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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 75 (1991)

Issue: 1. (January)

First Page: 139

Last Page: 155

Title: Occurrence of Fracture-Hosted Impsonite and Petroleum Fluid Inclusions, Quebec City Region, Canada (1)



Impsonite--a high-rank solid petroleum residue--occurs as fracture fillings in Cambrian-Ordovician deep-water sedimentary rocks of the Taconian Allochthon in the region surrounding Quebec City. A material of similar appearance occurs as solid, opaque, black inclusions in small euhedral quartz crystals that formed contemporaneously with the impsonite. Raman microprobe spectroscopy reveals that this material is comprised, in part, of disordered graphitic carbon. Sheaths of nonfluorescent fluid (gas or supercritical vapor) surrounding some of these solid inclusions may represent volatile products generated during postentrapment thermal maturation. Raman spectra reveal the presence of CH[4] and possibly CO[2] and C[2]H[6].

Petroleum fluid inclusions, entrapped in the quartz contemporaneously with the impsonite precursor, are comprised of a heterogeneous and variable mixture of organic phases, including a viscous fluorescent liquid and a nonfluorescent supercritical fluid. These fluid inclusions occasionally contain a reddish brown translucent solid (bitumen?), a colorless birefringent solid (paraffin?), and may also include an aqueous phase. Two distinct populations of fluid inclusions can be distinguished based upon their fluorescence spectra. The mode of occurrence and the composition of these materials suggest that the fracture-fill impsonite is a solid residuum of the petroleum in the inclusions.

Reflectance of the fracture-fill impsonite increases from north to south (1.3-1.9%) and crosscuts nappe boundaries, whereas reflectance of dispersed organic matter from the same strata increases from south to north (1.7-2.2%) and changes abruptly at the nappe boundaries. Dispersed organic matter reflectances indicate pre-Taconian maturation beyond the petroleum window, whereas the emplacement and maturation of fracture-fill material is post-Taconian. Strata underlying the nappes contain probable source rocks that are still within the oil window. Petroleum may have migrated into overlying strata through fractures, whereupon it was destroyed by subsequent catagenetic processes. The interpreted paragenetic sequence of pretectonic maturation, nappe emplacement, maturation, and oil generat on from the underlying autochthon, and subsequent degradation of the petroleum may serve as a model for oil- and gas-producing fold-and-thrust belts elsewhere.

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