Analysis of large thermal maturity datasets: Examples from the Canadian Arctic Islands

International Journal of Coal Geology 77 (2009) 436-448

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Analysis of large thermal maturity datasets: Examples from the Canadian Arctic Islands Keith Dewing ⁎, Hamed Sanei Geological Survey of Canada, 3303-33rd Street NW, Calgary, Alberta, Canada T2L 2A7

A R T I C L E

I N F O

Article history: Received 7 January 2008 Received in revised form 22 April 2008 Accepted 28 April 2008 Available online 17 May 2008 Keywords: Thermal maturity Vitrinite reflectance Rock-Eval Arctic Canada Sverdrup Basin

A B S T R A C T This paper describes an approach for verifying thermal maturity data in a large historical dataset from the Canadian Arctic Islands. A compilation of more than 6000 maturity measurements (vitrinite reflectance and Rock-Eval Tmax) collected over the span of three decades involved a rigorous assessment of data quality. Some common anomalies in interpreting thermal maturity dataset include: (i) elevated thermal maturity due to Cretaceous igneous intrusion in the region, (ii) reworking of refractory material from older rocks into younger strata during the Triassic period, (iii) suppression of vitrinite reflectance and Tmax in hydrogen-rich samples, (iv) low maturity values due to cross-contamination by the younger sediments during drilling process (caving), and (v) offset maturity values obtained from different maturity measurements. The study discusses various independent checks to verify the obtained maturity parameters. The comparison between thermal maturity data with the sonic velocity of shale resulted in a satisfactory correlation. While such a correlation may vary in different sedimentary basins, it produces a useful independent assessment of thermal maturity. The results indicate that increased heat flow during the Jurassic–Early Cretaceous rifting of the Canada Basin may have caused the elevated maturity beyond the expected burial level as suggested by the discrepancy between thermal maturity and sonic velocity data. Given the fact that vitrinite reflectance records only the maximum temperature to which the enclosing rocks were exposed, deviation of the collected reflectance values from the current depth of burial serves as an indicator for the amount of geological uplift. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved.

1. Introduction Thermal maturity data are a key component of basin analysis and hydrocarbon assessment. These data provide constraints on the maximum depth of burial and paleogeothermal gradient in an exploration borehole. In a frontier area, like the Canadian Arctic Islands where drilling is very widely spaced and petroleum systems poorly known, generating new exploration plays requires the best possible thermal maturity data to estimate the areas in which source rocks are likely to produce hydrocarbons, and whether oil or gas is expected. In anticipation of renewed interest in hydrocarbon exploration in the Arctic, all the previously existing thermal maturity data from the Canadian Arctic Islands were compiled and released digitally (Dewing et al., 2007a; Obermajer et al., 2007). These measurements of thermal maturity parameters were made over many years in support of regional mapping studies, analysis of coal potential, and graduate theses. These data include data collected by Panarctic Oils Ltd and stored in the library collections at the Geological Survey of Canada (Leythaeuser and Stewart, 1986), analyses from TeckCominco Ltd. (Héroux et al., 1999), data from graduate theses (Riediger et al., 1984; Bustin, 1986; Gentzis, 1991; Nunez-Betelu, 1993), as well as samples run by the Geological Survey of Canada as part of published (e.g., Powell, 1978; Goodarzi et al., 1987, 1989; Fowler et al., 1991; Brooks ⁎ Corresponding author. E-mail address: [email protected] (K. Dewing).

et al., 1992; Gentzis and Goodarzi, 1993a,b; Stasiuk and Fowler, 1994; Stewart, 1999), and unpublished regional or stratigraphic studies. Two techniques were used to determine the level of thermal maturity. The first was vitrinite reflectance measurements. Analyses were done in-house by a variety of operators using different generations of equipment, and by commercial laboratories under contract. The second technique involved doing pyrolysis experiments using Rock-Eval™. Almost all the analyses were done in-house, but on different generations of equipment. Compiling these disparate data into a single, reliable dataset posed a significant challenge. This paper describes our methodology for compiling our data, some common problems and an approach to verifying the dataset using independent parameters such as the sonic velocity from well logs. Lastly we try to use the datasets to resolve the heating caused by burial from elevated heat flow, and calculate uplift. 2. Geological setting Strata from four main geological assemblages are recognized (Fig. 1): crystalline basement and Mesoproterozoic sedimentary basins and volcanic rocks; Ediacaran to Devonian passive to convergent margin sedimentary strata (including undeformed Arctic Platform, and Franklinian Mobile Belt); Carboniferous to Cretaceous rift and basinal sediments and volcanic rocks of the Sverdrup Basin; and Cretaceous– Tertiary rift and passive margin sediments of the Banks Basin and Arctic Continental Margin.

0166-5162/$ – see front matter. Crown Copyright © 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2008.04.009

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Fig. 1. Geological map of the Canadian Arctic Islands based on Okulitch (1991). The oldest rocks are exposed in the south and east. Strata get progressively younger to the northwest.

