Mid Cretaceous sand supply to offshore SW Nova Scotia: tectonic diversion of Labrador rivers during Naskapi Member deposition

Transcription

1 1 Mid Cretaceous sand supply to offshore SW Nova Scotia: tectonic diversion of Labrador rivers during Naskapi Member deposition Final Report OERA Research Project number Total project duration: 10 September March 2018 Georgia Pe-Piper Department of Geology, Saint Mary s University, Halifax, NS B3H 3C3 (902) gpiper@smu.ca Submitted 26 th March 2018

2 2 2. Summary Deep water areas of the SW Scotian Basin are a current target of exploration for oil. Sources of sand to this area, that might form suitable reservoirs for oil, are not well understood. Some sand was supplied from the Meguma terrane of SW Nova Scotia. Two other potential sand sources are (a) by episodic diversion through the Bay of Fundy of a river that drained from Labrador in the Cretaceous (the Sable River) and built the sandstone reservoirs of the Sable Project; and (b) by rivers draining Maine and perhaps New Brunswick. The Sable River was postulated to have flowed through the Bay of Fundy for about 10 million years in the Mid Cretaceous. Little is known about supply from Maine, but that can be tested by study of samples from the COST reference wells drilled on the US side of Georges Bank in the 1980s. The study tested the hypothesis that tectonic diversion of Labrador rivers during the Aptian resulted in sand supply through the Bay of Fundy to the Shelburne sub-basin, allowing shales to accumulate farther east in the Scotian Basin. It also documented the changing supply of sand through time from eastern New England and New Brunswick to the COST G-2 well. The main part of this study was delivered as an M.Sc. thesis (and derivative journal publications) by Ms Isabel Chavez. The study has shown that there was little or no diversion of the Sable River through the Fundy Graben. The Mid Cretaceous Naskapi Member shales in the Scotian Basin did result from diversion of the Sable River to somewhere unknown for about 10 million years. The COST G-2 well on Georges Bank was supplied by local sediment from Maine in the Jurassic and by a larger drainage basin probably encompassing eastern New England and New Brunswick in the Cretaceous. There was thus modest potential for sandy sediment to be supplied to the SW Scotian Basin, with a moderate level of petrographic maturity.

3 3 3. Table of Contents 4. Introduction Discussion of Methodology, Objectives and Results Dissemination and Technology Transfer Conclusions and Recommendations Publications Employment Summary Bibliography/References Introduction The purpose of this study was to reduce exploration risk in the southwest Scotian Basin by better defining the types and amounts of sand available for reservoir rocks in the area. The PFA (2011) showed that the SW Scotian Basin was the most prospective part of the basin for oil. This part of the basin had previously attracted little attention and interpretation is hampered by the small number of wells in the area. The scope of this study was restricted by this paucity of wells. The Upper Jurassic to mid Cretaceous interval in the COST G-2 well, on the US side of Georges Bank, was studied in detail for the timing and petrology of the major sandstone units, in order to determine the provenance of the sand. In addition, particular attention was paid to the character of the Naskapi Member on the Scotian Shelf and the petrology and provenance of its sands, because at the beginning of the project it was thought that during Naskapi Member deposition, large amounts of sand might have been supplied through the Fundy Graben to the SW Scotian Basin. This study showed that this hypothesis was wrong. The M.Sc. thesis of Dan Dutuc showed that in the mid Jurassic to early Cretaceous, sand supply to wells such as Mohawk and Mohican on the western Scotian Shelf was almost entirely

4 4 from the Meguma Terrane (Dutuc et al., 2017). If there was a major source of sand to the Shelburne sub-basin, it must have come either from Maine or by diversion of rivers draining Labrador and the inboard Appalachian terranes of Atlantic Canada through the Bay of Fundy. Piper et al. (2011) proposed that the large Sable River was diverted in this manner during the Aptian, allowing the thin Naskapi Member shales to accumulate in most of the Scotian Basin. This hypothesis was supported by DionisosFlow modelling for the PFA. 5. Discussion of Methodology, Objectives and Results Our methodology is based principally on studies of the mineralogical and chemical composition of both sandstones and shales in offshore wells, to determine sources and dispersion of sediment and their relevance to petroleum reservoirs. Most aspects of the methodology have been summarized recently by Dutuc et al. (2017) and Pe-Piper et al. (2014). The study of the Naskapi Member used gamma and other wireline logs to correlate the Naskapi Member across the Scotian Basin. It built on experience gained from previous detailed studies of the Member from conventional core in the Panuke B-90 well (Chavez et al. 2016) which helped understand the lithologic and paleo-environmental significance of the wireline logs. Objectives: The study tested the hypothesis that tectonic diversion of Labrador rivers during the Aptian resulted in sand supply through the Bay of Fundy to the Shelburne sub-basin, allowing shales to accumulate farther east in the Scotian Basin. It aimed to compare the record of sand supply to the Georges Bank Basin to recently published work on the SW Scotian Basin (Dutuc et al. 2017), in the hope that it could show whether rivers from Maine contributed sediment to the Shelburne sub-basin, despite the Yarmouth Arch. It was hoped that the new data would allow conclusions about the quantity and quality of supply of reservoir sand to the deep water Shelburne sub-basin. It was also hoped that there might be improved information on basement tectonics and volcanism in the mid Cretaceous, both significant for thermal modelling of the basin.

