Post-Paleocene Evolution of Regional Groundwater Flow-Systems and Their Relation to Petroleum Accumulations, Taber Area, Southern Alberta, Canada

ABSTRACT Three regionally extensive fluid-dynamic systems have been mapped in a 23 500 km2 portion of the Western Canada Sedimentary Basin in southeastern Alberta. The "Modern Land Surface System" reaches depths of 300 m and is well adjusted to the modern relief. The subjacent "Erosional Rebound System" occupies most of the 600-m thick predominantly shale Colorado Aquitard, including its often gas-bearing sand members. It is characterized by extensive regions of subhydrostatic pressures. Below the Colorado Aquitard is the "Cypress Plain System" which has strong upward-directed forces and mild but definite lateral components. The lateral components oppose the modern relief and are oriented from the centre of the study area toward its western and eastern boundaries, where most of the known oil fields also are located. The three dynamic systems are interpreted to be transient stages of sequentially superimposed gravitational and dilatational force fields. The gravitational fields depend on topography, and the dilatational field results from elastic rebound of the Colorado Aquitard in response to erosional removal of 700 m overburden during Pliocene and Pleistocene times. The flow pattern that existed during the time of the Cypress Plain was generated after Paleocene (58 Ma) or Eocene (36 Ma) time by a paleorelief dominated by the northeast-southwest trending Bow Island Topographic High. It was composed of, and maintained as, cross-formational gravity-flow systems throughout the Miocene (i.e. for approximatley 30 Ma). Oil migrated to, and was entrapped in, the discharge regions of these systems. Subsequently, dilation of the Colorado Aquitard created the hydraulic sink in the Erosional Rebound System and induced gas to exsolve and accumulate in the sands. Based on a one-dimensional solution of the diffusion equation, 10 Ma and 0.1 Ma were calculated to be required for the respective pore pressures below and above the Colorado Aquitard to adjust to changes in the land surface. These times, when compared with inferred ages and durations of paleotopographies, agree with the hypothesized evolutionary history of flow systems. There was sufficient time for flow in the Cypress Plain System to adjust to the post-Eocene relief, but not enough to re-adjust to the Modern Land Surface initiated during middle Pleistocene (1 Ma) time. In contrast, the Modern Land Surface System had 10 times the 0.1 Ma required for it to adjust to the modern relief; thus its apparent steady state is expected. EVOLUTION POSTPALEOCENE DES SYSTEMES REGIONAUX D'ECOULEMENT D'EAUX SOUTHERRAINES ET LEURS RELATIONS AUX ACCUMULATIONS DE PETROLE, REGION DE TABER, ALBERTA, CANADA RESUME Trois systemes fluides-dynamiques d'extension regionale ont ete cartographies dans une region de 23 500 km2 du bassin sedimentaire de l'ouest canadien, dans le sud-est de l'Alberta. Le "Systeme de surface terrestre moderne" atteint des profondeurs de 300 m et est bien ajuste au relief moderne. Le "Systeme de rebond erosif" sousjacent occupe la plupart des 600 m de l'Aquitard Colorado compose surtout d'argile et incluant aussi des gres souvent satures en gaz naturel. Il se caracterise par des regions extensives de pressions soushydrostatiques. Sous l'Aquitard Colorado se trouve le "Systeme de plaine Cypress" possedant de grandes forces a mouvement vertical ainsi que des composantes laterales faibles mais bien definies. Ces composantes laterales s'opposent au relief moderne et sont orientees a partir du centre de la region vers ses limites occidentales et orientales ou se situe aussi la majorite des gisements petroliferes connus. Les trois systemes dynamiques sont interpretes comme des etapes temporaires de champs de forces gravitationels et dilatationels superposes en succession. Les champs gravitationels relevent de la topographie, et le champs de dilatation resulte du rebond elastique de l'Aquitard Colorado suite a l'enlevement par erosion de 700 m de couverture durant le pliocene et le pleistocene. Le type d'ecoulement en existence a l'epoque de la plaine Cypress fut cree apres le paleocene (58 Ma) ou l'eocene (36 Ma) par un paleo-relief domine par l'axe positif nord-est/sud-ouest de Bow Island. Il se composait de, et se maintenait comme, un systeme d'ecoulement gravimetrique interformational durant tout le miocene (i.e., environ 30 Ma). Le brut aura donc migre vers les regions de decharge de ces systemes ou il aura aussi ete piege. Subsequemment, la dilatation de l'Aquitard Colorado crea un puits hydraulique dans le "Systeme de rebond erosif" amorcant l'exsudation du gaz et le piegeage dans les gres. A partir d'une solution unique a l'equation de diffusion, des durees de 10 Ma et 0.1 Ma sont requises pour permettre l'egalization des pressions interstitielles au-dessous et au-dessus, respectivement, de l'Aquitard Colorado et ce, en reponse aux changements de topographie. Ces durees sont conformes avec l'hypothese sur l'histoire de l'evolution des systemes d'ecoulement lorsque comparees aux ages et durees deduites des paleo-topographies. Il y avait suffisamment de temps pour que l'ecoulement dans le "Systeme de plaine Cypress" s'ajuste au relief posteocene mais pas pour qu'il s'ajuste au "Surface terrestre moderne" initie durant le pleistocene moyen (1 Ma). Par contre, le "Systeme de surface terrestre moderne" beneficia de 10 fois les 0.1 Ma requises pour s'ajuster au relief moderne, expliquant ainsi son regime d'apparence stationnaire. Traduction: Auteurs et Jean Pelletier End_Page 339------------------------