Behavior of Inorganic Constituents During Early Diagenesis of Peats

WILLIS, JAMES J., Baylor University, Waco, TX, and ALAN M. BAILEY, University of Southwestern Louisiana, Lafayette, LA

Extensive diagenetic changes occurring in peat during burial to approximately 1.5 km may result in the formation of lignite (or medium-brown coal). Examination of available literature indicates very little research dealing specifically with the behavior of inorganic (noncarbon-based) compounds during this early postburial diagenesis. Because these inorganic constituents determine both the quantity and quality of ash in coals and may act as potential diagenetic agents in associated rocks, their behavior throughout the peat-to-lignite transformation is considerably important. Laboratory simulations of the early postburial diagenesis of peat offer a new approach for the study of such processes.

In the present research, laboratory simulations have been used to study diagenesis of well-characterized peats, including several from the Gulf Coast region. During the experimental runs approximately 80 g of peat were compressed and heated in a PTFE-lined reaction cell from atmospheric conditions to 2100 psi and 60 degree C, thus approximately simulating conditions necessary for the formation of lignite. Pressure and temperature were increased on a stepwise pattern, and, prior to each increase, expelled solution from the previous interval was collected. Following completion of each experimental run, the residual solid plug was collected.

Extensive reorganization of organic compounds in the produced residual solids during laboratory simulations was documented by Rollins et al. In addition, extensive petrographic changes can be observed, including increases in gelatinous material, darkening of colors, and increases in reflectance.

Concentrations of major inorganic constituents (Na, K, Ca, and Mg) released in solution are typically constant or decrease with increased pressure and temperature for high-ash peats, whereas minor inorganic concentrations (Fe, Cu, Zn, Mn, and AL) increase, occasionally reaching a maximum and then decreasing. With low-ash peats, concentrations generally are much lower than high-ash peats, and both major and minor inorganic concentrations typically increase throughout the experimental runs. For most peats, the ratio of major to minor inorganics increases. The general behavior of inorganics during diagenesis of these peats is strongly related to the composition of the original peat and to the stage of alteration. In addition, organic acids, including acetic and others, are present within the solutions as determined by ion chromatography. These organic acids may act as organic complexors and thus contribute to the mobilization of inorganic constituents.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)