5 5 Results: The principal results are publicly available in an M.Sc. thesis by Isabel Chavez. Isabel also prepared two papers for the scientific literature arising from her study. Both were accepted after the reviewers recommended only minor revision. The abstracts for these two papers are given below: Late Mesozoic sediment provenance on Georges Bank: enlargement of river drainages to the Atlantic Ocean in the Late Jurassic Early Cretaceous Isabel Chavez, Georgia Pe-Piper, David J.W. Piper, and R. Andrew MacRae. AAPG Bulletin The detrital mineralogy of Upper Jurassic to Lower Cretaceous sandstones from the COST G-2 well in the Georges Bank Basin was studied to better understand the drainage system and determine sediment sources in the area. Heavy minerals were separated from cuttings and identified by chemical analyses on a scanning electron microscope. Raman spectroscopy was applied to titania minerals to identify the polymorphs (rutile, anatase and brookite). Upper Jurassic (Oxfordian) sands contain tourmaline, fluorapatite, Mn-almandine and grossular garnets, which are commonly found in the Central Maine and Coastal Maine belts, and pumice and trachytes, which were reworked from volcanics in the Long Island Platform. Zircon geochronology ( and Ma) supports sources from peri-gondwanan Appalachian terranes. Lower Cretaceous heavy minerals show that Oxfordian supply was diluted to 30%, mostly by additional ilmenite, zircon, tourmaline and staurolite, probably from central and northern Maine and New Brunswick. Rutile and anatase from metapelites are more common than in the Oxfordian, where mafic igneous sources predominate. Lower Cretaceous zircons include a few Mesoproterozoic grains that might be reworked from mid-paleozoic sediments in the Gander terrane of northern Maine and New Brunswick, although subhedral grains together with rare chromite may indicate minor or intermittent supply from the Sable River draining Labrador. Zircon geochronology alone gives an incomplete record of detrital sediment supply. Progressive enlargement of the catchment area supplying sediment to Georges Bank Basin resulted from increasingly humid climate in the latest Jurassic and active mostly NE-trending faults in the Early Cretaceous.

6 Correlation of the Aptian Naskapi Member of the Scotian Basin and its regional implications Isabel Chavez, David J.W. Piper, and Georgia Pe-Piper. Canadian Journal of Earth Sciences. The Naskapi Member of the Logan Canyon Formation, a 150-m-thick shale dominated unit, lies in between sand dominated units of Upper Mississauga Formation below and the Cree Member above. The great decrease in sediment supply has been suggested as due to tectonic and/or eustatic sea-level changes. Wireline logs and recent biostratigraphy of 27 wells from the Scotian Shelf and Georges Bank, mudstone geochemistry from the Naskapi and Cree Members, and modal composition and chemical variation of detrital heavy minerals in sandstones were examined to better understand the deposition of the Naskapi Member and its regional implications. Minor sandy intervals at the base of the Naskapi Member were correlated based on gamma and sonic log signatures from the type section in the Cree E-35 well to progressively more distant wells, on the assumption that the sands represent periods of lowered eustatic sea level. Correlation was confirmed by the distribution of highstand black shales in washed cuttings and biostratigraphic markers identified in some wells. The geochemistry of mudstones from the Naskapi Member resembles mudstones sourced from the Meguma Terrane, except for higher abundance of elements likely reworked in smaller amounts from the Upper Mississauga Formation. Based on the correlation and geochemistry of mudstones and detrital minerals, we suggest the diversion of Sable River through the Gulf of St Lawrence to either the Orphan Basin or towards western Canada was responsible for the decrease of sediment supply in the Scotian Basin during the deposition of the Naskapi Member shales Dissemination and Technology Transfer Isabel attended the OERA/NSDoE workshop in early April 2016, and presented the COST G-2 petrology results at the Oceans meeting that replaced the NS Energy R&D Conference in May She twice presented to Statoil staff at Saint Mary s University on her work, in 2017 and She gave a talk at the Atlantic Geoscience Society meeting in