2.1. Proterozoic basins 1.2–750 Ma Unmetamorphosed and undeformed Mesoproterozoic strata are preserved in several areas of the central and eastern Canadian Arctic islands (Fig. 1). These basins are likely remnants of a continental margin (Jackson and Iannelli, 1981), and are considered to be approximately 1200 Ma, although radiometric dates are sparse. A Neoproterozoic sedimentary basin is exposed on Victoria Island in the

western Arctic (Shaler Group; 1000–720 Ma, Rainbird et al., 1994). There are no known potential source rocks within any of these areas. 2.2. Lower Paleozoic Up to 11 km of Neoproterozoic to Devonian, clastic, carbonate and evaporite sediments were deposited in a northward-thickening wedge on the Arctic continental margin following mid-

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Neoproterozoic rifting at about 723 Ma (Heaman et al.,1992; Dewing et al., 2004). Lower Paleozoic passive-margin strata are divided into five tectono-stratigraphic groups. These are, from oldest to youngest: A) Ediacaran clastic slope and shelf deposits; B) Lower Cambrian clastics; C) Lower Cambrian to Upper Silurian carbonates; D) Upper Silurian to Lower Devonian synorogenic clastics; and E) Lower Devonian carbonates (Trettin, 1991). Organic-rich shales, that are potential hydrocarbon sources, occur in Upper Ordovician and Lower Silurian strata, and locally within the Upper Silurian synorogenic clastics. By early Middle Devonian time, the effects of plate convergence were widespread over most of the Arctic Islands. Over 5 km of shallow-marine and non-marine syntectonic clastic rocks were deposited in a foreland basin adjacent to a southeastward- and southward-advancing deformation front. These units are sandstone dominated but contain local coal and shale interbeds. The foreland basin was subsequently folded and faulted by the advancing orgenic front (Devonian Clastic Wedge on Fig. 2; Embry and Klovan, 1976). The youngest preserved strata of this succession are of Late Devonian (Famennian) age. This Middle Devonian to earliest Carboniferous continent–continent collision is known as the Ellesmerian Orogeny (Thorsteinsson and Tozer, 1970; Harrison, 1995). 2.3. Sverdrup Basin — Carboniferous to Cretaceous The Ellesmerian orogenic belt collapsed in the Early Carboniferous to form fault-bounded lakes and a narrow, inland sea that accumulated thick salt deposits (Beauchamp et al., 1989). This rift system opened to form a seaway (the Sverdrup Basin; Fig. 1) to the north of the lower Paleozoic shelf margin. The basin depocentre is displaced northwest and outboard of its lower Paleozoic predecessor. The axial region contains up to 3 km of upper Palaeozoic strata, up to 9 km of Mesozoic strata, and locally more than 3 km of Tertiary strata (Trettin, 1991). Upper Carboniferous and Lower Permian carbonates were deposited around the edge of the basin whereas argillaceous limestone and shale were deposited in the basin centre. Changing climate conditions in the Late Permian curtailed carbonate deposition and shale was deposited across almost the entire basin. The Permo-Triassic boundary is marked by an unconformity at the basin margins, but was conformable across the centre of the basin. Fluctuating sea level and basin tectonics caused the repeated advance and retreat of clastic deltaic strata during Early Triassic to Early Jurassic time. The largest deltaic system deposited over 1500 m of deltaic sandstones that form the principle hydrocarbon reservoir in the Arctic Islands (Lower Jurassic Heiberg Group). By the mid-Jurassic, the basin was restricted to between the emergent Sverdrup Rim on the northwest side of the Sverdrup Basin and the southern margin located on Ellesmere–Bathurst–Melville islands. Rifting of the Canada Basin (between Canada and Alaska–Russia) in the Early Cretaceous was associated with uplift and erosion in the western Arctic. This was followed by deposition of fluvial and deltaic sandstones across the basin and onto the newly forming continental margin. Starting in the Early Cretaceous and lasting about 60 Ma, flood basalts, gabbroic sheets and dykes were emplaced as a radiating dyke swarm in the eastern part of the Sverdrup Basin (Fig. 1). The peak igneous activity was synchronous with the accumulation of volcanic rocks on the Alpha Ridge in the Arctic Ocean and spreading of the Canada Basin in the Early Cretaceous (Fig. 2) During the Early Paleogene, the north Atlantic widened and Greenland rotated away from North America. Rotation of Greenland initially caused widespread extension across the Arctic Islands (Okulitch and Trettin, 1991). Extension was followed in the Eocene by translation and compression as Greenland moved north and west relative to North America. This compressive event is termed the Eurekan Orogeny

