This is the first of two papers illustrating the design and results of a steam drive pilot in the South Belridge Diatomite, Kern County, California. Steam drive on 5/8 acre spacing appears to be an economically viable alternative to waterflooding in the Diatomite; hence, it is being explored as a secondary recovery process. To interpret steam drive results quantitatively, we propose a computationally simple, high-resolution model that captures formation heating due to both steam/hot condensate convection and heat conduction, evolution of formation permeability, and changes in the size and shape of injection hydrofractures. From this model, we obtain formation pressure, temperature, the cumulative steam injection, the dynamics of hydrofractures undergoing steam injection, and, thus, a history match for the pilot. Model results can then be used to generate input data for a fully compositional three-dimensional simulator and even more detailed study. The two, separate, noncommunicating hydrofractures used for steam injection collectively span the entire diatomite reservoir column. Concentrating first on steam injection through the lower hydrofracture, we interpret results obtained between October 1989 and January 1994. It is discovered that steam did not flow into the outer portions of the hydrofracture wings for roughly 225 days of injection and that formation heating was asymmetrical. Over time, the hydrofracture grew, thereby improving volumetric heating. The most dramatic evolution of hydrofracture shape occurred across the bottom 150 ft of the lower fractured interval. The hydrofracture wings increased their length many fold in response to steaming. Further, the most permeable layers of diatomite demonstrated the greatest heating because steam preferentially entered and convected within these layers.