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GEOREF RECORD

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

Georgia Pe-Piper and Shawna Weir-Murphy
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
AAPG Bulletin (September 2008) 92 (9): 1153-1168

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.


ISSN: 0149-1423
EISSN: 1558-9153
Coden: AABUD2
Serial Title: AAPG Bulletin
Serial Volume: 92
Serial Issue: 9
Title: 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
Affiliation: Saint Mary's University, Department of Geology, Halifax, NS, Canada
Pages: 1153-1168
Published: 200809
Text Language: English
Publisher: American Association of Petroleum Geologists, Tulsa, OK, United States
References: 52
Accession Number: 2008-120914
Categories: Economic geology, geology of energy sourcesSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. cols., 1 table, sketch map
N44°00'00" - N46°00'00", W63°00'00" - W58°00'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by American Association of Petroleum Geologists, Tulsa, OK, United States
Update Code: 200846
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