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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 36 (1988), No. 1. (March), Pages 1-8

Shale Compaction in the Michigan Basin: Estimates of Former Depth of Burial and Implications for Paleogeothermal Gradients

Raymond Vugrinovich

ABSTRACT

The Michigan Basin is an intracratonic basin in which regionally extensive shales were deposited many times between the Precambrian and Pennsylvanian. This study was conducted to calculate former thicknesses of now eroded sediments using sonic transit time wireline logs of the Mississippian Coldwater Shale. Results indicate at most 850 m of additional sediment in the central Lower Peninsula. The depocentre was located in the vicinity of Saginaw Bay. Estimated sediment thickness decreases to less than 200 m in the northern and southern Lower Peninsula. The sediments were post-Desmoinesian in age and were removed prior to Kimmeridgian time.

Subsurface temperature measurements compiled by the author indicate that the average basinwide geothermal gradient in the Michigan Basin is 19.2°C/km. Recent studies contain evidence for higher paleotemperatures in the basin. The evidence includes the occurrence of late diagenetic dolomite in Devonian and Ordovician rocks, late-stage diagenetic dolomite and calcite in Silurian rocks, the occurrence of bituminous coal in Pennsylvanian rocks at depths of 75 m and less, fluorite occurrences in Devonian rocks, a K/Ar age of 382 million years on samples of altered Precambrian basalt, and the occurrence of oil, believed to have been generated in situ or in source rocks which have not been deeply buried. The present average geothermal gradient will not produce subsurface temperatures in the range of 60°C to 150°C and at the reconstructed depths of 875 to 2000 m indicated by the above studies, implying that the high paleogeothermal gradients deduced from thermal maturation studies are reasonable and did exist at one time.

LA COMPACTION DES ARGILES DANS LE BASSIN DU MICHIGAN

RESUME

Le Bassin du Michigan est une bassin intracratonique dans lequel des couches d'argile de grande etendue furent deposees a plusieurs reprises entre le Precambrien et le Pennsylvanien. Le but de cette etude etait de calculer l'epaisseur originelle de sediments maintenant erodes, en utilisant des diagrammes soniques de l'argile Mississippienne Coldwater. Les resultats indiquent au plus 850 metres de sediment en supplement, dans la partie centrale de Lower Peninsula. Le point de sedimentation maximum etait situe dans les environs de la Baie de Saginaw. On estime que l'epaisseur des sediments decroit a moins de 200 metres dans nord et le sud de Lower Peninsula. Les sediments etaient d'age post-Desmoinesien et ont ete erodes avant le Kimmeridgien.

Des mesures de temperature de la subsurface compilees par l'auteur indiquent que le gradient geothermique moyen a la grandeur du Bassin du Michigan est de 22,6°C/km. Des etudes recentes temoignent de paleotemperatures plus elevees dans le bassin. La preuve comprend la presence de dolomie diagenetique tardive dans des roches Devoniennes et Ordoviciennes, de dolomie et de calcite diagenetique tardive dans des roches Siluriennes, de charbon bitumineux dans des roches Pennsylvaniennes a des profondeurs inferieures a 75 metres, la presence de fluorite dans des roches Denoniennes, une date K/Ar de 382 millions d'annees determinee sur des echantillons de basalte Precambrien altere, et la presence de petrole, qu'on croit avoir ete engendre sur place ou dans des roches meres qui n'ont pas ete profondement enfouies. Le present gradient geothermique moyen ne peut produire des temperatures de subsurface d'ecart (60°C a 150°C), et a des profondeurs (875 metres a 2000 metres) reconstituees, qu'indiquent les etudes ci-haut mentionnees, ce qui signifie que les gradients paleogeothermiques eleves deduits des etudes de maturation thermique sont raisonnables et existaient dans le passe.

Traduit par Marc Charest


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