Heavy oil accumulations in the Jeanne d'Arc Basin: a case study in the Hebron, Ben Nevis, and West Ben Nevis oil fields
Authors: Shimeld J W , Moir P N 
Published: November 2010

The possibility of encountering heavy oil (gravity less than or equal to 25 degree API) has generally been perceived as a significant risk to hydrocarbon exploration in the Jeanne d'Arc Basin, a risk that is heightened by the lack of comprehensive published
information on the subject. Hence, the processes responsible for heavy oil in the basin are evaluated in this report through integration of a broad range of studies, primarily focussing on the data-rich Hebron, Ben Nevis, and West Ben Nevis oil fields.

Compilation of new and previously published geochemical data demonstrates that biodegradation is the principal cause of heavy oil discovered in the basin to date, although early generation of heavy oil from sulphur-rich source rocks is suspected in a small number of cases. The main factors
controlling biodegradation through time are reservoir temperature, access to nutrients, salinity, hydrogen sulphide concentration, and the relative timing of key events such as hydrocarbon generation and charging of the reservoirs. Other causes of heavy oil are possible, but tend to be localized
phenomena that are difficult to evaluate quantitatively given the current level of information.

When evaluating the processes that may have led to biodegradation, events of the mid-Turonian ( about 90.5 Ma) prove to be very important. By this time, as confirmed by organic petrology and fluid inclusion studies, the major reservoirs of the Ben Nevis and earlier formations had been deposited and
had received a significant charge of hydrocarbons. The spatial distribution of paleotemperature, investigated both through forward modelling of basin processes and through inverse modelling of apatite fission track data, was favourable for biodegradation in every reservoir that is known to contain
biodegraded oil (i.e., temperature was greater than 60 degrees C). Also, seismic nterpretation reveals that significant erosion and possibly karstification occurred during the mid-Turonian. Indeed, with the exception of King's Cove A-26, the Petrel member is less than 20 m thick in every well
that encountered biodegraded oil. In certain portions of the basin the Petrel Member and underlying Nautilus Shale, which are conventionally considered seals, may have been breached by circulating meteoric water that supplied nutrients to the oil-bearing reservoirs. Together, these observations form
a template for evaluating the risk of biodegradation that can be applied quantitatively, after appropriate refinement of the models, to areas of the Jeanne d'Arc Basin that are outside the study area or to post-Petrel reservoirs.

Several results are of broader interest. In particular, fluid inclusion and geochemical evidence suggest two distinct pulses of oil entered Ben Nevis Formation reservoirs. Possibly related to this, apatite fission track analysis indicates that rapid heating during the Early Cretaceous was followed
by a period of significantly cooler temperatures between approximately the Aptian and Eocene and then by renewed heating that continues to the present-day. Further work is necessary to explore the significance of these results.