The relations between porosity, preferred clay mineral orientation, and mineralogy with respect to present burial depths up to about 1,500 feet were investigated for samples of shales of Pennsylvanian age in western Kentucky. Statistical tests showed that porosity decreases slightly with depth but is strongly influenced by increases in illite content and preferred orientation. Preferred orientation is independent of depth but increases wiith increasing clay content. Illite and chlorite are inversely correlated and illite increases with depth. This suggests the conversion of chlorite to illite.