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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

¹ Department of Geology, Saint Mary's University, Halifax, Nova Scotia, B3H 3C3, Canada; gpiper{at}smu.ca
² Department 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

Georgia Pe-Piper graduated from the University of Athens, Greece, and gained her Ph.D. in volcanic geochemistry from the University of Cambridge, United Kingdom. She is best known for her work on igneous rocks, but for many years, her teaching responsibilities included petroleum geology. Her current interests include the application of mineralogical studies to the provenance and diagenesis of sedimentary rocks.

Shawna Weir-Murphy graduated from Saint Mary's University in 2004 with an M.Sc. degree in applied science (geology) with the co-operative education option. Her thesis was on the Orpheus graben, offshore Nova Scotia. Since 2004 she has been employed as a geologist with EnCana in the former Atlantic and Mackenzie delta groups and currently in the Integrated Oil Division working on a steam-assisted gravity drainage project.

ABSTRACT

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.