A new study by Applied Petroleum Technology (APT) presents strong evidence for a previously unknown Lower Jurassic petroleum system on the Southern Grand Banks of Newfoundland, offshore eastern Canada. This area has seen little exploration for 30 years. The study characterizes oil shows and potential source intervals in this area and calibrates their thermal maturity.
The Southern Grand Banks contains several Mesozoic basins that were formed by rifting and seafloor spreading associated with the fragmentation of Pangea in the Late Triassic that led to the formation of the Atlantic Ocean. This area was mostly explored in the 1966-1975 period with 27 unsuccessful wells, although some minor shows and stains were noted. All these wells were drilled on 2D seismic data of variable data quality, which may in part explain the lack of success to date. Only one well (St George J-55) in 1986 has been drilled in the area since then. A major reason for the lack of interest is that Upper Jurassic sediments are absent in much of this area because they were eroded during the Avalon Unconformity, which was the result of regional uplift. Rocks of this age are the principle source for much of the hydrocarbons in the North Atlantic area, including the producing regions of offshore eastern Canada such as the Scotian Shelf and Jeanne d’Arc basins.
The study suggests older rocks, within the Lower Jurassic that are equivalent to known source rocks in western Europe, have generated hydrocarbons. While this data does not mean economic quantities of petroleum are present in the area, it does provide evidence that they could exist and, hence justify, further exploration on the Southern Grand Banks.
The Southern Grand Banks gained the company’s attention because of some results in a large regional North Atlantic study, that included almost all the basins surrounding the North Atlantic, which APT completed in 2017. Oil staining was noted in samples from a subset of wells from the Southern Grand Banks area, the analysis of which provided geochemical evidence suggesting that the analyzed oil shows belong to a very different petroleum system to that documented elsewhere in offshore Atlantic Canada. It was decided to follow-up and see if there was any interest from the industry. After some positive feedback, it was decided to go ahead with the project that would create a new high-quality geochemical database to help further elucidate the potential of the Lower Jurassic in this area.
The study wanted to answer the questions:
- Were the reported oil shows in older reports genuine and do they show consistent geochemical characteristics?
- Do Lower Jurassic sections contain viable source rocks and how do these compare to the oil shows?
- How did the area evolve in terms of burial and thermal history?
The study concentrated on wells drilled in the Carson, Horseshoe, southern Jeanne d’Arc, and Whale basins.
Older information such as the ‘End of Well reports’ and wireline logs were first examined which enabled the identification of possible intervals to sample for staining and potential source rocks. APT used a proprietary approach to invert the available petrophysical logs to continuous logs of total organic carbon (TOC) and hydrogen index, which both helped focus where the sampled and gave a quantitative assessment of the net thickness of the potential source rock horizons penetrated. Samples were obtained from the Canada Newfoundland Labrador Offshore Petroleum Board (CNLOPB) Core Storage Centre. Although some of the wells were almost 50 years old, based on the condition of the cuttings boxes, some of these wells at the CNLOPB facility appeared to have never been sampled previously, indicative of the previous lack of interest in this area.
The samples were analyzed at APT’s laboratories in Oslo. Sediment samples were screened for source rock quality by TOC and Rock-Eval analyses. Hydrocarbons were extracted from stained intervals and potential source rock intervals for characterization by gas chromatography and gas chromatography-mass spectrometry. This allowed oil-source correlation, provided information on the depositional environment of the source rocks and the maturity of the staining hydrocarbons. The thermal maturity of sampled sediment intervals was determined using vitrinite reflectance and spore color. Apatite thermochronology data was also obtained to further constrain the thermal history of the area. Results were combined with those obtained for the area in the 2017 study.
Evidence of genuine oil staining was obtained from Lower Jurassic samples from a number of wells. These exhibit consistent geochemical signatures that clearly differentiate them from eastern Canada Upper Jurassic oils, including those from the central and northern Jeanne d’Arc basin. This suggests a common source rock, with biomarkers suggesting this is most likely a marl deposited under restricted conditions. Relatively thin Lower Jurassic source rock intervals within the Iroquois Formation were identified in two wells with TOC contents up to 6.7% and Type II-III organic matter.
Source rock extracts from these intervals have similar, but not identical, geochemical characteristics to the oil stains and are lower thermal maturity. It is suggested that more mature equivalents of these Lower Jurassic source rocks could be the source of the oil staining, implying that migration over some distance has occurred. When coupled with the oil stain observations, it is thought the source rocks that were responsible for the stains likely improve in quality away from well control. This is because the stains show characteristics indicating that their source rocks were deposited under more anoxic conditions than the organic rich intervals sampled in wells which were drilled on highs.
In the study, new vitrinite reflectance, spore color and apatite thermochronology data have been integrated and geological scenarios tested by 1D basin modeling. This has helped resolve the burial / thermal histories of the wells studied which do exhibit some complexity due to the presence of multiple unconformity events. The maturity gradients in the wells examined were relatively low, suggesting modest geothermal gradients. The Avalon unconformity does not appear to be an important factor for the maturity of the Lower Jurassic in most wells with a basal Tertiary unconformity, or very late Tertiary burial and uplift (glacial related?), more significant for understanding the timing of hydrocarbon generation.
Upper Jurassic intervals are the most prolific source rocks in the North Atlantic/North Sea area. However, Lower Jurassic source rocks are well known in western Europe, including those in the Paris basin, the Cleveland basin of northeast England, offshore Ireland basins, the Lusitanian basin, the conjugate margin to offshore Newfoundland and onshore and possibly offshore Morocco. There has also been recent evidence to suggest that there may be Lower Jurassic sourced hydrocarbons elsewhere offshore Atlantic Canada on the Scotian Shelf and Slope. These Lower Jurassic source rocks have largely developed within intra-continental basins formed during the incipient rifting of the North Atlantic. The data available to date suggests that while the occurrence of potential source rocks occur throughout the Lower Jurassic stratigraphy, the actual distribution of source rocks is more complex and is a controlled by a number of factors, meaning that their occurrence is discontinuous. The Southern Grand Banks Lower Jurassic petroleum system exhibits both similarities and differences with European analogues, as might be expected from the approximately time equivalent settings that span a range of distinct basins across a range of palaeolatitudes and paleogeographies.
The Southern Grand Banks area was previously not thought to be prospective because of the lack of Upper Jurassic source rocks. The results of this study indicate that the absence of these rocks need not be an impediment to exploration in this area. It confirms the presence of an older Lower Jurassic petroleum system that may have generated economic quantities of oil, as similar age rocks in western Europe have done on the other side of the North Atlantic. Establishing data that support the recognition of hitherto unrecognized Lower Jurassic source rocks on the western side of the Atlantic conjugate also has significance for regional exploration across the area.
Martin Fowler is head of APT’s Canadian operation. He has been at APT since 2013. Before that he worked at the Geological Survey of Canada for 25 years, including on offshore eastern Canada, and Talisman Energy.
Julian Moore joined APT in 2015 and is director of the company’s UK operation. Prior to that he worked at BP for 12 years, principally in Exploration.