James J. Willis

Behavior of Inorganic Constituents During Early Diagenesis of Peats

ABSTRACT 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 (non-carbon-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 potentially important in several areas of study. 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 several well-characterized peats. 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°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 extracted from the cell intact. Extensive reorganization of organic compounds in the produced residual solids during laboratory simulations was documented by Rollins et al. (1991). 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 increasing 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 all peats, the ratio of major to minor inorganics decreases. The general behavior of inorganics during the 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. These organic acids may act as organic complexors and thus contribute to the mobilization of inorganic constituents.

Field Examination of Exposed Evaporite-Related Structures, United States and Mexico: Relations to Subsurface Gulf of Mexico Examples

ABSTRACT Considerable exploration attention has been devoted to salt-withdrawal minibasins and sub-salt traps in the offshore Gulf of Mexico (GOM) province. Investigation is inherently by geophysical and petrophysical means, utilizing sonar, seismic and gravity data, and scattered well logs, with obvious resolution limitations. Outcrop studies in Texas, Utah and Mexico provide an additional dimension, leading to an improved understanding of GOM examples. Outcrops along I-10 between Kerrville and Sonora, Texas, provide a remarkable suite of evaporite-withdrawal (dissolution) structures, including varying exposure levels of withdrawal synclines, related secondary structures, and the primary weld surface. Paradox Basin exposures in Utah include diapirs and evaporite anticlines, revealing intense internal deformation, and various stratigraphic-structural interactions. The La Popa basin in Mexico hosts evaporite-cored anticlines, diapirs, withdrawal synclines, and a previously-recognized secondary weld. These outcrop analogues provide excellent insight into GOM structures. For example, GOM welds form potentially important hydrocarbon pathways, but the coarse scale of seismic resolution, though revealing significant character changes, leaves many questions unanswered. The Edwards Plateau and La Popa welds exhibit remarkable diversity along their traces, even over relatively short distances. Some segments exhibit enhanced permeability through brecciation, fracturing, or sand-sand juxtaposition, whereas shale smearing or remnant salt limits fluid transmission along other portions.

Structure and Stratigraphy of the Calcasieu Lake Salt-Withdrawal Minibasin, Onshore Louisiana

ABSTRACT The Calcasieu Lake salt-withdrawal minibasin, central Cameron Parish, Louisiana, forms an E-W elliptical shape of ~15 mi by 10 mi. Complex normal-fault systems occur along the basins periphery, commonly down-to-the-basin and flanking adjacent salt-cored uplifts, including Hackberry, Big Lake, Sweet Lake and Creole. Calcasieu Lake salt dome, a comparatively small piercement structure, occupies the basins center. Sub-salt exploration is currently a major play offshore, yet sub-salt production beneath prominent overhangs of the Calcasieu Lake salt dome was established nearly a quarter century ago in October, 1966, producing ~6.0 BCFG and 300 MBO. Calcasieu Lake minibasin formation was preceded by emplacement of an allochthonous salt sheet into Upper Eocene Jackson strata, probably similar to the modern salt canopy offshore. Rapid sedimentary loading in the Oligocene caused salt remobilization, forming or enhancing peripheral salt uplifts, and an eventual salt weld to form. The Middle Miocene marked renewed sedimentation and basin formation, likely related to formation of the now-detached Calcasieu Lake salt dome by remobilization of salt from a central residual mound. In particular, a sandrich depositional sequence, up to 3,000 ft or more in thickness, is confined to the basin itself, without equivalent strata on peripheral highs. Local upturning of Middle Miocene basinal strata by the Calcasieu Lake salt dome allowed for the sub-salt production at Calcasieu Lake. The paucity of wells drilled along the basins flanks leaves open many exploration opportunities to test these sand-rich strata as they onlap and pinch-out up the flanks. End_of_Record - Last_Page 409-------