BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 53, NO. 4 (DECEMBER, 2005), P

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1 BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 53, NO. 4 (DECEMBER, 2005), P Post-Early Devonian thermal constraints on hydrocarbon source rock maturation in the Keele Tectonic Zone, Tulita area, NWT, Canada, from multi-kinetic apatite fission track thermochronology, vitrinite reflectance and shale compaction D.R. ISSLER Natural Resources Canada Geological Survey of Canada rd Street NW Calgary, AB T2L 2A7 dissler@nrcan.gc.ca A.M. GRIST Department of Earth Sciences Dalhousie University Halifax, NS B3H 4J1 L.D. STASIUK Natural Resources Canada Geological Survey of Canada rd Street NW Calgary, AB T2L 2A7 ABSTRACT New thermal maturity (%Ro, Rock-Eval pyrolysis), shale compaction and apatite fission track (FT) data were integrated into thermal history models for the East MacKay I-77 petroleum exploration well located approximately 80 km southeast of Norman Wells, Northwest Territories. The study well is in the Keele Tectonic Zone where multiple phases of anomalous Phanerozoic subsidence and uplift have resulted in an Upper Cretaceous Cenozoic foreland succession resting unconformably on Devonian strata. This major unconformity, which developed during pre- and post-albian time, displays a thermal maturity discontinuity ( %Ro) and represents approximately 270 m.y. of missing geological record. Linear shale compaction across this unconformity suggests that maximum burial occurred during the Cenozoic, whereas thermal maturity data imply that maximum temperatures were reached sometime between the Devonian and Early Cretaceous. Detrital apatite grains from a single sample from the Upper Devonian Imperial Formation of the I-77 well yielded two FT age populations (90.4±6.1 Ma and 222.2±22.5 Ma; ± one standard deviation) with different thermal annealing properties based on their chlorine content. An inverse multi-kinetic FT annealing model was developed and used to determine thermal histories that are consistent with the FT parameters and other geological constraints. Model results suggest that hydrocarbon generation from Devonian rocks at the I-77 well location occurred during the early Mesozoic prior to the development of Late Cretaceous Cenozoic structures. Peak FT model temperatures are 124±10ºC within the Early Triassic to Middle Jurassic ( Ma), <75ºC during the Albian ( Ma) and 97±9ºC during the Paleocene-Early Eocene (65 50 Ma). The Cretaceous Cenozoic thermal history was modelled using a simple burial history with the present geothermal gradient (32ºC/km) held constant; a range of geothermal gradients (31 42ºC/km) and maximum burial depths for the pre-aptian thermal history fit Devonian maturity data. If maximum burial was during the Cenozoic, then Mesozoic peak maturity was achieved under a higher geothermal gradient than present. Although hydrocarbon generation pre-dates structural trap development near the I-77 well, Devonian source rocks retain significant hydrocarbon potential. Given the complicated geological history of the central Mackenzie Valley, deeper Cenozoic burial elsewhere in the region may have generated hydrocarbons from Cretaceous and reactivated Devonian source rocks. RÉSUMÉ Une nouvelle maturité thermique, (%Ro, pyrolyse Rock-Eval ), un compactage d argile litée et des données de trajectoires de fissions apatites (FT) ont été intégrés à des modèles thermiques historiques pour l exploration pétrolière du puits East MacKay I-77 situé approximativement à 80 km Sud-Est de Norman Wells, dans les Territoires du Nord- Ouest. L étude du puits se trouve dans la zone tectonique de Keele dans laquelle des phases multiples de subsidences et d ascendances anomales du Phanérozoïque ont abouti à une succession d avant-chaîne du Crétacé-Cénozoïque supérieur recouvrant de façon discordante les strates du Dévonien. Cette discordance majeure, qui s est développée au cours de la période du pré- et post-albien, démontre une discontinuité de la maturité thermique (de 0.55 à 0.75 %Ro) et représente un manque d enregistrement géologique approximatif de 270 m.y. Le compactage linéaire d argile litée à travers cette discordance suggère qu un enfouissement maximal se soit produit durant le Cénozoïque, alors que les données de maturité thermique impliquent que des températures maximales ont été atteintes épisodiquement entre le Dévonien et le Crétacé précoce. Des grains d apatite détritique prélevés d un seul échantillon de la Formation Impériale du Dévonien 405