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(Thorsteinsson and Tozer, 1970; Harrison et al., 1999). Deformation generated long wavelength folds and reactivated salt diapers across the entire Sverdrup Basin (Harrison and Brent, 2005; Jober et al., 2007). 2.4. Thermal impact of geological history The Phanerozoic history illustrates two main burial events and three thermal events. The first burial was during the Middle and Late Devonian as thick accumulation of strata were deposited in a foreland basin, burying the lower Paleozoic and older successions. The second burial event was the progressive sedimentation in the Sverdrup Basin, reaching a maximum in the Eocene, just before the Eurekan Orogeny. The first thermal event was rifting of the Sverdrup Basin. The second thermal event occurred as the Canada Basin formed between the Mid Jurassic and Early Cretaceous, creating the present-day Arctic Margin. The third thermal event was the widespread intrusion of igneous rocks in the Early to Late Cretaceous related to creation and spreading of the Alpha Ridge. 3. Methodology Pyrolysis experiments on 6320 samples were conducted using Delsi Rock-Eval II and Rock-Eval VI units equipped with a total organic carbon analysis module (Obermajer et al., 2007). A typical Rock-Eval pyrolysis involves the programmed temperature heating of a pulverized rock sample at the rate of 25 °C/min, first in the pyrolysis oven (in helium atmosphere), and then in the oxidation oven. A Flame Ionization Detector (FID) measures the quantity of hydrocarbons released from the samples during the pyrolysis process. In the first pyrolysis stage, the bulk sample is heated to 300 °C, when naturally occurring hydrocarbons (free and adsorbed) are volatilized (S1, mg HC/ g rock). During the next stage, the oven temperature is steadily increased to 600–650 °C in which thermal decomposition of kerogen occurs (S2, mg HC/g rock). The temperature measured at the maximum of the S2 peak is referred to as Tmax (°C). Simultaneous to measurement of hydrocarbons, the online infrared (IR) detector(s) continuously measure the quantity of organic CO2 and CO (in Rock-Eval 6) generated during pyrolysis, comprises the S3 peak (mg CO–CO2/g of rock). The final stage involves oxidation and combustion of the residual organic matter up to 600 °C (850 °C in Rock-Eval 6). The total organic carbon (TOC; wt.%) in the sample is quantified as sum of the total pyrolysable organic matter and residual carbon (Espitalie et al., 1985). Organic matter reflectance determinations for 6292 samples from 140 oil and gas wells and 1857 measurements from 346 outcrops or mining drill core are compiled in Dewing et al. (2007a). In most cases, only the average random reflectance and number of measurements were recorded. In this dataset, there is great variability in the number of measurements that were made on vitrinite from each sample (Fig. 3). Ideally, every individual measurement would be recorded so that the maximum, minimum, average, and standard deviation could be plotted. This would allow for statistical comparison of samples and to look for sub-populations. Unfortunately, the amount of work capturing and analysing the data is onerous and has only been done in the case of highly problematic wells or intervals in wells. Often the lab making the measurements will choose a population to measure based on its reflectance, introducing a strong operator bias to the dataset. Only 176 exploration boreholes have been drilled in the Canadian Arctic Islands. Consequently, there were relatively few problems as the old paper records were made digital. ‘Tombstone’ information, such as drill hole name, location, and total depth are easy to verify. The main issue with data quality assurance was feet-to-metre conversions. Canada changed from the imperial to metric systems during the early 1980s. Shortly after the changeover, samples collected at the well in metres were frequently sent to labs who reported back assuming the

Fig. 2. Time-stratigraphic successions in the Canadian Arctic Islands. There were three main phases of burial (PreCambrian, Cambrian to Devonian, Carboniferous to Eocene). Three heat events affected parts of the Phanerozoic succession (Carboniferous, Jurassic and Cretaceous).

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Fig. 3. Histogram showing number of individual readings made on vitrinite from each sample from the Arctic Islands dataset.

depths were still in feet. The lab results (reported as feet) were then converted into metres causing the samples to plot erroneously in the upper third of the well prior to data verification. 4. Results and discussion 4.1. Correlation of maturity parameters Thermal maturity parameters can be obtained from a number of different techniques. Using maturity parameters derived from differ-

ent techniques not only increases the amount of data available for thermal maturity profiles, it also provides a check on the quality of the data, since a large discrepancy between the values and trends of different parameters indicates a problem with at least one of the datasets. The vast majority of our data comes from vitrinite reflectance and Rock-Eval pyrolysis. In order to plot all the data on a single diagram, two questions need answering: a) how much organic matter has to be in a rock to give reliable Rock-Eval measurements, and b) what is the conversion from the Rock-Eval Tmax parameter to an equivalent vitrinite reflectance? Some Rock-Eval data cannot be used in developing a thermal maturity profile. For instance, samples that are dominated by Type I (i.e., algal-derived) kerogen have a simple molecular structure and the bonds rupture at very close to 440 °C. The sharp spike in generated gas and a resulting Tmax of 440 °C is indicative of the bond strength, rather than the level of thermal maturity (Bordenave et al., 1993; Pepper and Corvi, 1995). A certain amount of organic matter is required to generate a reliable S2 peak. Peters (1986) recommends that samples with less than 0.2 mg HC/g rock be rejected. The reliability of Tmax relative to the S2 content was tested using the dataset from the Arctic Islands using one-dimensional, downhole variograms (e.g., Issaks and Srivastava, 1989). To perform the variography, dataset was filtered to include only samples with Tmax in the range 410 °C–550 °C and those with Type II or Type III kerogen. The Tmax column in the data spreadsheet was replicated and the first cell of the replicated column deleted so that a Tmax value is adjacent to the Tmax value of the next deepest sample. The dataset was then filtered to include only samples that occur within 10 m of each other. A total of 2111 sample pairs occur within the filtered dataset.

Fig. 4. Downhole variography of Tmax. The data compares the Tmax of a sample to the Tmax of the next deepest sample, where the separation between samples is 10 m or less and Tmax is in the range 410–550 °C. A. Comparison of Tmax(i) with Tmax(i + 1) where S2 is in the range 0.1–0.2 mg HC/g rock. Note the poor correlation of adjacent Tmax values. B. Comparison of Tmax(i) with Tmax(i + 1) where S2 is in the range 0.3–0.4 mg HC/g rock. The scatter is less than in graph A. C. Comparison of Tmax(i) with Tmax(i + 1) where S2 is in the range 0.7–0.8 mg HC/g rock. Correlation between adjacent Tmax samples is excellent. D. Semivariogram of the ‘moment of intertia’ [γ(h)] for each 0.1 interval of S2. The moment of intertia is a measure of the fatness of the cloud of points. Note that the correlation of adjacent Tmax values is excellent if S2 is greater than 0.5 mg HC/g rock. The correlation decreases rapidly once S2 is less than 0.38 mg HC/rock.

