Improved Reservoir Characterization of a Nisku Carbonate Ramp Margin Using Sequence Stratigraphic Concepts, Upper Devonian, Central Alberta, Canada
Sequence stratigraphic analysis of lithologic (core/cuttings) and wireline log data from the Nisku Formation within a mature field in central Alberta provided the depositional framework to identify exploration potential along the ramp margin trend. Further integration of sparse well coverage and geophysical (magnetics/seismic) data resulted in characterization of multiple porosity fairways associated with ramp margins developed at different time intervals along the regional exploration trend.
Mud-rich carbonates (variably dolomitized) in slope to basinal settings do not have reservoir potential. The ramp margin trend has very coarse skeletal carbonates (stromatoporoid/rugosan coral) with primary and secondary porosity that is improved by early dolomitization of the skeletal sand matrix. Ramp interior strata are mud-prone, with scattered thin beds of skeletal carbonate debris, suggesting deposition within a generally low energy setting. Porosity tends to be limited, with reservoir potential limited to thin, dolomitized, grain-rich beds. Further updip, the ramp interior strata transition into dolomitized algal laminites and other peritidal carbonate facies.
Integration of core and cuttings descriptions with wireline logs revealed multiple stacked, upward-shoaling successions along the margin trend. The ramp margin was aggradational in some areas, but prograded basinward elsewhere along trend. Progradational packages were verified on limited available 2D seismic data. In addition, pulses of argillaceous siliciclastic material appear to be shed basinward of the ramp margin trend from localized fluvial point sources during lowstands. Each upward-shoaling package is interpreted as a third-order sequence, with improved reservoir development of marginal facies developing during the sequence highstands. Where progradational, highstand margins form a series of laterally discontinuous reservoir-prone packages that are bounded updip by non-porous ramp interior facies. Transgressive portions of sequences are thinner and dominated by skeletal carbonate sands with more localized coarse constructional fabrics.
The implications of this study are that there are numerous potential locations for stratigraphic traps along the ramp margin trend, often with variable lateral connectivity between the porous ramp margin carbonate facies. Most of these packages likely are below regional 2D seismic resolution, but have potential to trap significant volumes of hydrocarbon.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009