Tectonic controls on preservation of Middle Triassic Halfway reservoir facies, Peejay Field, northeastern British Columbia; a new hydrocarbon exploration model

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ABSTRACT Abundant hydrocarbons have been produced from the Middle Triassic Halfway Formation of northeastern British Columbia; however, the facies relationships and geological history of this strata are poorly understood. To address these issues, 132 cores and well logs from 345 wells were examined from the Peejay Field of northeastern British Columbia in ordre to: 1) establish a depositional model; 2) identify the origin of all reservoir facies; and 3) construct an exploration model to better predict the trend of reservoir facies. Middle Triassic deposits of the Peejay Field comprise four west-southwestward prograding shoreface parasequences which form a progradational parasequence set. Palaeoshoreface deposits, referred to as Lithofacies Succession I (L.S.I), have been truncated and replaced with sharp-based cross-stratified bioclastic grainstone and litharenite tidal inlet fills, referred to as Lithofacies Succession II (L.S.II). Tidal inlet fills trend north-northwest to south-southeast and exhibit the best reservoir quality in the field. Successively younger parasequences are thicker and contain coarser grained lithofacies. However, the youngest parasequence (P.A. 4) varies laterally in thickness due to postdepositional erosion. Isopach maps and cross-sections through the Middle Triassic strata reveal the occurrence of a post-Halfway erosional surface that truncates successively older strata to the northeast. Superimposed on the northeastward thinning of Middle Triassic regional isopach trends are local north-south-trending thickness anomalies interpreted as post-Halfway block faulting. The combination of post-Halfway erosion and block faulting has resulted in reservoir facies preservation being completely controlled by structure. Based on reconstruction of the geological history, a hydrocarbon exploration model has been developed for the Peejay Field in order to predict the genesis, distribution and degree of preservation of reservoir facies. RESUME Une abondance d'hydrocarbures ont ete produits de la Formation Halfway du Triassique moyen dans le nord-est de la Colombie-Britannique; toutefois les relations et l'histoire geologique des facies de ces couches ne sont pas bien comprises. Grace au bon controle sur les puits, 132 intervalles carottes et les diagrapphis de forage de 345 puits provenant du champ Peejay du nord-est de la Colombie-Britannique ont ete etudies afin : 1) d'etablir un modele de sedimentation; 2) d'identifier l'origine de tous les facies de reservoirs et 3) de construire un modele d'exploration dans le but de mieux predire la direction des facies de reservoirs. Les depots du Triassique moyen du champ Peejay se composent de quatre parasequences de zones infratidales en progression vers la mer en direction ouest-sud-ouest formant un jeu de parasequences en progression. Les depots de zones infratidales anciennes, ci-apres appelees Lithofacies Succession (succession de lithofacies) I (L.S.I), ont ete tronques et remplaces par du calcaire a debris anguleux jointifs bioclastiques avec stratification entrecroisee et des remblais de passes de litharenites, ci-apres appelees Lithofacies Succession (succession de lithofacies) II (L.S.II). Les remblais de passes se dirigent vers le nord-nord-ouest au sud-sud-est et indiquent la meilleure qualite de reservoir pour le champ. Les parasequences successivement plus jeune sont plus epaisses et contiennent des lithofacies a grains plus grossiers. Toutefois, la parasequence la plus jeune (P.A. 4) varie laterellement en epaisseur a cause de la post-sedimentation en erosion. Les cartes isopaques et les sections entrecroisees a travers les couches du Triassique moyen revelent l'occurence d'une surface en erosion du post-Halfway qui tronquent successivement des couches plus anciennes vers le nord-est. Superimposees sur l'amincissement vers le nord-est des directions regionales isopaques du Triassique moyen se trouvent des epaisseurs d'anomalies en direction regionale nord-sud interpretees comme des blocs qui se faillent du post-Halfway. La combinaison de l'erosion du post-Halfway et des blocs qui se faillent a donne comme resultat la conservation de facies en reservoir etant entierement controle par la structure. End_Page 595------------------------ Selon la reconstruction de l'histoire geologique, un modele d'exploration d'hydrocarbure a ete developpe pour le champ Peejay afin de predire la genese, la distribution et le degre de conservation du facies de reservoir. Traduit par Marie-Louise Tomas