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

L. M. Pratt, R. C. Burruss, D. L. Gautier

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_org), organic carbon isotopic ratios, and Rock-Eval pyrolysis. C_org contents of Sharon Springs concreti ns (range 1.3 to 2.1%) show a strong positive correlation with C_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_max = 423°C; R_o ~ 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_org are higher for the concretion (77 mg/g) than for the shale (53 mg/g), and the amounts of nonhydrocarbons (asphaltene and NSO compounds) and aroma ic 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. The 50% lower aromatic, NSO, and asphaltene content of the shale may reflect loss of more water-soluble components during compaction. Additional analyses are in progress to evaluate this hypothesis and to evaluate possible maturation effects due to clay catalysis or other mineralogically controlled organic-inorganic interactions.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.