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.
AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England