Organic geochemistry of shales - II. Distribution of extractable organic matter in the siliceous Mowry Shale of Wyoming
The average content of extractable organic matter in the Lower Cretaceous Mowry Shale is about 400 ppm in the northwestern part of Wyoming and it increases southeastwardly, reaching a value of 2100 ppm in the Douglas area. Thus, the regional variations parallel those of the organic carbon content reported previously. Vertical variations at a given locality likewise parallel variations in the carbon content. The results suggest that the extractable matter is indigenous. Near Rawlins, however, the extractable content correlates poorly with the carbon content, suggesting that the extractable matter is not indigenous. The exceptional character of the extractable matter from the Rawlins area is also indicated by its relatively low content of hexane-insoluble matter. The ratio of the extractable content to the organic carbon content is higher, the higher the carbon content. Possibly the conditions favorable to the preservation of greater amounts of organic matter also favored the formation of greater proportions of bitumen, or organic source material which produced greater proportions of bitumen was deposited in areas in which greater amounts of organic matter were preserved.