Authigenic illites have been found in pores of many sandstones which are known to be hydrocarbon reservoirs. Typical samples are from the Norphlet Formation in southern Mississippi, the Wilcox Formation in south Texas, and the Lance Formation in Wyoming. Illites in these sandstones are mainly in the form of laths with perfectly developed morphologies. Laths have widths of 0.1 to 0.3µ and lengths ranging up to 30µ. Scanning electron (SEM) images show that these "hair like" illites fill many of the pores of these sandstones, and cause a serious reduction in permeability. Elemental analysis of the laths with EDAX energy dispersive X-ray analyzer shows that Si, Al, and K are the major constituents and Mg is the minor constituent in the chemical composition of the la hs. Transmission electron images show that illite laths have grown from an amorphous core which is rather similar to an irregular smectite aggregate. X-ray diffraction patterns display the coexistence of discrete illite (9.9A) with an illite/smectite mixed layer with a distinct reflection varying from 10.5 to 11.0A. In the sample from the Wilcox Formation the laths form regular arrangements which lead to the development of platelets.

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