Abstract: An improved water base drilling mud composition comprises a placid shale clay from strata of the Pennsylvanian system of the Paleozoic Era which consists essentially of chlorite, illite and kaolinite mineral components, including at least trace amounts of .alpha.-quartz, calcium carbonate and gypsum. The placid shale clay is characterized by a relatively low volatile matter content, typically six percent by weight. Additionally, it is characterized by a relatively high percentage by weight of high density compounds such as aluminum oxide (Al.sub.2 O.sub.3) and a relatively small amount of low density compounds such as magnesium oxide (MgO) and sulfur trioxide (SO.sub.3). The placid shale clay is preferably used in combination with a gel viscosifier such as bentonite. In addition to serving as a weighting agent for increasing the density of the mud, it has been discovered that the placid shale clay flocculates the bentonite which produces an increase in the viscosity of the mud.