Demineralization and Kerogen Maceral Separation and Chemistry

Abstract

Although there are exceptions, the abundance of kerogen is generally small in most oil shales and petroleum source rocks. The presence of the inorganic matrix, which constitutes most of the rock, complicates both the characterization of the kerogen and investigation of pyrolysis chemistry. Kerogen isolation and subsequent separation of its components, or macerals, is important to gaining a fundamental understanding of its chemistry. Techniques to separate kerogen from rocks include both physical and chemical methods. Chemical methods are more effective but may alter the organic matter.

The mineralogy of the rock, for the most part, determines the overall effectiveness of the demineralization reagents. Oil shale contains three broad classes of minerals: carbonates, silicates, and sulfides. The common carbonates are soluble in dilute acids; silicates are soluble in HF. Common sulfides include pyrite and marcasite which are resistant to HC1/HF attack but can be removed with oxidizing or reducing agents. A new procedure for demineralization has been developed which uses HF and HBO³ to form aqueous BF³ which is used to remove neo-formed salts. This approach is effective and reduces organic matter alteration.

High speed density gradient centrifugation (DGC) is used to remove pyrite and isolate and concentrate the different kerogen maceral groups. This technique utilizes a dense media of CsClloaded into a spinning centrifuge to form an in-situ density gradient. Demineralized kerogen particles are then dispersed across the gradient. This method effectively segregates the principal macerals present. Analytical pyrolysis andFT-i.r. analysis on feedstock and separated treated macerals do not indicate any strong alteration of organic matter by this approach.

Our research has focused primarily on Upper Devonian oil shales, including the New Albany Shale from the Illinois basin and the Cleveland Member of the Ohio Shale from the Appalachian basin. These shales are classified as marinites and contain a mixed maceral assemblage. The most abundant maceral present in both shales is bituminite, with both primary forms of alginite, telealginite and lamalginite present in abundance. Vitrinite is an important constituent in both shales. Inertinite is also abundant in the leveland but sparse in the New Albany.

The various maceral components fell in all three geochemical type trends. With the alginites plotting in the Type I field, the bituminites in Type II and the inertinites in Type III. Compared to terrestrially deposited macerals of similar rank, the marine materials were found to be considerably enriched in hydrogen. Comparisons between the New Albany and Cleveland materials found the New Albanymacerals to be petrographically and geochemically more degraded than the Cleveland. The New Albany alginites were “red shifted” and had less hydrogen that the alginites of the Cleveland. The New Albany vitrinites were higher in hydrogen than those of the Cleveland suggesting that more of this maceral was derived from the degradation of bituminite precursors.

The relative states of preservation are ascribed to the depositional environment, with the Cleveland deposited under upwelling conditions and the New Albany under stagnant density stratified basin conditions. The Cleveland also had iron limited pyrite, while that of the New Albany had much high concentrations of carbon limited pyrite. More reactive organic matter was consumed in the formation pyrite in the New Albany than the Cleveland resulting in a higher degree of degradation. This and the general enrichment of hydrogen in the shale illustrates the importance of the environment of deposition relative to material source in determining the overall chemistry and oil generating capacity of the kerogen.