The productive intervals at Terra Nova comprise Late Jurassic sandstones of the Jeanne d'Arc Formation which were deposited in a braided-plain to braid-delta setting. Depositional cycles, initiated by periodic tectonic uplift, began with a regressive surface of erosion and resulted in the formation of high aspect ratio incised valleys. Seismic imaging of the valleys is excellent to the south and west of the Terra Nova oil field where the basal sequence bounding unconformity erodes into limestone resulting in valleys up to 200 m deep, and from 0.5 to 5 km. wide, but is poor within the field due to incision primarily into shales or sands, the velocity of which is similar to the valley fill, and a complex structural overprint.

The valleys were filled by the rapid progradation initially of conglomeratic alluvial fans and fan deltas, and conglomerate-filled channels from the southeasterly margins of the basin, and of sands (sandy braid-plain to braid-delta) from the uplifted head of the basin to the southwest into an anoxic to brackish basin to the north and northeast. As the sediment supply diminished after each tectonic episode, each depositional sequence was drowned as a result of subsidence. Successive depositional sequences are variably preserved as a consequence of the amount and rate of tectonic uplift sediment supply and other factors.

Detailed and quantitative description of the Terra Nova cores resulted in the clear recognition that there were two types of braid-plain/braid-delta deposition. Some of the progradational units display high proportions of high-angle cross-bedding while other intervals are dominated by or have significantly higher proportions of horizontal to slightly inclined planar parallel bedding. These differences are interpreted to reflect differences between upper and lower flow regime conditions within and between individual progradation events. While sedimentologically significant, these findings also impacted the building of a geostatistical model of the reservoir with respect to the identification of geologically-based reservoir units and the assignment of petrophysical properties, and to a consideration of possible capillary trapping effects on reservoir performance.

Based on the observations from core, two outcrop analogs were examined: the Castlegate Sandstone (Book Cliffs, Utah) and the Salt Wash Member, Morrison Formation (Shootering Canyon, Utah). Additionally, comparison was made to a modern braid-plain/braid-delta at the head of Ekalugad Fjord, Baffin Island. The analogs were used to better understand the possible lateral heterogeneities and hierarchies of lateral correlation lengths within such a depositional setting for use in constructing a geostatistical geological model.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah