Although steam injection is widely used in hydrocarbon extraction, little is known of the effects of steam on reservoir properties. This study documents mineralogic changes resulting from cyclic steam injection into the Potter sand (upper Miocene) at Midway-Sunset field. Samples were obtained from two cores recovered from wells located 200 m (650 ft) apart on the 120-acre ARCO Anderson-Goodwin lease, Section 21, T31S/R22E (Fig. 1). This lease is located on the northwest edge of Midway-Sunset field (the reader is referred to the Introduction in this publication and R. L. Gardiner’s article on the Potter for general location maps for this lease). The first well was drilled in 1984 prior to initiation of cyclic steam injection; the second well was drilled in 1991 following cumulative injection of 1,000,000 bbls of steam.