A steam-injection pilot in the Yates field is evaluating the acceleration of gravity drainage in the gas cap of the fractured San Andres reservoir. As steam is injected, downhole geophones monitor microseismic events caused by thermal expansion of the rock to estimate the area and vertical extent of the heated zone. Concurrently, time-lapse carbon/oxygen logs are used to monitor changes in oil saturation behind the thermal front. Data will be combined to estimate the volume of oil mobilized from the matrix to the fracture system as a result of steam injection.