BATEMAN, JASON A., EDWARD H. DAVIES, Mobil Oil Canada, J. F. (Rick) SARG, Mobil E&P Technical Center, and JOHN W. SNEDDEN Mobil Exploration Norway
Abstract: Lowstand Exploration Plays of the Jeanne d'Arc Basin (Jurassic-Lower Cretaceous), Offshore Newfoundland, Canada
The Jeanne d'Arc Basin is a Mesozoic failed-rift basin situated within the Grand Banks, 100 km off the east coast of Newfoundland, Canada. Sediments in the Jeanne d'Arc range in age from Late Triassic to the Tertiary. Depositional sequences were initiated by rifting associated with the break up of Pangea and the formation of the Atlantic Ocean. To date, 40 exploration wells have been drilled into major normally-faulted and salt diapir-related structural traps resulting in 16 discoveries, the most notable being the Hibernia and Terra Nova fields with recoverable reserves of 650 and 400 million barrels respectively (Fig. 1).
Renewed exploration interest in the basin has led to the acquisition and synthesis of new 3D and old 2D seismic data, and the application of sequence stratigraphy. Depositional sequences have been defined through the integration of biostratigraphy, core and log motif analysis, and seismic stratigraphy. Detailed descriptions of 1.4 km of core have verified a number of the major sequence boundaries and maximum flooding surfaces (mfs). The boreal Northwestern European ammonite zonation, as applied in North Sea exploration, has been utilised to calibrate the maximum flooding surfaces. This scheme has been successfully transferred across the North Atlantic to the Jeanne d'Arc Basin through high resolution biostratigraphy, principally utilizing palynology. The mfs surfaces were identified from the abundances and diversity peaks of the associated microfossils and related to maxima in v-shale curves. This helped constrain the sedimentary packages and enabled their subdivision into third-order sequences. Log motif analysis was incorporated in identifying systems tracts candidates. The resulting chronostratigraphic framework constrains lithostratigraphic and geophysical data that have been used to define the primary play types and major depositional fairways (Fig. 2). Three new play concepts have resulted from the regional sequence stratigraphic framework. Stratigraphic-type plays are contained within potential incised valley fill deposits, distal lowstand lobes, and updip lowstand pinchouts that are associated with structural elements. Additional potential occurs where depositional systems onlap against salt diapirs.
Dip and strike sections, hung on selected regional flooding surfaces as datums, have revealed local variations in the stacking patterns and sedimentary style within individual sequences. These relationships reflect a complex interplay between regional tectonics, sediment supply, eustatic control and relative subsidence rates. Regional gross thickness and net sand maps focussed on the prospective Upper Jurassic (Jeanne d'Arc) and Lower Cretaceous (Hibernia) strata serve to delineate major siliciclastic fairways throughout the basin, for both the proven highstand (HST) and transgressive systems tracts (TST), and the untested lowstand systems tracts (LST). Several prospective deepwater LST reservoirs have been identified in a series of depocenters defined by trans-basinal fault trends and salt tectonics. (Fig. 3).
The early Kimmeridgian to middle Tithonian, Jeanne d'Arc second-order supersequence (JDS) contains the major reservoirs at Terra Nova field and comprises four third-order depositional sequences. Exposure of the underlying Tempest second-order sequence containing the Rankin Limestones and the Egret organic-rich shales supplied abundant carbonate and shale-prone clastics during the initial development of the Kimmeridgian unconformity and deposition of the lowermost third-order depositional sequence of the JDS. Their relatively deep position in the basin, and their lithological composition diminishes the prospectivity of these sediments (i.e., poor reservoir, gas prone). The subsequent transgressive sands of the second third-order sequence are interpreted to comprise a wave-dominated system. Low angle, high amplitudes reflections onlap over a gentle gradient and initiate the filling of the axially aligned incised valleys of the Kimmeridgian unconformity. Prospectivity is manifested in the Hebron sands. The third sequence of the JDS contains the Terra Nova LST and TST sands which complete the filling of the peripheral Kimmeridgian incised valleys. Braided stream complexes, lateral alluvial fans and avulsing deltaic lobes of a river-dominated system feed coarse sediment into the deep basin resulting in the preservation of multiple-stacked reservoir packages separated by shales and unconformable surfaces. The HST of this highly prospective supersequence contains the Fortune Bay shales and acts as the regional top seal. The final third-order sequence of the JDS was initiated by thin TST sands (Hibernia stray sands) which are overlain by a HST regressive architecture of prograding clinoforms.
The late Tithonian to Berriasian second-order Hibernia Sequence contains the major reservoirs at the Hibernia field. Four major mfs's were identified and help subdivide this supersequence into four third-order sequences. The lowermost sequence comprises an untested basinal lowstand wedge that onlaps laterally and proximally and contains shoreface and distributary channel reservoirs. A south-to-north axial drainage pattern with a strong lateral depositional component off structural highs bounding the basin to the east and west is indicated. The next sequence is represented by stacked deltaic clastics that prograde into the basin primarily from the lateral axes. Large variations in the gross sand thickness of the second and third sequences locally control the prospectivity of these depositional units throughout the basin. The east side tends to be sand-prone, whereas the west side has a tendency to be more shale-prone. The final sequence is represented by thick regional HST carbonates (B Marker) and shales (Whiterose Formation) that may provide effective vertical seals. Massive thickening of the Hibernia Sequence in the north-central part of the basin reflects the overall N-NE plunge of the basin.
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