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Passive Margin
Deep-Water Clastic Reservoir Characterization from
Rift to Drift: Lessons from
Marsaglia, Kathleen M.,
Sand composition exerts a first order control on diagenesis and ultimate reservoir character on passive
margins. Sand provenance models suggest that during continental rifting, sand
could be: 1) quartzofeldspathic, derived from
uplifted plutonic basement rocks; 2) metamorphiclastic,
derived from metamorphic basement rocks; 3) sedimenticlastic,
derived from sedimentary cover rocks; 4) volcaniclastic,
derived from syn-rift volcanic centers; or 5) any
combination of the above. Sand composition can be used to constrain dispersal
pathways and paleogeographic reconstructions.
Furthermore, with the advent of seafloor spreading and formation of a
proto-oceanic trough/ocean, passive margin sediments become progressively more quartzose and mature according to these models. With
respect to reservoir potential of these sand units, composition can directly
affect cementation, for example, controlling the number of potential quartz
grain nuclei for quartz overgrowth cementation. Dissolution of unstable
components provides not only secondary porosity, but also affects the type and amount
of authigenic cementation. Thus there is an expected
compositional trend in rift-to-drift successions, which in turn should produce
predictable trends in reservoir characteristics within passive margin
successions.
Ocean Drilling
Program (ODP) results from Iberian-Newfoundland conjugate margins provide some
new insights into these sand provenance models and potential sandstone
reservoir evolution. ODP Leg 210 continuously cored and recovered sandy gravity
flow deposits from 800 to 1800 meters below the seafloor (mbsf)
that can be directly compared with sandy sedimentary successions cored by
several ODP legs on the Iberian conjugate margin (0-700 mbsf).
The former are passive margin sediments whereas the latter span the
rift-to-drift history of the margin.