abundance indicated by X-ray diffraction analysis. Reasons for this include: kaolinite does not occur in sufficient quantity in thin section to account for a greater abundance than chlorite; and the paleo-temperatures in the Cozzette Formation were probably too high for kaolinite to remain stable. Therefore, after discussion with Sandia personnel it was decided that further investigation of these clay minerals was necessary. The simplest method of determining whether kaolinite or chlorite is present is to digest clay-size separates in acid. Chlorite is soluble in acids such as nitric and hydrochloric while kaolinite is relatively insoluble in these acids. X-ray diffraction (XRD) analyses can be performed on clay separates before and after acid treatment. Characteristic XRD patterns for kaolinite have 7 A [001] and 3.55 A o 0 [002] reflections, whereas chlorite XRD patterns have 14 A [001], 7 A [002], 4.7 A [003] and 3.55 A [004] reflections. In high iron chlorite minerals the [001] and [003] XRD reflections are depressed and may be nonexistent. The [002] and [004] XRD reflections in these chlorites is enhanced giving the appearance of a kaolinite XRD pattern. If chlorite is the principal clay mineral it would dissolve in acid. As a consequence its corresponding a 0 XRD reflections would not be present and any residual 7 A and 3.55 A reflections would be due to kaolinite.