PRACtically Speaking
The Newsletter of Petroleum Research Atlantic Canada - August 2006Research Profile
The Thermal Effects of Buried Salt Structures on Petroleum Migration and Accumulation
Dr. Marcos Zentilli, Dalhousie University
Dr. Marcos Zentilli is Emeritus Professor of Geology at the Department of Earth Sciences, Dalhousie University, Halifax. In 2004, Dr. Zentilli received funding from PRAC with contributions from Corridor Resources Ltd. and Geostorage Ltd. These funds where matched by NSERC as a Collaborative Research and Development Grant (CRD). The project involved Dalhousie University PhD student (now Post-doctoral Fellow) Alexander M. Grist; Adjunct Professor Dr. Yawooz Kettanah; five undergraduate students; and collaboration with Drs. Marie-Claude Williamson and Hans Wielens of the Geological Survey of Canada (Atlantic), Dartmouth, NS.
Figure 1 - Microscopic petroleum fluid inclusions in Osprey Salt (Well H-84, Grand Banks, NF) and Pugwash Salt (Pugwash Salt Mine, NS). Unpublished data by Kettanah, Wielens and Zentilli.
Many of the richest petroleum basins in the world, such as the Middle East and the Gulf of Mexico, are associated with salt structures; offshore Atlantic Canada is no exception. Rock salt, gypsum and potash salt were deposited in thick beds by evaporation when ancient warm seas dried up. The basins were later buried by sands and silts carried by rivers. However, rock salt is plastic, therefore when it is buried by layers of sedimentary rocks; it is squeezed laterally and upwards.
This pressure causes the rock salt to rise, which deforms the rocks above. Often the rock salt will reach the surface, as it did in the active underground salt mines in the Magdalen Islands (Quebec), Pugwash (Nova Scotia) and Sussex (New Brunswick). Offshore Newfoundland and Nova Scotia, the presence of salt at depth has had a positive effect on petroleum maturation and the deformed strata around them often form attractive oil and gas reservoirs, some in relatively deep water.
Figure 2 - Geology student Evan T. Brown inspects plastically deformed salt underground in the Pugwash Salt mine, Nova Scotia, while collecting samples for fluid inclusions study. SMU student supported at Dalhousie University by Shell Canada Ltd.
Dr. Zentilli’s team has established the only geochronology laboratory in Canada that can apply two age-dating methods that also determine thermal histories of rocks to temperatures between 60ºC and 125 ºC (a temperature range called the “oil window” that indicates which source rocks can generate and release oil and below and above which only gas is generated). These techniques are apatite fission track (AFT) analyses and uranium-thorium-helium ((U-Th)/He) age-determination methods. AFT analyses include both the determination of an AFT age and modeling of time-temperature histories based on the measured fission-track length distributions. Other complementary specialized techniques applied in the same lab are fluid inclusion microthermometry and fluorescence microscopy of hydrocarbon inclusions in salt.
The objective of Dr. Zentilli’s project is to better understand the thermal effects of buried salt structures on petroleum source and reservoir rocks in the geological past and present, and thus be able to guide petroleum exploration in areas with substantial salt accumulations In addition to determining the age of the source and reservoir rocks, results include determining the heat effects of salt and volcanic rocks and permeability of salt.
The Heat Effects of Salt Structures
Because rock salt conducts heat more effectively than other sedimentary rocks in a basin, the heat of the Earth escapes preferentially upward through salt structures. This causes rocks above these structures to heat up, which makes these areas more prospective at shallower depth than the surrounding rocks (e.g. Primrose Field, Scotian Basin). Conversely, in a region where source rock strata are hotter than the “oil window”, they are expected to produce only gas. Under a heat-conductive salt structure, the source rocks will be cooler and thus yield oil or condensate. This is the case in hydrocarbon field in the Sussex area of New Brunswick. Predicting these geothermal anomalies is important in a play within salt basins. Using the AFT and (U-Th)/He techniques, Dr. Zentilli’s team has detected and dated the geothermal anomalies above salt structures in the Magdalen Islands and the Sverdrup Basin. Figure 1 represents significant heating to well within the “oil window” in two basins that contain hydrocarbons. Thus the study demonstrates that the technique from the Halifax lab can detect paleogeothermal anomalies related to the presence of salt at depth, and determine the geological age of last heating. The team has also modeled positive and negative affects of volcanic activity on the hydrocarbon prospectivity of the Sverdrup and Magdalen Maritimes Basin.
