Comparison of I/S Transformation and Maturity of Organic Matter at Elevated Temperatures

The smectite-to-illite diagenetic transformation has been documented in 5 different sedimentary basins using X-ray diffraction (XRD). Intermediate reaction products coexisting because of the effect of kinetics on this reaction have been characterized using decomposition of XRD patterns and comparison with calculated patterns. The nature and relative abundances of the various subpopulations of particles are shown to vary as a function of the geothermal gradient and of the age of the sediment. In all sedimentary basins that experienced a low steady geothermal gradient the physico-chemical characteristics (coherent scattering domain size [CSDS], junction probabilities) of intermediate mixed-layered illitesmectites (I-S) are similar. However, both the relative abundance and the crystallinity of the end-member illite increase as a function of the age of the sediment.In basins that have experienced a higher geothermal gradient, the CSDS of the I-S subpopulation is higher for a given illite content, indicating a slightly different reaction pathway. This difference in the characteristics (peak position and width) of elementary peaks may be used to infer the presence of such a high geothermal gradient when no other data are conclusive. In this case the growth of the illite endmember is favored over the growth of intermediate I-S phases even in young basins. Illitic phases formed from the alteration of kaolin minerals exhibit characteristics similar to the reaction products of the smectite-to-illite diagenetic transformation in the case of a high geothermal gradient. In contrast with what is observed in shale diagenesis, the characteristics of the illitic subpopulations describe a continuum with absolutely no gap in between subpopulations. In sandstone reservoirs, the various subpopulations crystallize simultaneously from a kaolin precursor. As a consequence, no kinship is expected between the various subpopulations.

Section: Relative Abundances Of I-s and Illite Subpopulationssupporting

confidence: 94%

Late-Stage Diagenesis of Illitic Clay Minerals as Seen by Decomposition of X-Ray Diffraction Patterns: Contrasted Behaviors of Sedimentary Basins with Different Burial Histories

Lanson

Velde²,

Meunier

1998

“…Consequently, the overall thermal-maturity level reflected by the clay mineralogy of the bentonites is less than the thermal maturity reflected by vitrinite. The differences in temperature indicated by vitrinite reflectance versus the measured percentages of illite in I-S is enigmatic though not unique (e.g., Velde and Lanson, 1993). At Cerro Negro, this phenomenon may be attributed to the retardation of the progressive formation of I-S by one or some combination of the following: the presence of K-poor pore fluids, a high silica content derived from the adjacent sandstones and shales (Abercrombie et al, 1994), limited availability of K + by diffusion into thick bentonites (Altaner et al, 1984;Altaner, 1989), and heating in the absence of fluids.…”

Section: Discussionmentioning

confidence: 99%

The Kinetics of the Smectite to Illite Transformation in Cretaceous Bentonites, Cerro Negro, New Mexico

Elliott

1999

Abstract--The thermal effects, as well as the survivability and origins of microorganisms in Cretaceous rocks, are evaluated from the timing and extent of the smectite to illite transformation in Cretaceous bentonites collected from cores outside the thermal aureole of the Pliocene Cerro Negro volcanic neck. Overall, randomly ordered mixed-layered illite-smectite (I-S) is the predominant clay mineral in these bentonites, and the K-Ar ages of I-S range from 36 to 48 Ma (21 analyses, two additional analyses were outside this range). Increased temperature from burial is thought to be the primary factor forming I-S in these bentonites. Kinetic model calculations of the smectite to illite transformation are also consistent with I-S formed by burial without any appreciable thermal effects due to the emplacement of Cerro Negro. In a core angled toward Cerro Negro, the percentages of illite layers in I-S from the bentonite closest to Cerro Negro are slightly higher (32-37%) than in most other bentonites in this study. The KAr ages of the closest I-S are slightly younger as a group (38-43 Ma; Average 41 Ma; N = 4) than those of I-S further from Cerro Negro in the same core (41-48 Ma; Average = 44 Ma; N 6). A small amount of illite in this I-S may have formed by heat from the emplacement of Cerro Negro, but most illite formed from burial. Vitrinite reflectance, however, appears to record the effects of heating from Cerro Negro better than 1-S. Tentatively, the temperature of this heat pulse, based on vitrinite data alone, ranged from 100 to 125~ and this is most evident in the CNAR core. The upper temperature, 125~ approximates the sterilization temperatures for most microorganisms, and these temperatures probably reduced a significant portion of the microbial population. Thermophiles may have survived the increased temperatures from the combined effects of burial and the intrusion of Cerro Negro.

“…Mixed-layer I/S is regarded as a mixture of one or more of these phases. Based on XRD studies and combined simulation-decomposition approaches for modeling XRD patterns, Lanson and Champion (1991) and Velde and Lanson (1993) postulated multi-phase assemblages in sediments of the Paris Basin. Rettke (1981) attributed the diverse expandability and ordered mixed-layering of the US in the Lower Cretaceous Dakota Group shales of the Denver-Julesburg basin to variability within original, unaltered detrital clay.…”

Section: Sensitivity Analysismentioning

confidence: 99%

Clay Mineral Diagenesis and Thermal History of the Thonex Well, Western Swiss Molasse Basin

Schegg

Leu²

1996

Abstract--Results are presented of a diagenetic study from the 1300 m thick Oligocene Molasse deposits penetrated by the Th6nex geothermal exploration well (Geneva, Switzerland). The x-ray diffraction (XRD) studies of fine-grained rocks indicate the following diagenetic changes: a decrease of illite/smectite (US) expandability from approximately 90% to 30% with depth, a decrease of the amount of US in the clay mineral fraction, and the appearance of corrensite at depths >750 m. The transition from random US to ordered I/S occurs at the base of the Th6nex well (1200 to 1300 m) and is associated with a coal rank of about 0.7% Rr (mean random vitrinite reflectance) corresponding to paleotemperatures of 110 to 115 ~ Corrensite appears at a vitrinite reflectance value of 0.6% Rr and a corresponding paleotemperature of 100 ~ The amount of post-Molasse erosion is estimated to be approximately 2 km. Thermal history modeling of the Th6nex well suggests maximum paleotemperatures of 80 to 115 ~ and an average paleogeothermal gradient of 27 ~ during Late Miocene maximum burial conditions.