Incipient metamorphism and the organic geochemistry of the Mancos Shale near Crested Butte, Colorado

Abstract

The purpose of this study is to examine the changes in the organic matter of a typical organic shale caused by extremely low grade, or incipient, metamorphism and to interpret the geothermal history of the study area. The samples are from the Mancos Shale, taken from an area near Crested Butte, Colorado, where the sediments have been intruded by igneous rocks of Tertiary age and subjected to contact metamorphism. Samples were taken from near the main intrusive trend outward, along the East River and the Slate River, and from near Glenwood Springs, about fifty miles to the north, in order to provide an approximately gradational temperature profile. The bitumen was extracted from the crushed rock by soxhlet extraction with benzene. The extracts were separated into saturate and aromatic fractions via elution chromatography with hexane and benzene, respectively. The saturate fractions were analyzed using gas chromatography. Organic carbon was determined by combusting crushed samples in an induction furnace and measuring the evolved CO2. The principal conclusions drawn from this study were: (1) Metamorphism drastically reduces the amount of extractable bitumen in organic shales. (2) Hydrocarbon content is much more drastically reduced by metamorphism than is organic carbon, i.e., metahydrocarbon morphism reduced the ratio. (3) No relationship organic carbon between metamorphism and saturate/aromatic ratio was found in the Crested Butte samples. (4) Aromatic fractions from metamorphosed samples were lighter in color than those from unmetamorphosed samples. (5) The n-paraffins between C16 and C33 from the unmetamorphosed samples were rather evenly distributed, while those from metamorphosed samples were predominately C16, C17, C18, and C19. Metamorphism decreases the average molecular weight of the n-paraffins. (6) The carbon preference index for the unmetamorphased samples was close to 1. The hydrocarbons were probably somewhat mature before metamorphism occurred. (7) Metamorphism causes an increase in the ratio of iso- and cycloparaffins to normal paraffins. (8) Increased induration of shale and the disappearance of swelling clays were associated with metamorphism. (9) All of the noted changes occurred in approximately the same region, about five to seven miles from the main intrusive trend. The samples from between five and fifteen miles of the intrusive trend resembled the unmetamorphosed samples from Glenwood Springs more closely than they did the samples from within five miles of the trend. Thus, the profile probably does not represent a gradation in temperature, but rather defines two zones, one metamorphic and one submetamorphic. (10) The region of metamorphism may delineate the boundaries of a subjacent igneous mass related to other regional structures.