7 7 Fredericton in February 2017 on her Naskapi Member work. She was also invited to present at a Geological Survey of Canada workshop on petroleum prospectivity in deep water areas offshore eastern Canada in Spring We have maintained informal communication with NSDoE and CNSOPB. The results of her work are being widely disseminated in her two journal publications. Isabel applied the petrological experience she had gained during a 4-month summer internship with Nexen in In the winter of she has worked with the NS Dept. of Energy on seismic interpretation in the Shelburne sub-basin. When Statoil Canada became fully aware of Isabel s work, they made a financial contribution towards the costs of zircon geochronology and have shown an ongoing interest in the result of the study. 7. Conclusions and Recommendations The Naskapi Member resulted from diversion of the Sable River either westward down the St Lawrence Valley or eastward along the Humber Valley. The sediment of the Naskapi Member was derived from local rivers draining the Meguma terrane and lowstand reworking of upper Mississauga Formation sediment. There is no Sable River sand deposit at that time in the Shelburne sub-basin. Sedimentation in the Naskapi Member was strongly influenced by sea level fluctuations. Georges Bank Basin received local sediment in the Oxfordian but the catchment area supplying sediment was greatly enlarged by the Early Cretaceous, but was mostly or entirely limited to eastern New England and New Brunswick. There was not a large source of sand to the Shelburne sub-basin, but present wells do not provide any control for the Shelburne delta seismic package apparently supplied by a river draining the Fundy Graben. This supply has the greatest potential to source deep-water sands. We propose that with the knowledge arising from the work in this study at COST G-2, previous work at Mohawk by Dutuc et al. (2017) and the modelling experience of Sangster (2018), the next step should be to attempt to model sand supply to the Shelburne sub-basin in the Upper

8 Jurassic and Lower Cretaceous. This will allow a better evaluation of sand availability for the Shelburne sub-basin, but would not have been possible without the present study Publications CHAVEZ GUTIERREZ, I Early Cretaceous sand supply to offshore SW Nova Scotia: tectonic diversion of the Sable River during Naskapi Member deposition. M.Sc. thesis, Saint Mary s University. CHAVEZ. I., PIPER, D.J.W and PE-PIPER, G Correlation of the Aptian Naskapi Member of the Scotian Basin and its regional implications, Canadian Journal of Earth Sciences (on line) CHAVEZ. I., PE-PIPER, G., PIPER, D.J.W and MACRAE, R.A Late Mesozoic sediment provenance on Georges Bank: enlargement of drainage to the Atlantic Ocean in the Late Jurassic Early Cretaceous. AAPG Bulletin (revision returned) 9. Employment Summary Name Isabel Chavez Position M.Sc. Student? Ph.D., M.Sc., under -grad Full or Part Tim e Scientific Contributions Yes M.Sc. FT Delivering the Source of funds NSERC, Work 24 months on project student project OERA, NS Grad Schol. Kerry Wallace student Yes under- FT Organising data, OERA 5 assistant grad and archiving samples, PT research support

9 9 11. Bibliography/References Chavez, I., Piper, D.J.W., Pe-Piper, G. and Zhang, Y., North Atlantic climatic events recorded in Aptian Naskapi Member cores, Scotian Basin. Cretaceous Research, 60, Dutuc, D.C., Pe-Piper, G. and Piper, D.J.W., The provenance of Jurassic and Lower Cretaceous clastic sediments offshore southwestern Nova Scotia. Canadian Journal of Earth Sciences, 54(1), Pe-Piper, G., Piper, D.J.W. and Triantafyllidis, S., Detrital monazite geochronology, Upper Jurassic Lower Cretaceous of the Scotian Basin: significance for tracking firstcycle sources. Geological Society, London, Special Publications, 386(1), pp Piper, D.J.W., Bowman, S.J., Pe-Piper, G. and MacRae, R.A., The ups and downs of Guysborough County the mid Cretaceous Naskapi Member in the Scotian Basin: eustacy or tectonics? Atlantic Geology, 47, Sangster, C.R, Hawie, N., Pe-Piper, G., Saint-Ange, F. and Piper, D.J.W., Application of Predictive Modeling to the Lower Cretaceous Sedimentary Sequences of the Central Scotian Basin. CSPG Annual Convention (extended abstract)