2 406 D.R. ISSLER, A.M. GRIST and L.D. STASIUK supérieur du puits I-77 ont donné deux âges de populations FT (90.4±6.1 Ma et 222.2±22.5 Ma; ± une déviation standard) avec des propriétés de températures de recuit différentes basées sur leurs teneurs en Cl. Un modèle inverse multi-kinétique FT de recuit a été développé et utilisé afin de déterminer les historiques thermiques compatibles avec les paramètres FT et d autres contraintes géologiques. Les résultats du modèle suggèrent que la génération d hydrocarbures, provenant des roches du Dévonien présentes dans le puits I-77, est apparue durant le Mésozoïque précoce avant le développement des structures du Crétacé-Cénozoïque tardif. Les pointes de températures FT du modèle indiquent 124 ± 10ºC à l intérieur du Triassic précoce au Jurassique moyen ( Ma), <75ºC durant l Albien ( Ma) et 97±9ºC durant le Paléocène-précoce Eocène (65 50 Ma). L historique thermique du Crétacé-Cénozoïque a été modélisé en utilisant une histoire simple d enfouissement avec l actuel gradient géothermique (32ºC/km) maintenu constant; une gamme de gradients géothermiques (31 42ºC/km) et des profondeurs d enfouissement maximales concernant l historique thermique du pré-aptien concordent avec les données de maturité du Dévonien. Si un enfouissement maximum s est manifesté durant le Cénozoïque, alors la pointe de maturité du Mésozoïque s est accomplie sous un plus fort gradient géothermique qu actuellement. Bien que la génération d hydrocarbures indique une époque antérieure du développement de pièges structuraux proches du puits I-77, le roches mères du Dévonien conservent un potentiel notable d hydrocarbures. Compte tenu de l historique géologique compliqué de la Vallée centrale de Mackenzie, l enfouissement plus profond du Cénozoïque ailleurs dans la région, aurait pu générer des hydrocarbures provenant du Crétacé et avoir ainsi réactivé les roches mères du Dévonien. Traduit par Gabrielle Drivet INTRODUCTION With increased hydrocarbon exploration activity in frontier regions north of the mature Western Canada Sedimentary Basin (Mossop and Shetsen, 1994), it has become apparent that new geological data are needed to help minimize exploration risk. For example, in the Tulita (formerly Fort Norman) area of the Northwest Territories (Fig. 1), timing of hydrocarbon generation with respect to trap formation is a major uncertainty affecting exploration decisions. Multiple phases of tectonism and associated erosion have eliminated substantial portions of the stratigraphic record from the region. Major erosion ( 1 km of strata) in the study area occurred within four main time intervals: latest Silurian to earliest Devonian, post-devonian to pre- Aptian, late Albian to early Cenomanian, and Cenozoic (post-paleocene) (Fig. 2). Limited organic geochemical and thermal maturity studies have previously identified mature Paleozoic source rocks and marginally mature Cretaceous source rocks (Snowdon et al., 1987; Feinstein et al., 1988a, b, 1991; Snowdon, 1990; Earnshaw and Grant, 1992). However, these data provide poor constraints on the timing of maximum temperature and hydrocarbon generation (e.g., Feinstein et al., 1996) and thus the thermal history of this region is generally poorly understood. In this paper, we present new apatite fission track (FT) and organic maturity (%Ro, Rock-Eval Tmax) data for the Northrock et al. East MacKay I-77 well, situated southwest of Tulita, Northwest Territories, between the MacKay Range and the north south trending East MacKay Fault (Fig. 1). The FT data, from a single sample of Devonian sandstone from a dominantly carbonate and shale succession, are intriguing because they comprise two separate grain-age populations with different annealing temperatures that behave as independent thermochronometers, thereby enhancing resolution of the post-devonian thermal history. Furthermore, the results of multi-kinetic FT modelling suggest that maximum temperatures within Paleozoic strata were achieved sometime during the Early Triassic to Middle Jurassic at this location even though strata of this age are absent from the study area and surrounding regions. Previous interpretations had assumed that maximum temperatures in this area coincided with maximum burial during Late Cretaceous Cenozoic orogenesis (Feinstein et al., 1996). GEOLOGICAL SUMMARY Figure 1 shows the location of the East MacKay I-77 well and major tectonic and structural elements within the study region. Regional stratigraphic relationships are shown in Figure 2. Aspects of the geology of the study area have been described in numerous publications (e.g., Cook, 1975; Cook and Aitken, 1975; Yorath and Cook, 1981; Aitken et al., 1982; Gabrielse and Yorath, 1991; Dixon, 1999). Overviews of the geology and tectonic history of the study region by MacLean and Cook (1999) and Williams (1989) form the basis for the summary below. TECTONIC HISTORY OF KEELE TECTONIC ZONE The East MacKay I-77 well lies within the north-trending Keele Tectonic Zone (KTZ; Fig. 1), a spatially and temporally variable region that experienced multiple phases of amplified subsidence (where sediments are deposited in a trough ) and emergence (where sediments are eroded across an arch ) throughout the Phanerozoic (MacLean and Cook, 1999). The KTZ is underlain by Neoproterozoic strata of the Mackenzie Mountains Supergroup which are themselves overlain unconformably by variable thicknesses of Lower Cambrian strata (Fig. 2). Early Cambrian extension resulted in the thick accumulations of Cambrian to Middle Ordovician strata of the Mackenzie Trough (Williams, 1989) (Fig. 3). During the latest Silurian to earliest Devonian, the trough evolved into the pre- Devonian phase of the Keele Arch (Fig. 2) with perhaps 1 km of strata eroded from its crest (Cook, 1975; Williams, 1989).

3 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 407 Fig. 1. Map showing the main tectonic and structural elements of the study area (modified from MacLean and Cook, 1999). Geology is from Yorath and Cook (1981). Location of Canadian Interior Plains on inset map of Canada is from Bostock (1970). The lower panel shows the location of the East MacKay wells and a seismic section shown in Figure 3. Grey shaded region approximates the Cretaceous expression of the Keele Tectonic Zone as defined by the absence of upper Aptian Albian strata (modified from Williams, 1989). The dotted-dashed line shows the boundaries of the Brackett Basin (after Williams, 1989). Summit Creek B-44 is a recent gas/condensate discovery well.

4 408 D.R. ISSLER, A.M. GRIST and L.D. STASIUK Fig. 2. Regional stratigraphic chart modified from MacLean and Cook (1999), based on discussions with J. Dixon (pers. comm., 2002). The area underwent renewed subsidence and burial by Devonian (including organic-rich shale of the Hare Indian and Canol formations; Fig. 2) and possibly younger Paleozoic to early Mesozoic strata; the youngest preserved Paleozoic strata are Upper Devonian (Frasnian) in age (Imperial Formation; Fig. 2). Prior to late Aptian Albian time, the region was uplifted, tilted westward and deeply eroded, removing strata of unknown thickness and age. Subsidence and sedimentation resumed with upper Aptian Albian strata covering most of the northern Interior Plains (Fig. 1; Dixon, 1999) but, by late Albian to early Cenomanian, subsequent widespread erosion resulted in a patchy distribution of preserved Lower Cretaceous sediments. Differential uplift of the KTZ caused complete erosion of at least one kilometre of upper Aptian Albian strata (grey shading in Fig. 1; Cook, 1975; Williams, 1989) and variable erosion of the underlying Paleozoic succession. About one kilometre of Imperial Formation is preserved below the pre- Aptian unconformity in the Peel Trough west of the I-77 well (Fig. 1; Pugh, 1993; MacLean and Cook, 1999). An Upper Cretaceous Cenozoic foreland succession was deposited across the northern Interior Plains but much of it was removed by post-paleocene erosion. Feinstein et al. (1996) estimate that 1 to 3 km of Upper Cretaceous Cenozoic strata were eroded from the region. In the study area, an anomalously thick succession of Upper Cretaceous Paleocene strata (approximately 2 km) is preserved in the Brackett Basin (Sweet et al., 1989) which overlies the KTZ (Fig. 1). Many of the structures in the area formed in association with the movement of Cambrian salt during Late Cretaceous Cenozoic Cordilleran deformation but earlier-formed salt-influenced structures are documented as well (MacLean and Cook, 1999). EAST MACKAY SEISMIC PROFILE Figure 3 is an interpreted seismic profile (MacLean and Cook, 1999) showing the projected location of the East MacKay I-77 well and the positions of the older I-55 and B-45 wells (see Fig. 1 for seismic line and well locations). The East MacKay Fault juxtaposes Upper Devonian Imperial Formation in its footwall with Upper Cambrian to Middle Ordovician Franklin Mountain Formation in its hanging wall (Fig. 3); major fault motion is poorly constrained as post-late Devonian and pre-late Cenomanian to Turonian. Above the eroded hanging wall side of the fault, the Cretaceous Slater River Formation rests directly on the Franklin Mountain Formation; to the west, where a thicker Devonian section is preserved, it overlies the Imperial Formation. Clearly, the thermal history differs on either side of the fault and such variations are to be expected in the central Mackenzie Valley due to its complicated geological history.

5 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 409 Fig. 3. Interpreted portions of seismic line 105 (N.S.M. Resources, 1983) and line 36X (Chevron Canada Resources, 1992) through the East MacKay structure, showing the location of the Conoco et al. East MacKay I-55 and Candel DECKMG et al. East MacKay B-45 wells (after MacLean and Cook, 1999). The position of the Northrock et al. East MacKay I-77 well is only approximate because it is an oblique projection from several kilometres to the north. See Figure 1 for location. EMF - East MacKay Fault; MT - Mackenzie Trough; Pm - Mackenzie Mountains Supergroup; Cclc - Mount Clark and lower Mount Cap fms; Cg - Glossopleura unit (upper Mount Cap Fm); Css - Saline River Fm salt; Csu - upper Saline River Fm; COf - Franklin Mountain Fm; Db - Bear Rock Fm; Dbh - Bear Rock and Hume fms; Dhc - Hare Indian and Canol fms; Di - Imperial Fm; K - Cretaceous; Ksl - Slater River and Little Bear fms; Ke - East Fork Fm; T - Tertiary (part of Summit Creek Fm). The B-45 well encountered 20º API gravity oil, locally derived from bituminous shale of the Slater River Formation (Feinstein et al., 1988a), within the fractured Ordovician cherty unit of the Franklin Mountain Formation. Although the Slater River Formation is only marginally mature, it has a high sulphur Type II kerogen that can generate oil at relatively low thermal stress (Earnshaw and Grant, 1992). It seems reasonable to infer that the heavy oil at B-45 was generated during maximum Cenozoic burial of the Slater River Formation. In contrast, the I-77 and I-55 wells were abandoned, having encountered mature source rocks, but lacking suitable hydrocarbon-bearing reservoirs. Although reservoir porosity, permeability and sealing characteristics influence exploration success, the poorly resolved thermal history may be an important factor as well. SAMPLES AND ANALYSIS METHODS Cuttings samples from the I-77 well were analyzed using organic maturity (Geological Survey of Canada, Calgary) and apatite FT (Dalhousie University, Halifax) techniques to better understand the thermal conditions for hydrocarbon generation in the study region. A single FT sample was collected to provide time-temperature constraints, whereas 44 vitrinite reflectance samples were acquired to better resolve the depth variation in maximum paleotemperature (see Mukhopadhyay and Dow (1994) and Taylor et al. (1998) for methods and applications of vitrinite reflectance). Additional data on organic-richness and maturity are provided by Rock-Eval 6/TOC analysis (see Lafargue et al. (1998) and Behar et al. (2001) for details of the Rock-Eval 6 instrument, experimental procedures and applications to hydrocarbon exploration). Log data were used for sediment decompaction (method of Sclater and Christie, 1980, modified for linear compaction by Issler, 1987) and shale compaction analysis (Issler, 1992; Feinstein et al., 1996) in order to constrain burial heating rates and to estimate maximum burial depths, respectively. Details of the FT method relevant to understanding the data of this study are presented below.

6 410 D.R. ISSLER, A.M. GRIST and L.D. STASIUK APATITE FISSION TRACK ANALYSIS Overviews of the FT method and its applications (Wagner and Van den Haute, 1992; Gallagher et al., 1998; Gleadow et al., 2002) form the basis for the following discussion. Apatite FT thermochronology is based on the measured density and etchable length distribution of linear tracks of crystal damage in apatite produced by the spontaneous fission decay of trace amounts of 238 U. FT density provides a measure of mineral age with respect to a low-temperature thermal history; complete erasure of apatite FTs occurs at temperatures in the range of 100 to 150 C over geologic time (depending on apatite chemical composition and heating rate). Corresponding etchable track length distributions contain information on the thermal history of the mineral because track lengths shorten by the temperature-dependent annealing of FT damage. FT ages record geologically meaningful events (e.g., passage through a specific temperature) only for the case of simple rapid cooling below the total annealing temperature. For more complicated thermal histories, as presented herein, FT ages represent partially annealed ages that should be interpreted using an appropriate annealing model for age and track length reduction. Fluorapatite (most common form of apatite in igneous, metamorphic and sedimentary rocks, e.g., Pan and Fleet, 2002) is generally the least resistant to thermal annealing. Various cation and anion substitutions can influence apatite FT annealing (e.g., Green et al., 1986; Crowley et al., 1991; Carlson et al., 1999; Barbarand et al., 2003; Ravenhurst et al., 2003) but the effect of chlorine is best documented and probably accounts for much of the variability in annealing behaviour. In general, resistance to annealing increases with increasing chlorine content except at very high chlorine concentrations where resistance is less than for more intermediate compositions (Carlson et al., 1999; Gleadow et al., 2002; Kohn et al., 2002). Apatite solubility is related to its composition and, therefore, etch figures (cross-section of FT etch pits at the etched mineral surface) should show some correspondence to annealing behaviour. Dpar, the arithmetic mean maximum diameter of FT etch figures parallel to the crystallographic c-axis (Donelick, 1993; Burtner et al., 1994), is a popular microscope-based measurement for constraining FT annealing properties in the context of a multi-kinetic annealing model (Ketcham et al., 1999, 2000). Sample FT age and length analysis and kinetic parameter determination Four 4 oz. vials of washed well cuttings samples from a Devonian sandstone (Imperial Fm) in the I-77 well were obtained over the depth interval 1730 to 1780 m, and processed in multiple stages for FT age and length measurement. Apatite mineral separates were obtained using standard mineral separation procedures (jaw crusher, disk mill, magnetic and heavy liquids) and analytical methods for FT etching, sample irradiation and age and length measurement follow those of Grist and Zentilli (2003). The external detector method (Fleischer et al., 1964; Hurford and Green, 1982) was used for FT age determination; ages were calculated using a weighted mean zeta calibration factor (Fleischer et al., 1975; Hurford and Green, 1983) based on the Fish Canyon Tuff and Durango apatite age standards. FT lengths were obtained from three age mounts and two separate length mounts. The length mounts were irradiated with 252 Cf to create more etchant pathways to increase the number of observed horizontal confined tracks (Donelick and Miller, 1991) and thereby reduce the statistical uncertainty on length distributions. Initial sample processing produced silt- to sand-sized apatite grains with predominantly Cretaceous ages (Grist, 2000; Issler, 2002), and final processing yielded abundant silt-sized grains with Jurassic to late Paleozoic ages (Issler and Grist, 2005). FT lengths obtained from Cf-irradiated apatite mounts in this study have no corresponding age information to aid interpretation. Furthermore, the broad range of grain ages implies that there is more than one population of apatite. This range in ages may be due to differences in annealing behaviour or may simply represent different provenances. In order to group the length and age data into appropriate populations for modelling purposes, kinetic-related parameters for FT annealing (Dpar and elemental data) were obtained for each grain for which ages or lengths were measured. Cl and other selected elemental data, expressed as atoms per formula unit (apfu), were acquired using the JEOL 733 electron microprobe at Dalhousie University (see Issler and Grist, 2005, for details of analysis method and data processing). Dpar measurements were done using the same microscope configuration as for track length measurements (Grist and Zentilli, 2003) and, where possible, are based on an average of four measurements per grain. THERMAL MATURITY SAMPLE AND WELL DATA Rock-Eval /TOC analysis Figure 4 shows a plot of selected Rock-Eval parameters (Tmax, Production Index - PI, total organic carbon - TOC, Hydrogen Index - HI) with depth for well cuttings samples collected at 10 m intervals in the I-77 well (see Issler et al., 2005, for analysis procedures and the complete set of Rock-Eval data). In Figure 4, Rock-Eval 6 data are presented using the format for Rock-Eval 2 data (see Peters, 1986, for guidelines on interpreting Rock-Eval 2 data). Tmax is a thermal maturity indicator that, under ideal conditions of uniform organic matter type and abundance and continuous burial, shows a progressive increase with depth. Ideally, PI indicates the amount of in situ hydrocarbon generation as a function of thermal maturity and organic matter type and richness, provided that the sample has not lost (expulsion) or gained (impregnation through migration) hydrocarbons. HI measures the hydrogen content of organic matter and thus indicates source rock potential. Rock- Eval pyrolysis records the contribution of all organic material in the well cuttings samples; organic matter recycling and mixing and other forms of contamination associated with drilling and sample recovery can affect Rock-Eval parameters.

7 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 411 Fig. 4. Selected Rock-Eval pyrolysis parameters with depth for cuttings samples from the East MacKay I-77 well. Some key parameters are: S1 - hydrocarbons (HCs) volatilized at 300 o C during sample pre-heating; S2 - HCs evolved from sample during ramped heating from 300 to 600 o C; Tmax - temperature at peak HC generation on S2 curve; PI - Production Index (S1/(S1+S2)); TOC - total organic carbon; HI - Hydrogen Index (S2/TOC x 100). The vertical dashed lines indicate thresholds for organic maturity (Tmax=435; PI=0.1) and good HC source rock attributes (TOC>2 4wt%; HI>300).

8 412 D.R. ISSLER, A.M. GRIST and L.D. STASIUK Potential hydrocarbon source rocks of the basal Cretaceous Slater River Formation (radioactive shale) and the Devonian Canol and Hare Indian (including Bluefish Member) formations are indicated by their high TOC and HI values (Fig. 4). The high sulphur content of kerogen and its associated low activation energy for hydrocarbon generation has suppressed Tmax values within the basal Slater River unit (Fig. 4) (Earnshaw and Grant, 1992). Well cuttings from this organicrich interval were retained in the drilling mud due to a change in its density and viscosity (A. Stirrett, pers. comm., 2002) and this contaminated the organically-lean cuttings of the Imperial Formation (Fig. 4). Most of the Cretaceous section is contaminated by higher maturity, recycled Devonian and Carboniferous organic matter. Only coal samples (indicated by high TOC values) represent true in situ organic maturity for the Cretaceous section (Fig. 4). Vitrinite reflectance Figure 5 shows a plot of indigenous and equivalent vitrinite reflectance versus depth for the I-77 well (data in Table 1; also see Stasiuk et al. (2003) for data and methods). As an additional check on the measurements, average Tmax values for the Paleozoic formations (excluding the contaminated Imperial Formation samples) were converted to %Ro equivalents using the equation, (1) %Ro = Tmax derived using thermal maturity data from the Devonian Winterburn Group and Wabamun Formation of the Western Canada Sedimentary Basin (Stasiuk and Fowler, 2002; Fowler et al., 2003). For Cretaceous coal, Tmax was converted to %Ro equivalent using the polynomial equation, (2) %Ro = c 1 (Tmax) 3 c 2 (Tmax) 2 + c 3 (Tmax) c 4 where c 1 = x10 6, c 2 = x10 3, c 3 = and c 4 = Equation 2 was determined by fitting tabulated data for type III organic matter included with the MATOIL TM (1990) basin modelling software. Calculated %Ro values generally show a good correspondence with the measured values (Table 1). For the Cretaceous section, calculated %Ro is shown only for coal due to the dominance of reworked organic matter (Table 1). Tmax varies from 431 to 441 (about %Ro equivalent) for the majority of Cretaceous samples, whereas measured %Ro on primary vitrinite and bitumen ranges from 0.36 to 0.58%Ro. Equations were fitted to the %Ro-depth data in order to characterize maturity trends (Fig. 5). There is a 0.2% difference in vitrinite reflectance across the Cretaceous Devonian unconformity (Fig. 5). The FT sample reached 0.82%Ro during pre- Cretaceous burial (interpolated using Devonian maturation trend) and was reheated to 0.60%Ro equivalent during Cenozoic burial (determined by extrapolation of Cretaceous maturity trend below unconformity). Projection of the %Rodepth equations to an assumed zero coalification value of 0.2% gives 2470 m and 1230 m for pre-late Cretaceous and post- Cretaceous erosion, respectively. APATITE FISSION TRACK PARAMETERS Table 2 lists track count, age and kinetic parameter (Cl content, Dpar) data for 35 apatite age grains from the I-77 FT sample. Measured Dpar values were corrected to equivalent Dpar values of Carlson et al. (1999) for multi-kinetic modelling purposes. This is meant to compensate for differences in laboratory etching conditions and observer bias and was achieved by multiplying measured Dpar values by the ratio of Carlson et al. s (1999) average Dpar value for the Durango apatite age standard by our corresponding value (Table 2). The same approach was used to correct FT length measurements; the effect is to increase both FT lengths and Dpar values by 5%. Electron microprobe and Dpar data for FT age and length parameters are in Issler and Grist (2005). Fig. 5. Organic maturity profile for East MacKay I-77 as determined by organic petrology. Solid diamond symbols are true vitrinite reflectance whereas solid circles are vitrinite reflectance equivalent derived from primary solid bitumen reflectance measurements (Jacob, 1985; Stasiuk et al., 2003); both measurements show good agreement where they coincide. Horizontal bars represent standard deviations on multiple measurements. Fission track age data Figure 6 is a radial plot (Galbraith, 1988, 1990) of all apatite FT grain ages with Cl measurements (Table 2) that was generated using the program Binomfit (Brandon, 2002) which uses the binomial peak-fitting method of Galbraith and Green