Circulation of Fluids and Salt Structures
Traditionally, salt has been considered extremely impermeable, and thus a perfect seal for hydrocarbons and fluids of any kind, to the point that deep cavities in salt structures have been assessed as repositories for liquefied natural gas (LNG), oil, and industrial wastes. However, Dr. Zentilli’s team has documented the existence of several generations of hydrocarbon-rich brines in salt from offshore Atlantic Canada and salt mines in the Maritimes Basin (Figure 2), some of which have been entrapped at temperatures close to 100ºC. This discovery suggests that fluids were able to penetrate salt while the salt was at a few kilometres depth in the basin. These observations are compatible with recent experimental work done by others, which suggests that under certain conditions of depth (more than three kilometres), salt becomes as permeable to water-type fluids as sandstone. It appears that some fluids are able to circulate within and peripherally to salt structures. These fluids may play a role in carrying a large proportion of the earth’s heat detected precisely above salt structures. These results lead to a need for re-assessing hydrocarbon plays involving salt, and the suitability of salt as a repository for highly pressurized LNG. Salt crystals with fluid inclusions fracture at temperatures below -50ºC. The effect of the much colder LNG on the salt walls of the storage cavities needs to be investigated with regards to induced fracturing and the resulting leakage.
PRAC News
PRACtically Speaking’s First Anniversary!
PRAC issued its first bi-monthly newsletter in August 2005 and on its first anniversary we would like to know how we are doing. Please help us evaluate PRACtically Speaking by filling in a six question survey.
Board Approves Additional Funding
On July 5th, PRAC’s Board of Directors approved funding for an additional three projects received in Call 7. Successful applicants include:
Dr. Duncan McIlroy, Memorial University – Tidal Pumping, Matrix Infiltration and Early Diagenetic Cementation of Reservoir Sandstones
Dr. Paul Sylvester, Memorial University – Heavy Mineral Studies of Cretaceous and Tertiary Sandstones, Flemish Pass and Orphan Basins, Newfoundland and Labrador
Dr. Phil Bording, Memorial University – Atlantic Canada Basin Synthetic Imaging Modeling
PRAC will contribute $264,000 over three years to these projects. Together with three projects previously approved at Dalhousie University, Saint Mary’s University and Memorial University, Call 7 will sponsor research and development that will lead to a better understanding of the complexity of geology offshore Atlantic Canada as well as lead to innovative modeling techniques for application regionally and globally.
A total of $594,200 is committed to the Call which is leveraging an additional $2,173,986 in cash and in-kind contributions from external sources.
Energy Forum 2006
Delegates representing research, industry and government came to the Energy Research and Development Forum 2006 to forge partnerships and discuss how research and development will shape the future of energy in Nova Scotia.
Speaker biographies, abstracts and their presentations are posted online with kind permission from the authors. To see the presentations go to www.energyresearch.ca, click on the ‘Energy Research and Development Forum’ banner, and scroll down to ‘Biographies, Abstracts & Presentations’.
An online evaluation survey is available to Forum Participants in order to provide feedback on the quality of the 2006 Forum and to get suggestions for the next event. The survey can be accessed through the same menu selection at www.energyresearch.ca.
International Ocean Institute Training Program
Dalhousie University hosted the 26th International Ocean Institute training program on Ocean Governance: Policy, Law and Management from May 24th to July 21st.
PRAC organized a three day energy module as part of the program covering the global energy situation, oil and gas exploration and production, regulation of activities, environmental impacts and mitigation, non-conventional ocean energy resources, and the politics of oil and gas. PRAC would like to thank the fifteen speakers who did such an excellent job presenting to the class.
The training program attracted 17 participants from 16 different countries in Africa, Asia, the Caribbean, South America, and the South Pacific. They are generally mid-career professionals with a broad range of education and work experience in the social and natural sciences.
The course was intended to provide the participants with an overview of the many and varied aspects of the management of the earth’s ocean and resources, and to help them develop a familiarity with the broad range of issues encountered in marine affairs including those related to energy resources and their production.
Comments & Questions Welcomed!
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