Mines and Energy Resources South Australia (MESA) submitted four thin sections to PGPC for detailed petrological description. Samples were taken from the ?basement and overlying alteration zones in the wells Narcoonowie-1 and Putamurdie-1 which were drilled in the Warburton Basin. Basement is comprised of sediments of Cambrian to Ordovician age and at Narcoonowie-1 has been described by Gatehouse (1986) as sandstone from the Dullingari Group. Recent Drilling in the Cooper Basin has been hampered by difficulties identifying basement in the underlying Warburton Basin. MESA recognsed an extensive zone of alteration above basement which has probably contributed to this drilling difficulty. The purpose of this petrology study was to document the nature of weathering/ diagenetic alteration of basement and suggest possible mechanism causing the alteration.