Demian J. C. Robbins¹, S. George Pemberton¹

(1) University of Alberta, Edmonton, AB

ABSTRACT: Lateral Equivalence of Coquinal and Siliclastic Units in the Montney Fm., Western Alberta, Canada

The subsurface Lower Triassic Montney Formation of western Alberta, Canada, has been a significant hydrocarbon producer since the late 1950's. Despite four decades of study, the genetic origin of reservoir coquinas remains a matter of continuing debate.

Montney coquinas erosively cross-cut fine-grained siliciclastic deposits and are commonly interpeted to have been deposited independently of underlying siliclastic material. The sharp change from very fine quartz sand to coarse, dolomitized bioclasts is generally very distinct in gamma ray well logs. A past emphasis on large-scale regional studies, dominated by geophysical log data with a minimum of core-based input, has led geologists to map all occurrences of this low-gamma response in the Montney as coquina.

A detailed, core-based local study in the vicinity of Kaybob South, a major Montney hydrocarbon pool, revealed a number of misconceptions regarding the relationships between coquinal facies and siliciclastics. Laterally adjacent siliciclastics and coquinas exhibit interfingering and strong similarity of palaeoenvironmental parameters. In the northwest of the study area, a distinctive gamma response on well logs that had always been mapped as coquina was found to represent wholly siliciclastic sections in some wells.

Examination of Montney sediments recognized three facies associations. Two of these, Facies Association 2 and the Bioclastic Facies Association, correspond to the laterally adjacent siliciclastic and coquinal deposits. The former was deposited under high-energy, storm-dominated conditions, while those of the latter correspond to winnowed, channel-filling deposits. These suggest a barrier island or bar setting, an interpretation supported by isopach maps of the combined siliciclastic-coquina section.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado