Integrated analysis of vitrinite reflectance, Rock-Eval 6, gas chromatography, and gas chromatography-mass spectrometry data for the Mallik A-06, Parsons N-10 and Kugaluk N-02 wells, Beaufort-Mackenzie Basin, northern Canada

Core and cuttings samples were selected from the Mallik A-06 (Mackenzie Delta), Parsons N-10 (Tuktoyaktuk Peninsula) and Kugaluk N-02 (Anderson Plain) wells of the Northwest Territories for Rock-Eval/TOC and vitrinite reflectance analysis. Selected Rock-Eval samples were extracted and analysed using gas chromatography and gas chromatographymass spectrometry. Rock-Eval parameters are strongly affected by sample contamination from drilling mud additives and migrated oil. For the Mallik A-06 well, most Rock-Eval pyrograms in the Iperk, Mackenzie Bay and Kugmallit sequences show evidence of drilling mud contamination. Approximately 50% of the pyrograms in the Richards and Taglu sequences are anomalous and GC-MS and GC-MS-MS analysis for selected extracts indicate extensive contamination by migrated and biodegraded Upper Cretaceous (Boundary Creek/Smoking Hills formations) derived oil. For the Parsons N-10 well, coal samples within the Iperk and Aklak sequences have anomalous pyrograms that are probably related to their low level of maturity. However, most pyrograms from the Cenozoic (Iperk and Aklak sequences) and Upper Cretaceous (Mason River, Smoking Hills and Boundary Creek sequences) successions show evidence of significant sample contamination. The upper part of the Albian Arctic Red formation shows similar contamination but there is an abrupt change below a depth of 2195 m where most pyrograms appear to be normal within the Lower Cretaceous-Upper Jurassic succession. For the Kugaluk N-02 well, all pyrograms have been disturbed by contamination from oil-based mud and yield low Tmax values as a result. Tmax thermal maturity estimates for the least disturbed pyrograms show very good agreement with measured vitrinite reflectance values for the Mallik A-06 and Parsons N-10 wells. Measured mean random vitrinite reflectance varies from 0.23 (Iperk Sequence) to 0.65 %RoR (Taglu Sequence) for the Mallik A-06 well and 0.25 (Iperk Sequence) to 0.66 %RoR (Husky Formation) for the Parsons N-10 well. A number of vitrinite reflectance measurements for samples from the Richards and Taglu sequences in the Mallik A-06 well have been suppressed due to staining by migrated Cretaceous-derived oil. The Paleozoic succession in the Kugaluk N-02 well is overmature with measured vitrinite reflectance values ranging from 1.6 (Imperial Formation) to 2.0 %RoR (Landry Formation) over a 960 m depth interval. Extrapolated maturity is 2.7 %RoR in the Franklin Mountain Formation at the base of this well. There is qualitative agreement between the expected and observed degree of apatite fission track annealing for apatite fission track samples in all three wells and this provides independent support for the thermal maturity estimates. Exponential curves were fit to the %RoR-depth data and extrapolated to an initial surface value of 0.2 %RoR to obtain estimates on the magnitude of erosion at each well location. For the Mallik A-06 and Parsons N-10 wells, the estimated thickness of strata eroded prior to the deposition of the Iperk Sequence is approximately 700 and 1300 m, respectively. For the Kugalik N-02 well, the modest maturity gradient implies that up to 8 km of strata may have been removed by erosion. INTRODUCTION The Geological Survey of Canada (GSC) is involved with a multi-disciplinary, industrygovernment funded study of petroleum systems of the Beaufort-Mackenzie Basin. This multi-year research project was initiated in December 2000 and work was done under the former GSC Project Approval System (PAS) (2001-2003), and the Earth Sciences Sector (ESS) Northern Resources Development (2003-2006) and Secure Canadian Energy Supply 1 (2006-2009) programs. Work is continuing under the ESS Geo-Mapping for Energy and Minerals (GEM) Program. As part of this research, thermal maturity (vitrinite reflectance, Rock-Eval Tmax) data are being acquired for key petroleum exploration wells across the basin to help constrain quantitative models of thermal history and petroleum generation. Geological recycling of organic matter is a common problem in the Mackenzie Delta region and therefore samples may contain multiple populations of coal macerals with different thermal maturity. Also, because cuttings samples are used, there is the potential for drillingrelated mixing of sample material from different depths (e.g. borehole caving, recirculation of cuttings through mud system) and contamination by organic mud additives. Migrated oil is another form of sample contamination. Some types of drilling-related contamination can be avoided by using core samples and this was done where possible. Petrographic analysis of samples was used to distinguish anomalous vitrinite reflectance values (due to caving, recycling and suppression by oil staining, for example) from primary in situ values. Sample contamination has a significant adverse effect on the quality of results from bulk analytical methods such as Rock-Eval pyrolysis. Rock-Eval pyrograms were used to identify contaminated samples and selected samples were extracted for organic geochemical analysis (gas chromatography and gas chromatography-mass spectrometry) to investigate the nature of the contamination. To improve on the quality of data and interpretations, we use an integrated approach to thermal maturity evaluation by assessing complementary data sets. For example, the RockEval Tmax and vitrinite reflectance data in this report provide independent measures of thermal maturity. Also, apatite fission track (AFT) thermochronological data are available for many of the key wells used in the Beaufort-Mackenzie petroleum systems study (to be published elsewhere). Fission tracks are linear regions of crystal damage that form continuously through geological time by the spontaneous fission decay of trace amounts of U within apatite crystals (e.g. Wagner and Van den Haute, 1992; Gallagher et al., 1998; Gleadow et al., 2002). They form with the same initial length (approximately 16 μm) but undergo length reduction (thermal annealing) and corresponding AFT age reduction at elevated temperatures, producing a distribution of AFT lengths that depends on the thermal history of a sample. Any discordance between the degree of AFT annealing and the level of organic maturity for a sample indicates a problem in the data that requires further investigation. We have used this criterion to re-examine several wells with vitrinite reflectance data that have been reported by other groups and included with the National Energy Board (NEB) well history reports at the GSC in Calgary. WELL LOCATIONS AND STRATIGRAPHY Figure 1 shows the location of the three onshore study wells in the Mackenzie Delta region. The Mallik A-06 well is located near the northern coast of Richards Island close to the Mallik L-38 gas discovery well (Dixon et al., 1994; NEB, 1998) and the Mallik gas hydrate research wells (Dallimore et al., 1999, 2005). The Taglu gas field, which is one of the three anchor fields for the proposed Mackenzie Valley gas pipeline, is situated immediately to the west of Mallik A-06. The Parsons N-10 well is located at the southwest end of Tuktoyaktuk Peninsula within the Parsons Lake gas field, another anchor field for the proposed Mackenzie Valley pipeline (Figure 1). The Kugaluk N-02 well is situated south of Tuktoyaktuk Peninsula on the southeastern basin margin within the physiographic region known as the Anderson Plain.