Petro-Find Geochem Ltd has successfully conducted geochemical soil gas surveys for the exploration of bitumen reservoirs in the Athabaska oilsands region in Alberta and Saskatchewan. Petro-Find has extensive experience in the exploration of heavy oils along the Alberta-Sasktchewan border.
The McMurray Formation, which contain the bulk of heavy oil and bitumen deposits, lies on an angular unconformity that truncates the limestone and calcareous shales of the Devonian. In general, the McMurray formation accumulated in incised valleys that were formed by fluvial processes and subsequently transgressed by marginal-marine environments during an early Cretaceous sea-level rise. Thus, the Formation displays a continuum of sedimentary environments, from fluvial in the lower parts, to estuarine in the middle, to marine shoreface near the top.
The source of the oils is controversial, although it agreed the oils are biodegraded mature marine oils charged from pre-Cretaceous source rocks during the development of the Rocky Mountain Fold and Thrust Belt in late Cretaceous-Early Tertiary time. A foreland basin was developed as result of isostatic down warping of the crust, owing to the tectonic load imposed during mountain-building. When source rocks are present in the older platform sequences or in the foreland basin sediments themselves, this basin developed an enormous oil source kitchen, which produced vast volumes of oil. This oil migrated laterally up the sloping strata up to several hundred miles to structural and stratigraphic traps along the migration fairway and at the margins of the basin. At the flanks of the basin, reservoir sediments lie on unconformable older basement rocks and produce reservoirs that are shallow, cool and may have local meteoric water circulation, conditions ideal for biological activity. Biodegraded oils are trapped by immobility from the high density and viscosity resulting form their biodegradation.
The accompaning map of Alberta and Saskatchewan shows the bitumen-rich areas of Athabaska, Cold Lake and Peace River and the heavy oil province. Resources of bitumen, which cover an area of about 60,000km2, and heavy oil are well over a trillion barrels. Oils with a specific gravity of 10-20 API are classified as heavy oil and 10 or less, bitumen. The oils have been heavily biodegraded with up to 50% loss. Chemically, aside from the lengthening of hydrocarbons, the bitumen is deficient in hydrogen, relative to lighter oil. Therefore, this material must be upgraded by the addition of hydrogen to make lighter and more valuable synthetic crude.
These resources are regarded as unconventional because they cannot be recovered economically with conventional drilling and production/refining processes. In fact, some 10% of the Athabaska deposit is being produced by mining. While many of these technologies are well developed, bitumen/heavy oil can only be produced economically with high oil prices and removal of environmental and pipeline constraints. Thus, technology is under constant development to meet these challenges.
Traditionally,exploration has been carried out by pattern core drilling, which is expensive, time-consuming and provide relatively low resolution. Geophysical techniques can be used to help locate the thickest and richest of the oilsand deposits. Due to the shallow nature of the deposits, electromagnetic airborne techniques and electrical resistivity have been applied to find the best deposits. High resolution seismic techniques have shown promise for delineating finer details of the complex fluvial and estuarine sedimentary structure. (Source: Heavy and Bituminous Oils; D.R. Schmitt; University of Alberta)
A contour map of C2+ concentrations shows a discontinuous pattern along the edge of the Athabaska Oilsands deposit. It shows a NE trending fairway (mauve) with some major sweet spots (red). Several faults are indicated (can you see them?). The total concentration of C2-C4 alkanes and C2L-C3L alkenes or C2+ is the most useful indicator of the bitumen potential. The rationale for adding alkanes and alkenes is based on the fact that C2L (ethylene) and C3L (propylene), which occur in very small quantities in any naturally occurring petroleum, are significant in soil gas because of the intense biodegradation of ethane and propane, respectively.
The concentrations of methane and CO2 are also indicative of the underlying bitumen. It is postulated that both anaerobic and aerobic processes are at work, but this theory is best left to University researchers. A plot of methane and ethane plus ethylene is a polynomial curve with an R2 of 0.82. As previously explained the curve for normal oils is linear. The polynomial nature is due to the fact that the normal alkanes have been biodegraded to the extent that they have disappeared as CO2. The abnormally high average ratio of methane to ethane plus ethylene of 12 (which normally would characterize the hydrocarbons as condensates), is due to the high biodegradation of the light alkanes.
CO2 in soil gas is important as an overall indicator of the massive aerobic biodegradation of bitumen that is occurring even today. It defines the width and trend of the bitumen fairways but it is not a useful indicator of the bitumen potential directly underneath. This is because the CO2, produced in massive amounts, is soluble to some extent in ground waters in which it can migrate long distances laterally. In comparison, the light alkane/alkenes are only slightly soluble and migrate vertically as bubbles to the surface soils where they become trapped in the interstices of soils.
It is believed that soil gas sampling is a very cost-effective method to evaluate the bitumen potential in terms of both quality and quantity. This theory can be tested by more extensive soil gas surveys over a known bitumen reservoir where both the quality and quantity have been defined. Biodegradation produces systematic compositional changes including isotopic variations. N-alkanes are preferentially removed and iC4/nC4 ratios increase and delta13C2 and delta13C3 increase. Thus, isotopes in natural gas over a heavy oil or bitumen reservoir can be used to map out "sweet spots".
So far, soil gas surveys and seismic have played only a minor role in the development of the bitumen areas, mostly in the marginal areas where the reservoirs are not as thick and not as continuous. However, soil gas surveys have been successfuly applied in the exploration of heavy oil along the Alberta-Saskatchewan border, in both mature and little-explored areas. Soil gas surveys and seismic can also be used to find lost compartments and bypassed plays as well monitoring reservoirs in production using in situ technologies like steam assisted gravity drainage and cyclic steam stimulation.
PETRO-FIND GEOCHEM LTD
215 Mallin Crescent
Saskatoon, Saskatchewan, Canada S7K 7X3
Phone: (306)931-3156 Fax: (306)931-9773
PETRO-FIND GEOCHEM LTD