9 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 413 Table 1. Vitrinite and vitrinite-equivalent reflectance and Rock-Eval Tmax data for well cuttings from the East MacKay I-77 well.

10 414 D.R. ISSLER, A.M. GRIST and L.D. STASIUK Table 2. Apatite age and kinetic parameter data, East MacKay I-77 FT sample.

11 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 415 (1990) to resolve mixed age populations. Two statistically significant age populations are identified for the I-77 FT sample: 87.1±6.3 and 215.7±25.6 Ma (± one standard deviation; Fig. 6). The FT grain age data were plotted with respect to Dpar (Fig. 7A) and Cl (Fig. 7B) to determine whether these age components could be resolved simply on the basis of their inferred compositionally-related, thermal annnealing behaviour. There is a weak correlation between Dpar and Cl content for the I-77 apatite grains (similar slope to that derived from the kinetic relationships of Ketcham et al., 1999) but the data are scattered (Issler and Grist, 2005). Although Dpar has been shown to be a useful kinetic parameter in other studies (e.g. Burtner et al., 1994; Ketcham et al., 1999), age populations were best resolved for this sample using Cl content. In Figure 7A, grain ages were grouped visually over selected Dpar ranges and Binomfit was used to calculate age populations. This yielded an older (234 Ma) and younger (77 Ma) population with an inferred intermediate age population (138 Ma) that, within error, could belong to either of the other two populations. The older and younger grain ages overlap with respect to Dpar in Figure 7A and this, in part, may result from an unknown amount of sample contamination as suggested by the Rock-Eval data (Fig. 4; the borehole was uncased below a depth of 615 m). Also, the extension of the low-age population to higher Dpar values (Fig. 7A) can be attributed to OH substitution (Issler and Grist, 2005) as seen in the data of Carlson et al. (1999). As a result, this parameter was not used for multi-kinetic modelling purposes. In contrast, two age populations similar to those in Figure 6 are separated at a Cl value of apfu (0.45 wt%); similar to the Cl content of the Durango apatite age standard (Young et al., 1969). The youngest, fluorapatite population (kinetic population 1) consists of 20 silt- to sand-sized grains with a pooled FT age of 90.4±6.1 Ma; the older, chlorapatite population (kinetic population 2) is comprised of 11 silt-sized grains with a pooled age of 222.2±22.5 Ma (Table 2; Fig. 7B). Both age groups pass the χ 2 test for a uniform age population with high probabilities (Q>0.05, Table 2). Based on these results, 31 of the 35 grain ages were grouped into two age populations (Table 2). Grain 1 has the youngest FT age and, although it has no Cl data, its small Dpar value suggests that it is an end-member fluorapatite (Table 2). It has no associated track length data and was not included with kinetic population 1 because Binomfit analysis identified it as a separate age component. This relatively high precision grain was either completely reset during maximum Cenozoic burial or it has caved from higher up in the Cretaceous section. Grains 33, 34 and 35 (Table 2) are outliers as well; they have high Cl concentrations and high Dpar values (suggesting a higher resistance to annealing) but young FT ages (Fig. 7B), and are either caved from the Cretaceous section or have abnormal annealing behaviour. Visually, the low Cl, low precision grain 12 (201 Ma; Table 2, Fig. 7B) would appear to belong to kinetic population 2. However, it has no associated lengths and little effect on the calculated age of kinetic population 1, so it was retained in this grouping. Fission track length data Laboratory annealing experiments on apatite have shown that simultaneously formed fission tracks become shorter with increasing angle with respect to the crystallographic c-axis and that track length anisotropy increases with greater annealing (e.g., Green et al., 1986; Galbraith and Laslett, 1988; Galbraith et al., 1990; Donelick, 1991). This variation in FT length with track orientation can be minimized by projecting lengths to a common orientation parallel to the c-axis using a modified ellipse model (Donelick et al., 1999; Ketcham, 2003). Both measured (Fig. 8A) and projected (Fig. 8B) FT lengths show little structure with respect to Cl content for the I-77 FT sample; the similar mean FT lengths for both kinetic populations shows that they experienced similar degrees of annealing but at different absolute temperatures. The effect of contamination on track length measurements cannot be quantified easily, but is likely to be minor. Based on the age data (Table 2; Fig. 7B), potential contamination and mixing of age populations is on the order of 10 to 15%. However, only a single track length measurement was associated with the outlier grains identified above and this was not used for modelling. Furthermore, the general similarity in lengths between both age populations (Fig. 8) suggests that a small amount of random mixing should have little effect on length distributions. Fig. 6. Radial plot of FT ages for 34 probed apatite age grains from the sandstone cuttings sample of the Devonian Imperial Formation in the East MacKay I-77 well. Note that points closest to the age axis are based on the highest number of counts and, therefore, have higher precision than grains closer to the origin. A line drawn from the origin and through a given data point yields the FT age of the corresponding apatite grain. The two lines represent the ages of two apatite FT populations that were resolved using the computer program, Binomfit (see text).

