ABSTRACT

In much of southern Alberta the Lower Cretaceous Basal Quartz is a complex assemblage of mostly nonmarine sandstone and mudstone. Within the study area, which is centred on Horsefly Lake and Chin Coulee pools (twp. 7-9; rge. 14-17W4), the BQ comprises a minimum of five unconformity-bounded sequences that are differentiated by a unique assemblage of attributes, including mineralogy, micro- and macroscopic sedimentological textures and structures, reservoir-quality characteristics, and production history. In stratigraphic succession, these sequences are informally termed the "A" sandstone, "weathered Horsefly" sandstone, "unweathered Horsefly" sandstone, "old BAT" sandstone, and "young BAT" sandstone.

The oldest strata consist of the "A" and "weathered Horsefly" sandstone. As a result of extensive pedogenic alteration, primary sedimentary characteristics and reservoir quality have been largely destroyed. Reservoir development in these strata is the result of diagenetic leaching, and is limited in areal extent (limited reservoirs). The younger "unweathered Horsefly" sandstone, or simply "Horsefly" sandstone, and "BAT" sandstones from the principal reservoirs in the study area. However, in spite of their different ages, a consistent vertical assemblage of lithofacies exists for each sandstone unit a braided fluvial succession several metres thick (primary reservoir strata) is overlain abruptly by a several-metre-thick meandering fluvial succession. This consistent upward change is interpreted as resulting from repetitive changes in sediment supply and its control on the nature and spatial patterns of sedimentation.

Although the Basal Quartz was deposited over about 18-32 my, it is less than ~ 25 m thick within the Chin Coulee-Horsefly Lake study area. This suggests that Basal Quartz sediment accumulated under highly accommodation-limited conditions, which in turn caused net sedimentation rates to be very low. One of the most significant consequences of this limitation was the development of a particularly complex stratigraphic succession. Because of the limited accommodation space, each unconformity-bounded succession tends to incise from a similar stratigraphic horizon, which therefore requires careful regional mapping to determine relative ages. In addition, younger stratigraphic successions eroded some or most of the commonly extensively pedogenically altered older ones. This resulted in the formation of an extremely complex succession of strata and a highly compartmentalized reservoir system.

RESUME

Dans une grande partie du sud de I'Alberta, le Quartz de Base (QB) du Cretace inferieur est un assemblage complexe compose surtout de gres non-marin et de mudstone. A I'interieur de la region d'etude, qui est centree sur les champs du Horsefly Lake et de Chin Coulee (T7-9, R-14-17W4), le QB comprend un minimum de cinq sequences limitees par des discordances qui sont differenciees par un assemblage unique d'attributs, incluant la mineralogie, les textures et structures micro- et macroscopique, les textures et structures sedimentaires, les caracteristiques de qualite de reservoir et I'histoire de production. Dans la succession stratigraphique, ces sequences sont nommees informellement gres "A", gres "Horsefly altere", gres "Horsefly non-altere", gres "vieux BAT" et gres "jeune BAT".

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Les plus vieilles strates correspondent aux gres "A" et "Horsefly altere". A la suite d'un alteration pedogenique prolongee, les caracteristiques sedimentaires primaires et la qualite de reservoir ont ete amplement detruites. Le developpement d'un reservoir dans ces strates est le resultat d'un lessivage diagenetique et est limite en etendue (reservoirs limites). Le gres le plus jeune de "Horsefly non-altere" ou plus simplement le gres "Horsefly" et les gres "BAT" forment les principaux reservoirs de la region d'etude. Toutefois, en depit de leurs ages differents, un assemblage vertical et consistant de lithofacies existe pour chacune des unites de gres : une succession fluviale anastomosee de plusieurs metres d'epaisseur. (strates de reservoir primaire) est recouverte abruptement par une succession fluviale a meandre de plusieurs metres d'epaisseur. Ce changement consistant vers le haut est interprete comme resultant de changements repetes d'e-spaces d'ajustement et de sedimentation.

Meme si le Quartz de Base a ete pendant a peu pres 18 a 32 m.a., il est de moins de 25 m d'epaisseur a l'interieur de la region d'etude de Chin Coulee-Horsefly Lake. Ceci suggere que le sediment du Quartz de Base s'est accumule sous des conditions d'ajustement tres limitees, qui a leur tour ont cause des taux de sedimentation net tres bas. Une des consequences les plus significatives de cette restriction est le developpement d'une succession stratigraphique particulierement complexe. En raison de cet espace d'ajustement restreint, chaque succession limitee par une discordance tend a s'inciser a partir d'un horizon stratigraphique similaire, qui necessite une cartographie regionale soignee pour determiner les ages relatifs. De plus, les successions stratigraphiques plus jeunes ont erode certains ou la plupart des successions plus vieilles communement alterees pedogeniquement de facon etendue. Ceci a eu pour resultat la formation d'une succession extremement complexe de strates et un systeme de reservoir hautement compartimentalise.

Traduit par Lynn Gagnon