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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 41 (1993), No. 4. (December), Pages 407-421

Reservoir Geometry Influenced by High-Frequency Forced Regressions Within an Overall Transgression: Previous HitCarolineNext Hit and Garrington Fields, Viking Formation (Lower Cretaceous), Alberta

Stephen D. Davies,, Roger G. Walker

ABSTRACT

584 well logs and 139 cores have been used to establish a two-part stratigraphy for the Viking Formation in the area of the Previous HitCarolineNext Hit and Garrington reservoirs, Alberta. The lower part consists of a northeastward-prograding coastal succession of offshore and shoreface storm deposits, capped in places by nonmarine facies. This succession is dissected by a regionally-extensive transgressive surface of erosion (TSE) that rises stratigraphically southwestward (landward). It has at least 15 m of relative relief and is mantled by a transgressive lag of sandstone and conglomerate averaging 1 - 2 m thick (maximum 8 m). The upper stratigraphic unit consists of marine black shales with five tongues of coarse sandstone and conglomerate, each 1 - 3 m thick. These have previously been interpreted as tidal sand ridges but a tidal interpretation is very difficult to reconcile with the presence of thick interbedded black marine shales. Correlation of well-log and core cross sections suggests that these coarse tongues converge toward (and are interpreted to onlap) the underlying transgressive lag toward the southwest; they are not coarse bodies completely isolated in marine mudstones. The tongues do not have a ridge morphology. We interpret the tongues as extensions of the lower shoreface, formed during minor regressions interspersed with the main transgression. At these times of lowered sea level, the coarse sediment was transported seaward by storms and/or tidal currents and deposited abruptly on top of black mudstones. Thus the coarse reservoir rocks appear to result from forced regressions, which are in turn controlled by high-frequency oscillations of sea level.

RESUME

584 diagraphies de forage et 139 carottes ont servi pour etablir une stratigraphie en deux parties de la Formation Viking dans la region des reservoirs de Previous HitCarolineTop-Garrington en Alberta. La partie inferieure est une succession littorale, progressant vers le nord-est, composee de tempestides neritiques et infratidales, recouverte a certains endroits par des facies continentaux. Cette succession est dissequee par une vaste surface d'erosion transgressive regionale qui monte stratigrafiquement vers le sud-ouest (vers le continent). Elle possede un relief d'au moins 15 m et elle est recouverte d'un residu de deflation compose de gres et de conglomerats mesurant en moyenne 1 - 2 m d'epaisseur (maximum 8 m). L'unite stratigraphique superieure consiste en shales noirs marins avec cinq biseaux de gres grossiers et de conglomerats d'une epaisseur de 1 - 3 m chacun. Ceux-ci ont deja ete interpretes comme etant des chaines de sable intertidales, toutefois cette interpretation est difficile a concilier avec la presence de shales marins noirs a bancs epais. La correlation des diagraphies de forage et des carottes suggere que ces biseaux grossiers convergent vers (et transgressent) le residu de deflation sous-jacent en direction sud-ouest; ce ne sont pas des masses etendues entierement isolees dans des mudstones marins. Les biseaux n'ont pas une morphologie de crete. Nous interpretons ces biseaux comme etant des extensions de la zone infratidale inferieure formees pendant des regressions mineures durant la transgression principale. Pendant la periode ou le niveau de la mer etait plus bas, les sediments grossiers ont ete transportes vers la mer par des tempetes et/ou des courants de maree et deposes abruptement au-dessus des mudstones noirs. Ainsi, les reservoirs semblent etre le resultat de regressions forcees controlees par les oscillations a haute frequence du niveau de la mer.

Traduit par Marie Louise Tomas


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