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Because the thermal gradient in the Arctic Islands is low, it would be expected that the Tmax of two samples within 10 m of each other should be the same (Majorowicz and Embry, 1998). The data were then filtered by 0.1 mg HC/g rock increments of S2 and the crossplot of Tmax(i) vs Tmax(i + 1) drawn (Fig. 4 variogram). For samples with low S2 (0.1 b S2 b 0.2 mg HC/g rock; Fig. 4a), there is poor correlation between adjacent Tmax samples. The correlation improves as the S2 increases (Fig. 4b,c). Using the ‘moment of intertia’ as a measure of the fatness of the cloud of points on the cross-plot (Fig. 4d; Isaaks and Srivastava, 1989), it is possible to see that for this dataset, the correlation is excellent for adjacent Tmax samples if S2 is greater than 0.5 mg HC/g rock. The correlation decreases rapidly once S2 drops below about 0.38 mg HC/g rock. For this study, the Tmax of samples with an S2 value of less than 0.35 were excluded from consideration as valid thermal maturity points. Tmax was converted to %Ro equivalent using the polynomial equation, kRo Vi equivalent ¼ c1 ðTmaxÞ3 −c2 ðTmaxÞ2 þc3 ðTmaxÞ−c4

ð1Þ

where c1 =5.37827×10− 6, c2 =7.47084×10− 3, c3 =3.47547 and c4 =540.167. Eq. (1) was determined using data for type III organic matter included with the MATOIL™ (1990) basin modelling software, constrained at higher Tmax values by a few Rock-Eval–vitrinite reflectance pairs from the Arctic Islands dataset. A look-up table of Tmax to vitrinite reflectance equivalent (Ro Vi eq %) is shown in Table 1. Matched Tmax–vitrinite reflectance pairs in our dataset agree well with the values in the table. 4.2. Common errors in the data The thermal maturity value (vitrinite reflectance or vitrinite reflectance equivalent) for each sample was plotted against its depth for each drill hole. The stratigraphic contacts, unconformities and igneous intrusions were overlain on the depth–maturity plot (two examples, Chads Creek B-64 and Helicopter J-12, are shown in Fig. 5a and b). Each graph was inspected for discrepancies. Five problems were commonly noted in interpreting the thermal maturity datasets. 4.2.1. Anomalous data Anomalously high thermal maturity values are common in the Arctic Islands dataset. These largely relate to igneous intrusions, especially in the northeastern part of the islands close to the epicentre of Cretaceous volcanism (Gentzis, 1991). Plotting the stratigraphic contacts and igneous sills on the depth–maturity plot is sufficient to explain the vast majority of the anomalous data as there is an obvious spatial relationship between igneous sills and elevated values of thermal maturity (Raymond and Murchison, 1991). An example is Fig. 5a, b. 4.2.2. Caving Caving of material down hole is the most common source of anomalously low values of thermal maturity. As the well is drilled, material from the previously drilled part of the section can spall and be carried up with cuttings from deeper down the borehole. It is typically shale units that cave (as shown by the caliper log from the Pat Bay A-72 well in Fig. 6). When cuttings are selected for thermal maturity studies, it is not always possible to separate caved from true material. The result, as shown in Fig. 6, is that caved material will sometimes constitute the majority of data. Using the caliper log to show where the borehole has expanded due to caving, it is possible to infer the probable reflectance of each caved interval. Samples deeper in the section with anomalously low reflectance can be matched to the higher caved intervals (Fig. 6). 4.2.3. Misidentification of macerals Despite the best effort of the labs, dispersed organic matter (kerogen) are occasionally misidentified. This problem would appear to affect less than 0.1% of the data based on a comparison of duplicate

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Table 1 Conversion between Tmax (°C) and equivalent vitrinite reflectance (Ro Vi eq %) Ro Vi eq

Tmax

Ro Vi eq

Tmax

Ro Vi eq

Tmax

Ro Vi eq

Tmax

%

°C

%

°C

%

°C

%

°C

0.42 0.46 0.5 0.54 0.58 0.61 0.64 0.67 0.7 0.73 0.75 0.78 0.8 0.83 0.85 0.87 0.9 0.92 0.94 0.96 0.98 1 1.02 1.04 1.06 1.08 1.1 1.12 1.14 1.16 1.17 1.19 1.21 1.23 1.24 1.26 1.28 1.3 1.31 1.33 1.34 1.36 1.38 1.39 1.41

427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471

1.42 1.44 1.45 1.47 1.48 1.5 1.51 1.53 1.54 1.56 1.57 1.58 1.6 1.61 1.62 1.64 1.65 1.66 1.68 1.69 1.7 1.71 1.73 1.74 1.75 1.76 1.77 1.79 1.8 1.81 1.82 1.83 1.84 1.85 1.86 1.88 1.89 1.9 1.91 1.92 1.93 1.94 1.95 1.96 1.97