12 416 D.R. ISSLER, A.M. GRIST and L.D. STASIUK Fig. 7. Apatite FT grain ages (symbols plus one standard deviation error bars) versus (A) Dpar and (B) Cl content for the East MacKay I-77 FT sample. Age populations show more overlap with respect to Dpar than Cl. Possible causes for overlap may include sample contamination by cavings, analytical uncertainty and the effect of OH substitution on mineral etching. Old grains (filled squares) are silt size; younger grains (filled circles) range from silt to sand size (see text). Fig. 8. Plot of (A) measured FT length and (B) FT length projected parallel to the crystallographic c-axis versus measured Cl content. The vertical line at apfu Cl is used to divide two separate length populations. The horizontal lines represent mean track length values for each population.

13 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 417 SHALE COMPACTION Table 3 contains depth-averaged, sonic and density log data for organically-lean shale from the I-77 well. Density porosity is for intervals with borehole enlargement of <20 mm (determined by the density caliper) and bulk density corrections of 100 kg/m 3 or less (Table 3). Sonic porosity decreases linearly with depth at a rate of approximately 10%/km (Fig. 9), similar to the normal shale compaction trend for the Beaufort-Mackenzie Basin (Issler, 1992). Compaction appears to be continuous across the Devonian Cretaceous unconformity (density porosity values are similar above and below unconformity, Table 3), in accordance with the observations of Feinstein et al. (1996) for other wells in the region. This implies that maximum burial was during the Cenozoic or that pre-cretaceous and Cenozoic maximum burial depths were similar. Using an initial surface porosity of 40% at maximum burial (Beaufort-Mackenzie value), estimated Cenozoic erosion is almost 1750 m. This is approximately 500 m greater than the amount inferred from %Ro data (Fig. 5) but both methods show reasonable agreement within error. THERMAL MODELLING Two modelling approaches were used to constrain the post- Devonian thermal history at the I-77 well. The first method uses an inverse FT model with combined temperature, thermal maturity and stratigraphic constraints to derive a range of timetemperature paths that are consistent with the FT data. The second method integrates thermal maturity, temperature, shale Table 3. Well log parameters and calculated porosity values for selected shale intervals from East MacKay I-77.

14 418 D.R. ISSLER, A.M. GRIST and L.D. STASIUK compaction and FT constraints in simple forward thermal / burial history models for selected stratigraphic layers. APATITE FISSION TRACK INVERSE THERMAL MODEL Model description We use an updated version of the inverse FT model of Issler (1996a). Issler (1996a) modified an earlier version of Willett s (1997) FORTRAN model to make it more user-friendly and to extend its capabilities. Major changes include (1) development of a model set-up program for easier incorporation of temperature and rate constraints and other model parameter specifications; (2) development of a program for displaying and plotting model output; (3) random temperature selection based on rate of temperature change (using a fixed time grid of arbitrary spacing) rather than absolute temperature; (4) incorporation of pulse-type temperature histories that allow for specification of the number of random heating / cooling events over selected time ranges; and (5) a time step predictor function (Issler, 1996b) for accurate and faster evaluation of the FT annealing equation. The model used for this paper has been upgraded to include (1) %Ro data and calculations (EASY%R o kinetic model of Fig. 9. Sonic porosity versus depth for selected shale intervals for the East MacKay I-77 well. Note that compaction appears to be continuous across the Devonian Cretaceous unconformity, implying that maximum burial thicknesses were achieved during Cenozoic foreland burial. Sweeney and Burnham (1990) or the TTI calibration of Issler (1984)) for constraining the cumulative heating effects of FT thermal histories; (2) multi-kinetic annealing using Dpar or Cl content (Ketcham et al., 1999, 2000) with simultaneous modelling of up to four different kinetic populations in a sample; and (3) a non-directed Monte Carlo method (versus the controlled random search method of Willett, 1997) for random thermal history generation. A priori constraints are used to restrict model parameter space; this excludes mathematically possible but geologically implausible temperature histories that may satisfy the FT and %Ro data. These restrictions can decrease model run times by preventing the model from straying into unpromising regions of parameter space. To further reduce computation time, the model was modified to (1) perform annealing calculations backwards in a single pass; (2) predict and abort unsuccessful temperature histories (by reference to a priori bounds) before they are completed; and (3) calculate optimum time steps for the annealing model as a function of heating rate, temperature and kinetic parameter. The inverse model searches parameter space thoroughly for a set of statistically-acceptable thermal solutions that provide a measure of model uncertainty. Temperature histories are generated randomly within geologically-constrained a priori limits (see below; Fig. 10); the model converges when a specified number of successful solutions have accumulated. Solutions are evaluated according to the degree of misfit between calculated and observed apatite FT age (to within two standard deviations) and length parameters and vitrinite reflectance values (to within one standard deviation). Track length distributions are assessed using the Kolmogorov-Smirnov (KS) statistic (e.g., Miller and Kahn, 1962; Press et al., 1992) which compares a measured cumulative distribution (ordering of shortest to longest tracks) to a theoretical cumulative distribution. A significance level probability of 0.05 provides a pass / fail test of the null hypothesis that the measured and calculated distributions are the same. A priori model constraints Table 4 includes well temperature information, formation depths, lithology and corresponding stratigraphic ages. The study region is in the zone of extensive discontinuous permafrost (frozen layer is tens of metres thick; see papers in Dyke and Brooks (2000) and references therein). Therefore, a surface temperature of 0ºC was used for calculating the present-day geothermal gradient. Bottom-hole temperatures are of poor quality for the I-77 well; a maximum temperature of 71.3ºC from a drill stem test ( m) yields a linear present-day geothermal gradient of 32.6ºC/km and a present temperature of approximately 57ºC at the FT sample depth. Formation ages (Table 4) from the time scale of Gradstein and Ogg (1996) were used for constraining burial histories and heating rates. A recently published time scale (Gradstein et al., 2004) shows a significant revision in ages for the Late Devonian; the duration of the Frasnian Famennian interval has increased from 16 m.y. to 26 m.y. The absolute shift in age has a negligible effect on model calculations but it was factored into the estimation of

