Abstract: Described is a system for automatically recording information about the physical condition, content and location of earth formations penetrated during drilling of a well, the rate of drilling the well, the well depth, the condition and content of the drilling fluids, and procedures followed in the drilling operation. Immediate visual displays are provided which permit comparisons between graphically recorded input and output data representing, respectively, the condition and content of an identified sample of drilling fluid as it enters the formation from the drill bit and the same fluid sample after it has returned to the well surface. To this end, the volume, or "lag", of fluid required to pump a sample of fluid from the drill bit to the well surface is determined and means are provided to automatically determine the amount of fluid injected into the well between each incremental unit of advance of the drill bit.