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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 41 (1993), No. 4. (December), Pages 453-463

Sedimentological and Sequence Stratigraphic Model of the Falher "D" Pool, Lower Cretaceous, Northwestern Alberta

R.W.C. Arnott

ABSTRACT

Sequence stratigraphy has been increasingly used as a correlation technique by both academic and petroleum geologists. This approach puts primary emphasis upon the surfaces that bound genetically related stratigraphic successions. In this study of the Falher "D" Pool, the use of a sequence stratigraphic approach helps identify four depositional intervals within the Falher "D" interval; these are termed D1, D2, LS1 and D3. Conglomeratic strata, abundant only in the D2 interval, represent the principal gas-producing lithofacies in the "D" Pool. Deposition of gravelly sediment along the D2 shoreline was terminated by a relative sea level fall that shifted the local shoreline northward and created a sequence boundary. The sharp northern boundary of the "D" Pool is attributed to this event. Subsequently, following the reestablishment of stillstand progradation, the nearshore strata of LS1 were deposited above the sequence boundary. Progradation during this episode caused the shoreline to migrate beyond the northern limit of the study area. Nevertheless, at some later time, but still during Falher "D" time, the shoreline returned into the study area. Despite the deposition of a thick nearshore succession, strata of D3 consist only of nonreservoir sandstones. This suggests that not only were changes of relative sea level important in controlling the distribution of reservoir strata within the study area but, also, temporal changes in the nature of sediment being supplied to the Falher shoreline - namely the relative proportion of gravel to finer-grained sediments.

RESUME

Les universitaires et les geologues du petrole utilisent de plus en plus la stratigraphie sequentielle comme technique de correlation. Cette approche met l'ampleur sur les surfaces limitant genetiquement les successions stratigraphiques. Dans cette etude du gisement Falher "D", l'usage de la stratigraphie sequentielle aide a identifier quatre intervalles de sediments appeles D1, D2, LS1 et D3. Les strates conglomeratiques qui abondent seulement dans l'intervalle D2 sont le principal litofacies producteur de gaz du gisement "D". Les depots de sediments graveleux le long de la cote D2 se sont termines par une baisse relative du niveau de la mer qui a deplace la cote regionale vers le nord et a ainsi cree une limite de sequence. Le contact net au nord du gisement "D" est attribue a cet evenement. Par consequent, apres le retablissement de la progression stationnaire du rivage vers la mer, les couches littorales LS1 ont ete deposees audessus de la limite sequentielle. La progradation a cause le deplacement de la cote au-dela de la limite nord de la region etudiee. Plus tard, mais toujours pendant la periode de Falher "D", la cote est reapparue dans la region etudiee. Malgre une succession littorale epaisse, la couche D3 se compose uniquement de gres non reservoir. Ce phenomene suggere la presence non seulement de changements du niveau de la mer relatifs controlant la distribution des couches reservoirs dans la region etudiee, mais egalement de changements temporels dans la nature des sediments fournis a la cote Falher (c'est-a-dire la proportion relative de graviers et de sediments a grains plus fins).

Traduit par Marie Louise Tomas


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