15 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 419 burial heating rates (Table 5). Formation lithology data (Table 4) are from cuttings descriptions in well history reports (Roberts, 2000; Smith, 1971) and outcrop studies (Sweet et al., 1989; A. Sweet, pers. comm., 2002); percent lithology data for the siliciclastic units were grouped into shale and sandstone end members for decompaction calculations that used the linear porosity-depth functions of Issler (1996c). Table 5 lists present and decompacted formation thicknesses and corresponding burial rates for Cretaceous and Upper Devonian clastic units of the I-77 well. Also shown are estimated burial heating rates (product of burial rate and geothermal gradient) for a range of geothermal gradients within ±5, ±10 and ±15ºC/km of the present value (approximately 32ºC/km), representing uncertainties in geothermal gradient of approximately 16, 31 and 50%, respectively. These heating rates were used to set a priori bounds on FT thermal histories during times of deposition of preserved strata. Figure 10 illustrates the initial parameter search space for modelling the I-77 FT data. A priori constraints are defined for the following four major time intervals: (1) the pre-depositional cooling history of the detrital apatite grains ( Ma); (2) the post-depositional, pre-late Aptian phase of regional burial and exhumation ( Ma); (3) the Early to middle Cretaceous phase of regional burial and localized exhumation of the reactivated Keele Arch ( Ma); and (4) regional Late Cretaceous Cenozoic foreland burial and exhumation (95 0 Ma). The initial model time of 500 Ma was chosen arbitrarily to allow for the possibility of a pre-depositional thermal history for the detrital apatite; it is not a sensitive model parameter (see below). Broad temperature bounds were chosen to avoid over-constraining the model (Fig. 10). Based on measured apatite compositions and the Ketcham et al. (1999) multikinetic model, 150ºC is sufficient to totally anneal FTs in the chlorapatites of kinetic population 2, whereas 120ºC should Table 4. Geological constraints for the East MacKay I-77 well.

16 420 D.R. ISSLER, A.M. GRIST and L.D. STASIUK totally anneal FTs in the fluorapatites of kinetic population 1. Model nodal points were spaced non-uniformly along the time axis (Fig. 10); closest spaced intervals correspond to the younger part of the thermal history that is better resolved by the FT data. Temperature limits are reduced substantially at the boundaries between the above four time intervals for burial and/or exhumation (Fig. 10). The FT sample was at surface temperature (0 20ºC) at the time of deposition (about 370 Ma) and at near surface temperatures following major periods of exhumation at 115 Ma (0 50ºC) and 95 Ma (0 25ºC). At 115 Ma, stratigraphic relationships interpreted from seismic data (MacLean and Cook, 1999) imply that approximately 1 km of Imperial Formation was present across much of the study region. By 95 Ma, differential erosion of the Cretaceous Keele Arch removed one or more kilometres of previously deposited Lower Cretaceous strata and almost 800 m of underlying Imperial Formation, bringing the FT sample to within 250 m of the surface. Finally, the FT sample is required to be at a present temperature of 57ºC with an assumed error of ±10ºC. Rate constraints are applied to further restrict initial parameter space. The pre-depositional history is unknown and, because FT annealing has obscured the early temperature history, the model is not sensitive to this initial condition. The sample is required to cool randomly from high temperatures to surface temperatures at 10ºC/m.y. (Fig. 10). From 370 to 115 Ma, one phase of random heating and cooling is permitted; there is random heating only at rates between 1 and 4ºC/m.y. (Table 5) during deposition of the Imperial Formation ( Ma) and random heating and cooling at 5ºC/m.y. for the remainder of the interval. The 5ºC/m.y. limit avoids rapid temperature fluctuations and is reasonable for the geological setting and the maximum burial thickness (Fig. 5) and inferred paleogeothermal gradient (see below). The same limit is applied for single phase random heating and cooling during burial and exhumation of Lower Cretaceous strata ( Ma; Fig. 10). Foreland burial and exhumation (95 0 Ma) are modelled using single phase random heating and cooling with heating only between 1 and 4ºC/m.y. during deposition of preserved foreland strata (Table 5); subsequent random heating and cooling at 7ºC/m.y. (Fig. 10) allows for a possible rapid increase in burial and heating during the latter stages of orogenesis. Figure 10 shows an example of a random temperature history that was generated using the above constraints. As an additional constraint, cumulative heating from 370 to 0 Ma is assessed by comparing calculated and measured %Ro values. The measured value (0.82%Ro) was interpolated from Figure 5 and has an assumed error of ±0.08 %Ro, reflecting the typical 10% error for such measurements (Table 1). Another constraint was derived by extrapolating the Cretaceous thermal maturity trend a few hundred metres below the unconformity to the depth of the FT sample, giving 0.6%Ro with an assumed error of ±0.06%Ro (Fig. 5). Thermal histories from 95 to 0 Ma are required to yield calculated values that fall within the range, 0.6±0.06%Ro. Table 5. Stratigraphic thicknesses and burial and heating rate constraints for the East MacKay I-77 well.

17 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 421 BURIAL / THERMAL HISTORY MODEL First-order burial and thermal history models for the I-77 well were calculated using the Feinstein et al. (1996) method which incorporates (1) observed (Table 4) and estimated eroded (Fig. 5) stratigraphic thicknesses; (2) present (Table 4) and assumed past geothermal gradients; (3) vitrinite reflectance data (Table 1; Fig. 5); (4) constraints on timing of peak temperatures from apatite FT modelling results (below); and (5) constraints on time of maximum burial from shale compaction data (Table 3; Fig. 9). Sediment thicknesses and ages are used to construct burial history paths for different sedimentary layers in the well. Assuming linear geothermal gradients, these burial histories are converted to time-temperature histories that are used to calculate %Ro values (Sweeney and Burnham, 1990; Issler, 1984). A constant surface temperature of 10ºC was used for all model times except at 0 Ma where 0ºC was used. This simple model, which neglects sediment compaction and heat transfer processes, is justified, given the large gap in the geological record and the associated uncertainties on lithology, compaction parameters and paleotemperatures. Fig. 10. A priori constraints defining model parameter search space. Broad temperature limits (grey shade) are used with narrower limits for specific times: (1) surface temperatures at deposition; (2) near surface temperatures after major exhumation; (3) present temperature. Rate constraints are from regional tectonic considerations (maximum heating / cooling rates; number of events) and preserved stratigraphy (heating only constraints and magnitudes). Cumulative heating must predict %Ro values that fit observations. Example random temperature history (bold curve) shows (1) random cooling to surface temperatures at deposition (370 Ma); (2) heating during Imperial Formation deposition ( Ma) plus a random heating / cooling phase for pre-late Aptian burial / exhumation; (3) a heating / cooling phase for Aptian Albian burial and exhumation of Keele Arch ; (4) heating during deposition of preserved foreland strata (95 65 Ma) plus a random heating / cooling phase for subsequent Cenozoic burial / exhumation. The model used 144 nodal points spaced along the time axis time as follows: 10 m.y. intervals from 500 to 370 Ma; 5 m.y. from 370 to 280 Ma; 2.5 m.y. from 280 to 0 Ma.

