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.