472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516

1.98 1.99 2 2.01 2.02 2.03 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.1 2.11 2.11 2.12 2.13 2.14 2.15 2.15 2.16 2.17 2.18 2.19 2.19 2.2 2.21 2.22 2.22 2.23 2.24 2.25 2.25 2.26 2.27 2.28 2.28 2.29 2.3 2.3 2.31 2.32 2.32 2.33

517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561

2.34 2.34 2.35 2.36 2.36 2.37 2.38 2.38 2.39 2.39 2.4 2.41 2.41 2.42 2.43 2.43 2.44 2.44 2.45 2.45 2.46 2.47 2.47 2.48 2.48 2.49 2.49 2.5 2.51 2.51 2.52 2.52 2.53 2.53 2.54 2.54 2.55 2.55 2.56 2.56 2.57 2.57 2.58 2.58 2.59

562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606

samples from two or more labs. The problem, however, can be locally important if a lab consistently misidentifies a kerogen in one well. An example was encountered in two wells drilled in the lower Paleozoic succession (Fig. 7). One lab recognized bitumen, another vitrinite. While vitrinite-like organic matter is known from pre-Devonian strata (Buchardt and Lewan, 1990; Gentzis and Goodarzi 1990; Goodarzi et al., 1992a,b; Obermajer et al., 1996), it can appear similar to bitumen. Given the lack of reported vitrinite in any other wells or outcrop samples from these formations, it is inferred that one lab misidentified bitumen as vitrinite. In general, for similar reflectance, bitumen represents higher maturity than vitrinite (Gentzis and Goodarzi, 1990; Potter et al., 1993) and they result from very different sources and thermal history. 4.2.4. Reworking Resistant Devonian spores are common in Triassic strata, but can be fragmentary and misidentified as an indigenous maceral (NzoussiMbassani et al., 2005). Reworking of material from older, thermally mature rocks into younger strata is known to be common in the Triassic part of the Sverdrup Basin (Utting et al., 2004). The main sediment source in the Triassic of the Sverdrup Basin was from the south (Embry, 1991) where thermally mature strata of the lower Paleozoic succession were being eroded. Fig. 8 shows an apparent increase in reflectance at

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Fig. 5. Maturity–depth graphs of the Chads Creek B-64 well on northern Melville Island (76.386°N, 109.906°W) and the Helicopter J-12 well on Ellef Rignes Island (78.693°N, 100.614°W). The maturity data in these wells is consistent and easy to interpret. Note the impact of igneous sills on the maturity.

about 2200 m creating a significant jog in the maturity–depth plot. Geological explains for this change in slope (unconformity, heat pulse, thermal blanket; e.g., Huvaz et al., 2005) can be discounted on the basis of regional geological studies. Samples of the cuttings were re-examined and the high reflecting population was identified as reworked Devonian material. Once the reworked material and the caved material (around 1300 to 1600 m) is removed, a best fit curve can be applied. The difference in interpretation of the maturity–depth plot at the bottom of the hole is 1.26% Ro Vi equiv. vs 1.57% Ro Vi measured or a difference of Ro = 0.31%. Importantly, the different interpretations would put source rocks on either side of the oil–gas window at 1.35% Ro. 4.2.5. Suppression of Tmax and vitrinite The suppression of vitrinite and Tmax by organic matter containing high hydrogen content and bitumen saturation has been described by a number of authors (e.g., Price and Barker, 1985; Hao and Chen, 1992; Lo, 1993; Snowdon, 1995; Carr, 2000a,b). This is evident especially in the Caledonian River J-34 well on Bathurst Island, which shows a suppression of Tmax in the hydrogen-rich samples from the Cape Phillips Formation (Fig. 9). The samples containing higher hydrocarbon are marked by their sudden increase in (S1 + S2)/ TOC relative to the other samples showing a general downward trend in their hydrocarbon content with depth and maturity. These hydrogen-rich samples match areas of correspondingly suppressed Tmax of 10 to 15 °C (Fig. 9). The suppressed Tmax values are likely attributed to the thermal breakdown of more volatile bitumen and not

the actual kerogen in the hydrocarbon rich samples (Bordenave et al., 1993). A study of Carboniferous-age oil shales from Arctic Canada by Goodarzi et al. (1987) showed that vitrinite reflectance can be suppressed by as much as 0.4%. Vitrinites of sapropelic coals, deposited in a lower delta plain to marginal marine environment show an increased fluorescence intensity (perhydrous), a higher hydrogen and nitrogen content and a suppressed reflectance (Teichmuller, 1986; Gentzis and Goodarzi, 1994). Vitrinite reflectance is also affected by depositional environment (organic facies), matrix lithology, and the presence of high liptinite macerals (as in an oil shale; Hutton and Cook, 1980). Variations in vitrinite reflectance with matrix lithology have been reported by Mukhopadhyay (1992) and Goodarzi et al. (1988, 1993). Possible mechanisms proposed include a differential thermo-catalytic effect and thermal conductivity of the host matrices, absorption of migrated bituminous substances by the vitrinite matrix during catagenesis, or differences in vitrinite chemistry in different depositional environments. 4.2.6. Offset curves Finally, there are examples of offset curves where the thermal maturity as measured by vitrinite reflectance and Rock-Eval Tmax produce parallel curves, but which are offset by roughly 0.30% Ro (Fig. 10; Hazen F-54). In addition, there are typically a few vitrinite samples that support the interpretation based on Tmax, and a few Tmax that support the interpretation based on vitrinite. To resolve this kind of problem, a series of independent checks were developed to see which curve is reliable.