18 422 D.R. ISSLER, A.M. GRIST and L.D. STASIUK THERMAL MODELLING RESULTS APATITE FISSION TRACK THERMAL HISTORIES Inverse FT modelling yields a preferred thermal history (Fig. 11) that is consistent with measured apatite FT parameters (Fig. 12) and %Ro data (Fig. 13). An effective Cl concentration of apfu was used for modelling kinetic population 1 apatite FT parameters (average for all age and length measurements); 0.21 apfu Cl was used to model kinetic population 2 FT parameters (see Discussion for the determination of an effective Cl concentration). Figure 11 shows 300 acceptable Monte Carlo thermal solutions and their exponential mean as a representative good-fitting solution (Willett, 1997) that provides an excellent fit to observed FT age and length (Fig. 12) and vitrinite reflectance (Fig. 13) data; calculated objective functions (difference between calculated and observed data) are well below pass / fail thresholds (Fig. 12). Almost all Monte Carlo thermal histories (Fig. 11) have maximum temperatures for thermal peak 1 (370 to 115 Ma interval) occurring between 150 and 250 Ma (Triassic Jurassic); nine of 300 solutions (3%) give Permian ages but these are near the margins of acceptability (Fig. 14A). The vast majority of peak 1 times lie within Early Triassic Middle Jurassic (ca Ma); time of thermal peak 1 for the exponential mean solution is 215 Ma and the average for 300 solutions is approximately 213±20 Ma. For thermal peak 3 (95 0 Ma interval), times vary between 50 and 65 Ma (Early Eocene Paleocene; Fig. 14B). Maximum temperatures for thermal peaks 1 and 3 for the Monte Carlo thermal histories are normally distributed with average values of 123.5±3.6ºC and 96.3±3.1ºC, respectively (note that peak temperature distributions show no overlap; Fig. 15); corresponding exponential mean values are 113.5ºC and 93.5ºC. These 20 to 27ºC differences in peak temperatures reflect the significant differences in total annealing temperature for kinetic populations 1 and 2. Thermal peak 2 ( Ma interval) has an upper limit of 75ºC (Fig. 11). This is compatible with the amount of pre-existing Lower Cretaceous strata (about 1 km) and post-albian erosion of Devonian strata (about 0.8 km) inferred from seismic data. The magnitude of the thermal peaks derived from FT modelling is consistent with the burial and erosion thicknesses and temperatures from the burial and thermal history models below (Fig. 11). Fig. 11. Preferred FT thermal history showing statistically acceptable solution space defined by 300 Monte Carlo solutions (grey). The smooth exponential mean of 300 solutions (bold curve) provides a good fit to the FT and %Ro data. Apatite FT thermal histories are compatible with temperatures and maximum burial thicknesses inferred from burial and thermal models constrained by vitrinite reflectance data (see text).

19 MULTI-KINETIC APATITE FISSION TRACK THERMOCHRONOLOGY, NWT 423 Fig. 12. Comparison of observed (histogram) and modelled apatite FT length distributions for (A) the younger, fluorapatite population (kinetic population 1) and (B) the older, chlorapatite population (kinetic population 2). Bold curves represent calculated probability density functions (PDFs) for the exponential mean thermal history (Fig. 11); the grey shaded regions are defined by the PDFs for the 300 Monte Carlo solutions (Fig. 11). The narrower range of uncertainty on computed lengths for kinetic population 2 is due to the higher number of track length measurements (111 vs. 60). Note that track length measurements have been increased by approximately 5% to correct them to equivalent length values of Carlson et al. (1999) for use with the Ketcham et al. (1999) multi-kinetic annealing model (uncorrected length data in Fig. 8). Calculated FT ages must fit the measured age to within 2 standard deviations. BURIAL / THERMAL HISTORY FOR EAST MACKAY I-77 Figure 16 shows first-order reconstructions of the thermal and burial / erosion history for the I-77 well based on integrating all the methods and data used in this paper. Given the broad range of acceptable FT thermal histories (Fig. 11), we did not recast the exponential mean FT thermal history into an equivalent burial history. Such a burial history satisfies, but is not uniquely constrained by, the %Ro data because thermal maturity depends mainly on the time spent near maximum temperature (e.g., Hood et al., 1975). Instead, estimated eroded sediment thicknesses (Fig. 5) are assumed to accumulate at linear rates between their time of deposition and maximum burial; erosion rate is assumed to be linear between the times of maximum burial and the onset of post-exhumation burial. The times of maximum burial (200 Ma, 105 Ma, 55 Ma; Fig. 16) are within the ranges predicted by FT modelling (Figs. 11, 14); 200 Ma represents the Triassic Jurassic boundary and 105 Ma (Albian) is close to the average model time for thermal peak 2 (Fig. 11). The 55 Ma time represents the midpoint of the estimated time range for the end of Cordilleran orogenesis (Willett et al., 1997). These times are not sensitive model parameters and other values consistent with the FT modelling would yield similar results. In Figure 16A, a constant geothermal gradient of 32ºC/km (similar to present-day value) was used to model the entire Middle Devonian to Cenozoic thermal history. This gives an acceptable fit to the %Ro data if the thickness of eroded Devonian to Triassic sediment is approximately 3400 m Fig. 13. Frequency plot of calculated vitrinite reflectance values for 300 Monte Carlo thermal histories (Fig. 11) corresponding to the postdepositional (370 0 Ma) and post-albian (95 0 Ma) thermal history. Calculated %Ro values (histograms) are required to fit observed values (filled circles with error bars) to within one standard deviation. (Fig. 16A). Calculated %Ro values show good agreement with measured values for both the EASY%R o and TTI methods (Fig. 16A). Generally, %Ro values are lower for the TTI model because EASY%R o may slightly overestimate %Ro at maturity levels <0.9% Ro (Morrow and Issler, 1993; Barker,