PRACtically Speaking

Research Profile        

The Cretaceous rocks onshore Nova Scotia:

What they tell us about sources, diagenesis and reservoir properties of offshore sedimentary rocks

Dr. Georgia Pe-Piper, Department of Geology

Saint Mary’s University

Dr. Georgia Pe-Piper, geology professor at Saint Mary’s University, is using sands and clays in pits and boreholes on land to better understand the development of offshore oil and gas reservoirs.

Most of the reservoir rocks offshore Nova Scotia that host oil and gas fields are in sandstones of Early Cretaceous age. These sandstones accumulated in a large deltaic plain and coastal zone, the “Sable delta,” deposited from large rivers that have been compared with the modern Niger river of west Africa. The properties of these reservoir sandstones are known only from sparse cores and from indirect geophysical methods such as wire-line logs and seismic-reflection profiling.

Sands and clays of similar age to the reservoir rocks are found in pits and boreholes in several areas of Nova Scotia and New Brunswick, where they are termed the Chaswood Formation. The Chaswood Formation was deposited by the rivers that ultimately brought water and sediment to the Sable delta. Thus further study of the old river deposits of the Chaswood Formation is important for several reasons. They can provide new information about the source and hence original composition of sand in the Sable delta, before the breakdown of some minerals and the growth of new minerals through the process of diagenesis, which has a major influence on reservoir porosity. The processes of soil formation, weathering and diagenesis in the Chaswood Formation provide information on the climatic conditions at the time of Sable delta deposition, which influences early diagenetic processes. Lignites of the Chaswood Formation preserve a record of airborne volcanic ash in the Early Cretaceous; the same ash accumulated on the Sable delta and contributed to diagenesis there. The overall character of Chaswood Formation deposits provides an understanding of the landscape and type of rivers that were present in the Early Cretaceous.


 

With funding from the partners in the ExxonMobil Sable Project, PRAC and NSERC, Georgia and her team carried out detailed studies of the geology of the Chaswood Formation and of correlative offshore rocks in the Sable sub-basin and Orpheus graben. M.Sc. student Jean-Philippe Gobeil made a detailed study of the Chaswood Formation sands in the West Indian Road silica sand pit near Shubenacadie, N.S. Another M.Sc. student, Shawna Weir-Murphy, studied the correlative section from well cuttings and seismics in the Orpheus graben. Numerous undergraduate students were involved in studies of samples from both the Chaswood Formation and offshore wells. Boreholes were drilled into buried Chaswood Formation sediments at Brierly Brook, near Antigonish N.S., at Belmont, near Truro N.S. and at the Atlantic Silica Pit at Poodiac, N.B.

Above: Dated monazite silt grain in an electron microprobe image, showing spot age determinations in millions of years


Studies of the rare mineral monazite in the Chaswood Formation and the offshore Sable delta sandstones show that at least four rivers built the Sable delta. The age of individual monazite grains can be determined using the electron microprobe and with this knowledge, and information on what other distinctive minerals are present in sands, it is possible to recognise the general course of former rivers that transported these sands. Three smaller rivers drained from the highlands of central and northern New Brunswick across Nova Scotia to the Sable delta, while a larger river drained western Newfoundland and northeast Quebec flowing through Cabot Strait and southwestwards to the south of Cape Breton Island.

The new work shows that the Early Cretaceous rivers flowed in braided gravelly channels and that the Atlantic provinces were mountainous, with coarse-grained alluvial fans shed off rapidly eroding fault-bound uplands. Fossilised soils show that the climate was similar to that of present day southern Mexico and except where rapidly eroding on steep slopes, bedrock was deeply weathered. As a result, the mineral ilmenite is an important component of river sands. This ilmenite breaks down during river transport and soil formation and is unusually abundant in the Sable delta. Alteration of this ilmenite releasing reactive iron may be responsible for the early diagenetic formation of the mineral chlorite in pore space in sandstones in the Sable delta, which has prevented the growth of younger diagenetic minerals and thus maintained high porosity in gas reservoirs. q

Other PRAC-Funded Research

Harpoon Free Fall Cone Penetrometer (FFCPT)

Above: Arnold Furlong and Harold Christian with the Penetrometer
(Photo courtesy of Brooke Ocean Technology)

The exploration and development of deepwater sites on the continental slope present unusual challenges for offshore foundation engineering.  This project led by Harold Christian, of Christian Situ Geoscience Inc. in partnership with Brooke Ocean Technology Ltd. and the Geological Survey of Canada (Atlantic) adapted an existing piston core sampling system to act as a flexible method for carrying out advanced in situ geotechnical tests and for installing long-term pore pressure sensors.  The funding provided by PRAC allowed Christian Situ Geoscience Inc. and its team to develop new geotechnical and geophysical site investigation technology for deployment at reduced cost from vessels of opportunity that may be conducting preliminary geophysical surveys and seafloor core sampling.

The Harpoon Free Fall Cone Penetrometer (FFCPT) has the potential to add an in situ geotechnical test capability to any conventional piston or gravity coring system. The FFCPT mounts in place of the soil sampling cutting shoe and can operate autonomously in any water depth up to 3,500m. The FFCPT method involves dropping an instrumented cone penetrometer into the seafloor, using its gravitational mass as a means of developing sufficient downward force to cause shear failure of the sediments. A high speed data acquisition system continuously monitors the sediment shearing response, by means of a suite of sensors mounted inside the FFCPT tool.

 
Above: Free Fall Cone Penetrometer prior to being lowered over the side of a research vessel
(Photo courtesy of Brooke Ocean Technology)

The continuous profiling of the sediment column and generation of high-quality cone penetration test-type data means that reconnaissance geotechnical site investigations could be carried out at deepwater locations.

Harpoon FFCPT equipment is easily mounted and deployed using existing piston coring systems, making the technology transferable to any vessel capable of coring operations.

PRAC News                                

PRAC Announces Call 7

Targeted Call for Research Proposals

PROPOSAL DEADLINE: 4:30pm (AST), Tuesday, January 31, 2006

PRAC is seeking proposals that address five specific research themes related to offshore geoscience and reservoir engineering in Atlantic Canada. These specific themes are:

1. Evaluation of the distribution and quality of reservoir rocks in the shelf and deepwater slope offshore Atlantic Canada
2. Geophysical imaging, characterization and validation of lithologies, fluids and pressures within Atlantic Canada basins
3. Characterization, monitoring and enhancement of reservoir performance
4. Geoscience study and regional compilation of lesser known sedimentary basins with petroleum potential
5. Studies of shelf and slope geo-hazards

PRAC has allocated $600,000 to this call.  Projects can range from one to three years in duration and contributions from PRAC will be a maximum of $50,000 per year.  Applicants are required to secure additional funds from other sources to be eligible for PRAC funding.

Applicants must have already submitted a Letter Of Intent to PRAC in order to be eligible to submit a proposal.

For details on the call, please click here.