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Fig. 6. Comparison of the caliper log and the maturity–depth graph of the Pat Bay A-72 well on Lougheed Island (77.350°N 105.449°W). The hole was drilled with a 12.5 in. (31.75 cm) bit to 1300 m before being cased. The hole was then continued with a 8.75 in. (22.2 cm) bit to depth. The lower part of the hole was not cased. The caliper log shows how much the diameter of the hole expanded due to caving of shaly material (shaded boxes numbered 1 to 5). This material would have returned to surface, mixed with cuttings from deeper in the well.

4.3. Independent assessment of thermal maturity The first, and most obvious, check is comparison of the depth– maturity curve to those from nearby wells. The second quick check of the

data can be made by comparing the thermal maturity derived from vitrinite and Rock-Eval to fossil colour alteration parameters such as the conodont colour alteration index (CAI) or pollen-spore thermal alteration index (TAI) (e.g., Utting et al., 1989). While these checks

Fig. 7. Maturity–depth graphs of the Blue Fiord E-46 well on southern Ellesmere Island (77.258°N, 86.302°W) and the Russell E-82 well just north of Prince of Wales Island (73.858°N, 98.947°W). Given the similarity between ‘bitumen’ measured by one lab and ‘vitrinite’ measured by a second lab, it is likely that all the measurements were made on the same maceral.

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Fig. 8. Maturity–depth graph of the Cape Mamen F-24 well, west of Mackenzie King Island (77.555°N, 109.166°W). The rapid increase in measured maturity below 2200 m is due to recycled material from older, Devonian strata to the south. The true curve, shown by the dotted line, is believed to be about 0.30% Ro less than the recycled material.

may help point to problems with the data from a particular well, they seldom are sufficient to decide which data are suspect and which are reliable.

Fig. 9. Suppression of Tmax by organic matter containing high hydrogen content. Note the mirror image of the two curves. Samples rich in hydrogen (high (S1 + S2)/TOC) match areas of correspondingly low Tmax.

Fig. 10. Maturity–depth graph of the Hazen F-54 well, south of Mackenzie King Island (77.0546°N, 110.354°W). Rock-Eval Tmax and reflectance derived from vitrinite produce two parallel curves. In the absence of other data, it is impossible to know which is correct.

4.3.1. Gas wetness Another independent thermal maturity parameter is given by the gas wetness. This technique was used by the Geological Survey of Canada between 1970 and 1975 (Snowdon and McCrossan, 1973; Monnier et al., 1981; data in Dewing et al., 2007b). Samples of cutting were collected at the well site and canned. In the lab, a known volume (usually 100 ml) of cuttings was homogenized with water in a gastight blender. A sample of the headspace gas was then analyzed for methane, ethane, propane, and butane (C1 to C4). The technique was used to establish the thermal maturity based on the change from dry, bacterially-produced gas in shallow rocks, to wet thermogenicallyproduced gas in deeper rocks. The results of cuttings gas analyses are prone to a number of biases. Gas may be lost to the drilling fluids during transport to the surface, especially in porous, organic-poor, or unconsolidated rock types. There may also be alteration of the gas or gas generation by bacterial methanogenesis in the canned sample. Finally, the transition from dry-to-wet gas frequently occurs at a formational contact, implying that migration of wet gas from below may overwhelm the bacterial signature. Comparison of the change from dry to wet gas (95% methane) and the best-fit curve derived from vitrinite and Rock-Eval data (Fig. 11) shows that the dry-to-wet transition occurs at 0.5 ±0.13% Ro Vi eq. The considerable variation in the reflectance at which the dry-to-wet transition takes place is presumably a result of the biases described above. 4.3.2. Sonic interval transit time (SITT) The best independent check of thermal maturity comes from comparison with the sonic velocity of shale. As the organic matter in a shale is buried, its temperature and vitrinite reflectance increase. Burial also compacts shale, so that the velocity of sound through the rock will increase, implying that there should be a correlation between sonic velocity and thermal maturity. Mallick and Raju (1995) documented a positive correlation between increasing sonic velocity and increasing vitrinite reflectance for Tertiary strata in India. Fig. 12 (data in Table 2) shows the correlation of reflectance and sonic interval transit time (SITT) for several shale units in the Arctic Islands. The SITT measures the time for sound to travel a set distance and hence is the inverse of the sonic velocity. The SITT was determined at the base of three widespread shale units in the Arctic: the Jurassic Jameson Bay Formation, the Triassic

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Fig. 11. Histogram showing the thermal maturity, derived from vitrinite and Rock-Eval measurements, at the transition from dry, bacterially-produced gas to wet, thermogenically produced gas.

