Effects of Elemental Sulfur During Programmed Pyrolysis

Roger Sassen, Elizabeth W. Chinn

The effects of elemental sulfur during programmed pyrolysis (Rock-Eval method) of Eocene Green River shale and common rock matrix minerals were assessed experimentally. Mixtures of elemental sulfur and Green River shale with calcium carbonate, silicon dioxide, and calcium sulfate were analyzed. With weight-fraction Green River shale held at a near-constant value, geologically realistic contents of elemental sulfur result in substantial decreases in hydrogen index and substantial increases in oxygen index regardless of matrix chemistry. The effect is such that oil-prone type I kerogen would be interpreted as gas-prone type III kerogen. Only minor differences in analytical response were noted for the different matrix compounds without elemental sulfur. Results suggest that ulfur-bearing compounds are created during analysis and interfere with the response of detection devices in the Rock-Eval analyzer. The values of S₁ and S₂, measured by flame ionization, decrease with increasing sulfur content. The values of S₃ and S₄, measured by thermal conductivity, show a positive correlation with sulfur content. The flame ionization detector is sensitive primarily to hydrocarbons and does not detect sulfur compounds, thereby accounting for the observed decrease in S₁ and S₂. Thermal conductivity detection depends on the quantity of gas which passes through the detector. Additional sulfur compounds would therefore augment expected detector response. This effect has been observed in naturally occurring ource rocks containing sulfur subjected to programmed pyrolysis. This study suggests that results of programmed pyrolysis on rocks containing elemental sulfur must be interpreted with caution.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.