Abstract: High and Low Frequency Sedimentary and Geochemical Cycles in the Upper Devonian Carbonate System of Western Canada: Implications for Outcrop to Subsurface Correlation

F. S. P. van Buchem, G. P. Eberli, M. T. Whalen, E. W. Mountjoy, P. W. Homewood

The integrated study of both shallow water carbonates and adjacent basinal sediments in a 2nd order depositional cycle established a genetic relationship between carbonate reservoir and source rock formation, which has immediate bearing on the predictability at an exploration and production level.

The Upper Devonian (Frasnian) carbonate system in the Canadian Rocky Mountains has been subdivided in six 3rd order depositional sequences which are each characterised by 1) the geometrical style of the shallow water carbonates (retrograding, aggrading and prograding), and 2) a specific geochemical signature of the basinal sediments (carbonate versus TOC relationship, and outcrop spectral gamma-ray logs). This implies a direct relationship between carbonate productivity (and thus reservoir geometries), clay supply and organic matter preservation, the exact expression of which is dictated by the position of the 3rd order sequence in the overall, 2nd order, transgressive/regressive cycle.

These relationships have been found in two carbonate platforms (the Miette and Ancient Wall) 40 km apart, and in the subsurface Redwater reef 400 km to the east. All show the same number of 3rd order depositional sequences (6), a strong similarity in geometrical styles of the buildup margin, and the same position of the source rocks. During overall transgression a pulsed increase of the TOC content has been observed with highest values (up to 5%; super mature) in sequences 3 and 4. The concentration of the organic matter is mainly due to a diminishing carbonate supply and improved preservation conditions. At the 3rd order level highest TOC values occur during LST and TST due to a temporary shut off and/or low productivity level of the carbonate factory. The overall regression is again characterised by a dilution relationship, here the low TOC values reflect high sedimentation rate of both clays and carbonate.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France