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4.4.2. Amount of uplift The graph of vitrinite reflectance vs SITT (Fig. 12) indicates a uniform response of thermal maturity with depth for Mesozoic strata within the Sverdrup Basin. Given that vitrinite reflectance is not reversible from its maximum level and cannot decrease after it is set, it should be possible to determine the amount of uplift a sample has undergone by comparing the depth inferred by the vitrinite reflectance to the current depth of burial. The thermal maturity at the level of the Upper Triassic Gore Point Member of the Roche Point Formation was established for each well using the best fit line through the vitrinite equivalent reflectance points. The Gore Point Member is a useful marker because it is the only widespread limestone in the Sverdrup Basin so it is consistently chosen in wells, it is a good seismic reflector, and it is close to the two main oil-prone source rocks in the Sverdrup Basin. Fig. 13 (data in Table 3) shows the depth of the base of the Gore Point Member below sea level plotted against the vitrinite equivalent reflectance. A normal burial curve is established using boreholes drilled in areas with no structural complexity. This curve, shown by the shading

Murray Harbour Formation, and Devonian shales from the clastic wedge (Cape de Bray, Weatherall, Beverley Inlet formations). In each case, sandstone intervals were excluded using the gamma log response, and a best-fit curve established for the sonic log. The value of the SITT at the base of the formation was determined from the best-fit curve. Changing the sandstone–shale cutoff on the gamma log within a reasonable range has very little effect on the SITT curve. The correlation between vitrinite equivalent reflectance and SITT (Fig. 12) is reasonably good between vitrinite equivalent reflectance of 0.4% to about 1.0%, with the data scattered by about 0.1% on either side of the best fit curve. Once the shale is fully compacted the sonic velocity no longer increases with depth; the sonic velocity in this dataset is insensitive to burial beyond a vitrinite equivalent reflectance of about 1.0% Ro. Correlation between vitrinite equivalent reflectance and SITT will be different between sedimentary basins since the heat flow, rate of burial, and chemistry of the shales will differ. The correlation between SITT and vitrinite equivalent reflectance helps determine which of the offset curves in Fig. 10 is correct. The vitrinite equivalent reflectance at the base of the Jameson Bay Formation on the two curves are compared to the SITT at the base of the Jameson Bay Formation (open diamonds on Fig. 12). The lower reflectance curve in Fig. 10 matches what is expected based on the SITT. 4.4. Geological interpretation 4.4.1. Depth of burial versus heat flow Comparison of thermal maturity to SITT may aid in separating the effects of heating due to deeper burial from heating due to increased heat flow. Fig. 12 shows that the SITT (a proxy for compaction) and vitrinite reflectance (a proxy for thermal stress) correlate well for most boreholes in the Canadian Arctic Islands. This would imply that the thermal response relative to the depth of burial is consistent over most of the islands, and that there were no great variations in geothermal gradient. These lower Paleozoic samples in the western Arctic (squares on Fig. 12), consistently plot below the curve at lower depth of burial as inferred from the sonic interval transit time. This suggests that the lower Paleozoic samples are thermally matured at a higher temperature than would be expected from their burial compaction. Given the location of these wells in the western Arctic, in the area of Jurassic–Early Cretaceous rifting of the Canada Basin (Harrison and Brent, 2005), it seems likely that the elevated vitrinite reflectance was due to an increased thermal gradient during rifting rather than deep burial during the Ellesmerian Orogeny.

Fig. 12. Comparison of vitrinite reflectance (Ro%) and the sonic interval transit time (SITT; μm/s) for three shale units in the Canadian Arctic Islands. Data in Table 2. There is good correlation between SITT and reflectance for most samples up to a reflectance of about 1.0%. At this point the shale is fully compacted and its velocity does not increase with deeper levels of burial. Samples from the western Arctic (squares) consistently plot below the curve, indicating an anomalous heating event. The vitrinite equivalent reflectance at the base of the Jameson Bay Formation on the two curves in Fig. 10 is shown by the open diamonds.

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Table 2 Vitrinite reflectance equivalent (%Ro Vi eq.) and Sonic interval transit time (SITT; μm/s) at the base of listed formation in the Arctic Islands wells (see Fig. 12) Ro eq

SITT

Well name

Formation

Age

Ro eq

SITT

Well name

Formation

Age

0.42 0.43 0.45 0.46 0.47 0.47 0.48 0.53 0.54 0.55 0.55 0.58 0.58 0.59 0.59 0.62 0.63 0.64 0.66 0.66 0.67 0.68 0.68 0.7 0.7 0.71

36.4 38.0 32.1 35.3 31.9 31.1 33.0 30.4 30.2 33.5 33.3 32.6 28.9 30.5 26.9 30.9 30.2 26.8 26.0 25.8 24.8 25.3 28.5 26.7 27.4 28.1

Satellite F-68 Desberats B-73 Sutherland O-23 Depot Island C-44 Jameson Bay C-31 King Christian N-06 Emerald Skate B-80 Skate C-59 Drake Point D-68 Hecla J-60 East Drake I-55 Skybattle C-15 Balaena D-58 Cisco M-22 Collingwood K-33 Hazen F-54 Cisco B-66 Sutherland O-23 Whitefish A-26 Weatherall O-10 Cape MacMillan2K-15 Mocklin Point D-23 Elve M-40 Pat Bay A-72 Cape Norem A-80

Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Murray Harbour Fm Jameson Bay Fm Cape de Bray Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm Jameson Bay Fm

Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Jurassic Triassic Jurassic Devonian Jurassic Jurassic Jurassic Jurassic Jurassic

0.73 0.74 0.75 0.79 0.8 0.81 0.81 0.82 0.84 0.87 0.89 0.91 0.92 0.95 0.95 0.95 0.95 1.18 1.21 1.27 1.35 1.4 1.82 1.83 1.85 1.91 1.92

24.8 23.8 24.6 23.2 28.6 24.6 24.1 24.0 24.3 25.0 22.4 21.9 20.2 23.2 23.7 20.8 27.1 24.1 24.2 24.5 26.0 20.8 20.5 19.8 22.0 19.6 22.6

