Organic geochemistry of shales—I Distribution of organic matter in the siliceous Mowry shale of Wyoming

Abstract

Variations in the organic carbon content of the Lower Cretaceous, siliceous Mowry shale of Wyoming were surveyed. Detailed sets of samples from twenty-four outcrop sections were analysed. For each section, the average organic carbon content was calculated and the results wore plotted on a map and contoured. In general, the average increases in a southeasterly direction, which is the seaward direction. The average organic carbon concentration is about one per cent at the northwest limit of the region studied, increasing to almost three per cent at the southeast limit. The contours show that the organic carbon content reaches a maximum (2.5 to 3 per cent) in an area which overlies the present-day Casper Arch, probably extending eastward beyond the control area. The contours also suggest the existence of either a broad minimum or an area of essentially constant organic carbon content to the southwest. The vertical distribution in the Mowry shale in many places exhibits a maximum in the middle of the rock unit; this pattern is evident over most of the region studied. The gross pattern of vertical variability can be correlated with the electrical resistivity of the Mowry shales, high resistivity being associated with high organic carbon content. This association may result from the replacement of interstitial water and exchangeable clay cations by organic matter.