Water-based drilling fluids comprising different clays (bentonite, saponite, and sepiolite) have been autoclaved in the temperature range of 300–600°F (149–316°C) under appropriate confining pressures in order to simulate the conditions encountered while drilling deep oil and gas wells. The clay mineral saponite represents the magnesium analog of montmorillonite which is the aluminous clay mineral in bentonite. Both montmorillonite and saponite are characterized by a swelling silicate lattice and an excess layer charge. Morphologically, saponite and montmorillonite are similar and they consist of thin platelets and films mostly in the micron-size range. Sepiolites are similar in chemistry to saponites but they possess a fibrous morphology and a neutral lattice. These basic characteristics of the clays are found to lend different rheological and wall-building properties to the drilling fluids. Saponite fluids display much higher viscosities and significantly lower fluid losses at high temperatures. Saponite fluid was also more stable than bentonite fluid at high temperatures and in the presence of common Na, K, and Ca salts and hydroxides due to the nature of reactions between these compounds and bentonite. Mixing fibrous sepiolite with saponite enhances the viscosity of these fluids in the presence of common salts. At high temperatures sepiolite readily converts to a smectite and improves the fluid rheology and fluid losses.