Discrepancies between different organic maturity indicators in a coal series affected by an abnormal thermal event (Viséan, Niger)

doi:10.1016/0009-2541(93)90040-P

Chemical Geology

Volume 106, Issues 3–4, 25 June 1993, Pages 397-413

https://doi.org/10.1016/0009-2541(93)90040-P Get rights and content

Abstract

Classical petrographical and geochemical approaches have been used to study Viséan coal seams sampled in two areas 50 km apart, in northern Niger.

T_max-values given by Rock-Eval® pyrolysis and individual molecular ratios determined on hopanes and steranes are in agreement with the burial history of the two sets of samples studied. The difference of maturity of the two sample sets revealed by these various parameters is consistent with their current difference in burial depth (∼ 400 m). Only small changes of these parameters with depth were observed owing to the small depth interval sampled at both sites (15- and 40-m). These variations also appear to be related to normal burial diagenesis.

No maturity differences between the two sample sets are revealed by vitrinite reflectance data and atomic H/C ratios which are in the same range at both sites, but display very large variations with depth (R_v ≈ 0.60 −1.40% and (H/C)_at ≈ 0.80–0.90 to 0.55). These data indicate rapid entrance into the oil window, this conclusion also being supported by microspectrofluorimetric observations showing oil production and expulsion and by the presence of pyrobitumens containing large devolatilization vacuoles in the deepest samples. The abnormally high maturity gradient could have been caused by a hydrothermal event. The present observations on coals confirm and supplement previous conclusions drawn from the study of organic materials disseminated in ZnPb-sulfide ore-bearing sedimentary rocks.

The difference in sensitivity shown by the various maturity indicators is considered very likely to be due to differences in the kinetics of the transformations on which they each depend.

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^α: Present address: Laboratoire de Géodynamique Sédimentaire, Centre de Géochimie de la Surface du CNRS, Université de Strasbourg, F-64084 Strasbourg, France.

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Research paperDiscrepancies between different organic maturity indicators in a coal series affected by an abnormal thermal event (Viséan, Niger)

Abstract

Geochim. Cosmochim. Acta

J. Geochem. Explor.

Geochim. Cosmochim. Acta

Chem. Geol.

Geochim. Cosmochim. Acta

Fuel

Org. Geochem.

Coal Geol.

Applications de la pétrographie des organoclastes à l'histoire géologique et thermique des bassins sédimentaires carbonés

Mém. Soc. Géol. Fr.

Histoire géodynamique post-paléozoïque de l'Afrique de l'Ouest, d'après l'étude de quelques bassins sédimentaires (Sénégal, Taoudenni, Iullemeden, Tchad)

Microscopic measurement of the level of catagenesis of solid organic matter in sedimentary rocks to aid exploration for petroleum and to determine former burial temperatures — A review

Organic petrology applied to study of thermal history and organic geochemistry of igneous contact zones and ore deposits in sedimentary rocks

Diagenesis, 10. Quantification and modelling of organic diagenesis

Geosci. Can.

Détermination de paléotempératures maximales d'enfouissement de sédiments charbonneux à partir de données de pyrolyse

C.R. Acad. Sci. Paris, Sér. II

Géochimie des hydrocarbures gazeux: analyse et application à la prospection minière (bordures orientale des Causses et gisement ZnPb des Malines; Gard, France)

Bur. Rech. Géol. Min., Orléans, Rep.

Elemental analysis of kerogens (C, H, O, N, S, Fe)

Reflectance of vitrinite as a control of thermal history of sediments

Pentacyclic triterpenes of the hopane type as ubiquitous geochemical markers: origin and significance

La pyrolyse Rock-Eval et ses applications — Première partie

Rev. Inst. Fr. Pét.

Research paper
Discrepancies between different organic maturity indicators in a coal series affected by an abnormal thermal event (Viséan, Niger)