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AAPG Bulletin, V.
Early diagenesis of inner-shelf phosphorite and iron-silicate minerals, Lower Cretaceous of the Orpheus graben, southeastern Canada: Implications for the origin of chlorite rims
1Department of Geology, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada; [email protected]
2Department of Geology, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada; present address: EnCana, 421 7th Ave. SW, P.O. Box 2850, Calgary, Alberta T2P 2S5, Canada
Wells in the Orpheus graben encountered the most proximal part of the deltaic Lower Cretaceous rocks of the Scotian Basin. More distal sandstones are important gas reservoir rocks, with good reservoir quality where Fe-rich chlorite (chamosite) rims on framework grains have inhibited quartz cementation. Cutting samples from the Orpheus graben show the presence of Fe-rich sheet silicates (berthierine or chamosite) and early diagenetic phosphorite. These minerals were analyzed by electron microprobe, and their textures were mapped with backscattered electron images. Studies in the North Sea have shown a relationship between high phosphorus and the presence of good chlorite rims in reservoir rocks. The mineralization of pore-water phosphorus, instead of its return to seawater, is favored by Fe-rich sediments and sorption on iron oxides during shallow sea-floor diagenesis. The Fe, Ti, and P contents are uncommonly high in Scotian Basin shales compared with global average shale compositions. The uncommon occurrence of inner-shelf phosphorite in this study is interpreted to be a consequence of the same high Fe content of the sediment that also favors the formation of Fe-rich sheet silicates. In rapidly deposited deltaic sandstones of the offshore reservoirs, the dominance of type 3 kerogen led to sulfate depletion occurring at depths of tens of meters and a corresponding great thickness for overlying Eh (oxidation potential)-controlled diagenetic zones. The thick Fe-reduction zone allowed the formation of early diagenetic berthierine, which on burial formed the chamosite rims that resulted in the improved reservoir quality in sandstones. The distribution of phosphorus minerals may be an indicator of conditions suitable for berthierine formation.
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