Determination of trace elements in kerogens

Kerogen is the organic matter in sedimentary rocks that is insoluble in organic solvents. Kerogens are the precursors to petroleum, and their chemical composition is a determining factor in the type of crude oil generated during catagenesis. Recent work has shown that crude oils in sedimentary basins can be correlated using multivariate statistical analyses of trace-element abundances or through the relative abundances of the Ni(II) and VO(II) geoporphyrins. The role of kerogen in the genesis of metal complexes and other trace-element species in crude oils is not well understood but is important in developing oil-source rock correlation techniques based on metal abundances. The determination of trace elements in kerogens is difficult because of the insolubility of the matrix and because of residual mineral matter not removed in the conventional HCl-HF dissolution of the carbonate and silicate matrix of the host rock. The residual mineral matter thus contributes to the total trace-element concentration in the kerogen. Instrumental neutron activation analysis (INAA) has been used to determine trace elements in kerogens. Correction for the contribution of the mineral matter based on the low-temperature oxidation of the organic matrix has been described, but partial oxidation of fine-grained pyrite makes the correction for elementsmore » other than nickel and vanadium suspect. This paper reports a method of correcting for the mineral component of a kerogen by determination of trace elements by INAA in two different density fractions of the kerogen.« less