Sedimentology and stratigraphy of an ancient progradational terrigenous clastic shelf margin, Missisauga Formation (Upper Jurassic-Lower Cretaceous), offshore Nova Scotia, Canada.

摘要

Large-scale stratal architecture, structure, and commonly hydrocarbon distribution in the Mississauga Formation, Sable Subbasin, offshore Nova Scotia, can be satisfactorily explained by shelf margin progradation. Using an integrated subsurface dataset (2-D and 3-D seismic data, biostratigraphy, geophysical well logs, core), the physical characteristics and depositional history of the Missisauga Formation (Tithonian-Barremian) were studied on both local and regional scales. These data suggest that during deposition of the Missisauga, the shelf margin prograded southward from an initial position just basinward of the Venture Field (Tithonian) to a final position just basinward of the Glenelg and Alma fields (Barremian). Growth-faulted, storm-dominated deltaic sandstones deposited at or near the shelf margin during this process are interpreted to contain over half of the discovered in-place gas reserves offshore Nova Scotia. Because shelf-margin delta sand-bodies are typically shore-elongate, tend to occur in groups along a shelf margin and commonly correlate downdip to turbidite sand-bodies, recognition of the shelf-margin delta play-type will not only improve hydrocarbon exploitation strategies offshore Nova Scotia, but will provide an important framework to guide the identification of new exploration opportunities in genetically related parts of the stratigraphic section.; In the Venture Field, Tithonian shelf-margin delta lobes are stacked vertically, suggesting that growth-fault related subsidence at the shelf margin negated depositional topography and created topographic lows through which fluvio-deltaic systems preferentially flowed. Over several relative sea level cycles, the positive feedback between sediment supply and subsidence at the shelf margin potentially focused enough sediment to have constructed slope turbidite systems downdip, which in turn represent new exploration targets.; In the Glenelg Field, Barremian shelf-margin deltas were initially tide-influenced (dominated?) and then changed to a storm-dominated state, possibly because the shoreline initially prograded into a tidally resonant topographic depression, which upon being filled was converted to a wave-dominated setting. Along depositional strike of the main hydrocarbon-bearing sandstones at Glenelg, Barremian sandstone reservoirs in the Alma Field are interpreted to be storm-dominated shelf-margin delta deposits. However, unlike Glenelg, incised valleys were not identified at Alma, suggesting that downdip depocenters were fed by sediment that bypassed Glenelg during the Barremian.; Transgression at the end of the Barremian deposited mudstone of the Naskapi Member throughout the Sable Subbasin, forming a regional seal. Sharp-based, bioturbated shallow marine sandstones deposited locally during this transgression are an important play type in the western Sable Subbasin (e.g., Alma and Panuke fields).