Comparison of Organic Matter in Concretions and Adjacent Shale--Possible Means to Study Migration Processes: ABSTRACT

Abstract

The Sharon Springs Member (Upper Cretaceous Pierre Shale) near Canon City, Colorado, contains numerous layers of solid and septarian limestone concretions. High carbonate contents (71-85%), an uncompacted sedimentary fabric, and a slight decrease in volume percent calcite (apparent porosity) from core to margin in these concretions suggest rapid formation just below the sediment-water interface. Organic matter in concretions and adjacent shale samples have been visually characterized using transmitted light and fluorescence microscopy. About 50 samples of concretion and adjacent shale were analyzed using routine methods for organic carbon content (C/sub org/), organic carbon isotopic ratios, and Rock-Eval pyrolysis. C/sub org/ contents of Sharon Springs concretions (range 1.3 to 2.1%) show a strong positive correlation with C/sub org/ contents of adjacent shale (range 5.7 to 10.3%). The Sharon Springs at this locality is thermally immature with respect to hydrocarbon generation (T/sub max/ = 423/sup 0/C; R/sub 0/ approx. 0.5%). For one concretion-shale pair, bitumen was extracted from whole rock powder, HCl-insoluble residue, and acid leachate in order to recover bitumen entrapped or retained by calcite cement. Carbon isotope ratios of organic matter, hydrogen index, and gas chromatographic profiles of saturated hydrocarbons are indistinguishable. Extraction yields normalized to C/sub org/ are higher more » for the concretion (77 mg/g) than for the shale (53 mg/g), and the amounts of nonhydrocarbons (asphaltene and NSO compounds) and aromatic hydrocarbons in bitumen are 50% higher for concretion than shale. Early diagenetic cementation in Sharon Springs concretions prohibited compaction and markedly reduced porosity and permeability compared to shale. This difference in cementation between samples may have inhibited migration of maturation products from the concretion. « less