Abstract: Effects of Weathering on the Reflectance of Vitrinite in the Mowry Shale, Steinaker Reservoir, Utah

Michael D. Lewan

Weathering of shales typically occurs in two styles referred to as face and pedogenic weathering. Face weathering propagates laterally into the exposed face of a shale outcrop in the form of fissility. This style of weathering is dominated by physical processes, and causes only minor chemical changes to the organic and mineral components of a shale. Pedogenic weathering propagates downward from a topographic surface to form a saprolite horizon of leached and discolored shale. This style of weathering is dominated by chemical processes associated with soil formation, and causes significant changes to some of the organic and mineral components of a shale. Development of these two weathering styles on one another in many outcrops makes it difficult to differentiate their individual effec s on the reflectance of vitrinite in shales and potential source rocks. However, a roadcut through the Mowry Shale (Lower Cretaceous) on State Highway 44 near Steinaker Reservoir, Utah, provides an excellent exposure of both weathering styles, which are not superimposed on one another. Face weathering is well developed in the outer 30 cm of a quartzose shale in the upper part of the exposed section. The only change in vitrinite reflectance occurs in the outermost 9 cm of the fissile zone, where the mean random reflectance of indigenous vitrinite is 0.49%R_o. The remaining 21 cm of fissile rock and 50 cm of deeper nonfissile rock from the same shale bed have a higher mean random reflectance of 0.54%R_o with no systematic change. Mean random reflectance of the exogenous (i.e., recycled) vitrinites through this face weathering profile remains constant at a value of 1.10%R_o. Pedogenic weathering is most pronounced in argillaceous shales in the lower part of the exposed section. Samples of the same bed were taken through a 12.7-m saprolite horizon into an unweathered portion of the outcrop. Mean random reflectance of the indigenous and exogenous vitrinites remained essentially the same (0.52 and 1.09%R_o, respectively) through this pedogenic weathering profile. However, the exogenous vitrinite becomes dominant in the saprolite horizon as a result of preferential leaching of the indigenous vitrinite. Halfway into the saprolite horizon of the same bed, more than 50% of the original indigenous vitrinite is lost. This preferential loss is ikely to continue to the top of the saprolite horizon, where the higher reflectance of exogenous vitrinite could be mistakenly measured as that of the indigenous vitrinite.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994