North Sabine H-49 Skybattle C-15 Linckens Island P-46 Skybattle M-11 Kusrhaak D-16 King Point West B-53 Noice G-44 Hearne F-85 Stokes Range J-11 Dumbells E-49 Skate C-59 Charles Point G-07 Pat Bay A-72 Bent Horn A-57 Cape Fleetwood M-21 Charles Point G-07 Uminmak H-07 Intrepid Inlet H-49 Muskox D-87 Dundas C-80 Beverley Inlet G-13 Key Point O-51 Bent Horn N-72 Cape Fleetwood M-21 Castel Bay C-68 Bent Horn A-57 Apollo C-73

Jameson Bay Fm Murray Harbour Fm Jameson Bay Fm Murray Harbour Fm Cape de Bray Fm Cape de Bray Fm Jameson Bay Fm Cape de Bray Fm Weatherall Fm Jameson Bay Fm Murray Harbour Fm Weatherall Fm Murray Harbour Fm Beverley Inlet Fm Parry Islands Fm Cape de Bray Fm Cape de Bray Fm Cape de Bray Fm Cape de Bray Fm Weatherall Fm Cape de Bray Fm Cape de Bray Fm Cape de Bray Fm Cape de Bray Fm Weatherall Fm Cape de Bray Fm Cape de Bray Fm

Jurassic Triassic Jurassic Triassic Devonian Devonian Jurassic Devonian Devonian Jurassic Triassic Devonian Triassic Devonian Devonian Devonian Devonian Devonian Devonian Devonian Devonian Devonian Devonian Devonian Devonian Devonian Devonian

between the two black lines on Fig. 13, indicates the path that the Gore Point Member in the Sverdrup Basin took during burial. Most exploration for oil and gas took place on large, salt-cored anticlines, so most samples of the Gore Point Member come from uplifted areas. The salt-cored structures formed late in the basin history in the Eocene, during the Eurekan Orogeny, and hence post-date maximum burial (Fig. 2). This means that they once sat within the shaded area and have been uplifted, so that the reflectance is much greater than the current depth of burial would indicate. A minimum estimate of uplift can be derived from the vertical distance between the normal burial curve and the current position.

Fig. 13. Depth vs reflectance for the Gore Point Member of the Roche Point Formation (Triassic). The shaded area is the normal burial curve for the Gore Point Member in the Sverdrup Basin as defined by wells from structurally undisturbed areas. Samples of the Gore Point Member from large, salt-cored anticlines at Skate C-59 and Cape Allison C-47 have reflectances much greater than their current depth of burial indicates. The difference between the normal burial curve and the current position gives an estimate of the uplift amount.

For instance, samples of the Gore Point Member from large, saltcored anticlines at Cape Allison C-47 and Skate C-59 shown on Fig. 13 have uplifts of about 400 and 800 m, respectively. On-going stratigraphic and seismic studies seek to confirm or refute these estimates.

Table 3 Vitrinite equivalent reflectance (Ro eq %) at the base of the Gore Point Member of the Roche Point Formation, and elevation of base of the Gore Point Member in metres below sea level (see Fig. 13) Well Name

Ro eq. (%) at base of Gore Point Mbr

Base Gore Well name Point Mbr below sealevel (m)

Ro eq. (%) at base of Gore Point Mbr

Base Gore Point Mbr below sealevel (m)

Andreasen L-32 Brock C-50 Brock I-20 Cape Allison C-47 CapeMamen F-24 Cape Norem A-80 Chads Creek B-64 Collingwood K-33 Desbarats B-73 Drake B-44 Drake E-78 Drake F-16 Drake Point D-68 Drake Point K-79 Drake Point L-67 East Drake L-06 East Hecla C-32 East Hecla F-62 Emerald K-33 Fosheim N-27 Grenadier A-26 Hazen F-54 Hecla I-69 Hecla J-60 Intrepid Inlet H-49

0.51 0.7 0.69 1.01 1.08 1.1 0.62 0.71 0.47 0.58 0.58 0.56 0.59 0.67 0.6 0.63 0.6 0.66 0.51 0.9 0.76 0.73 0.61 0.57 0.46

1067.8 353.9 316.1 3175 2517.5 2202 1422.8 1844.1 797.1 1304.9 1347.8 1271 1327.4 1495.6 1311.9 1086.4 1209.8 1106 1595.8 29.1 1739.4 1640.2 1149.1 1225 847.5

0.62 0.84 1.06 0.71 0.56 1.48 0.62 0.85 0.65 0.87 0.64 0.78 0.86 0.64 0.54 0.8 0.7 0.67 0.57 0.64 0.92 0.58 0.73 0.84 0.61

1096.9 2294.5 3903.6 2463.2 1583.9 2359.2 525.8 3468.6 1174.7 2900.2 1029.1 2827.9 2683.8 1243.6 1247.6 1891.7 2676.1 2401.1 1144.8 1762.8 2560.5 1013.1 2464.3 2806.8 1152.1

Jameson Bay C-31 King Christian N-06 Kristoffer Bay B-06 Maclean I-72 Marryatt K-71 Mokka A-02 Neil O-15 North Sabine H-49 Northeast Drake P-40 Pat Bay A-72 Pollux G-60 Roche Point O-43 Romulus C-42 Sherard Bay F-14 Sherard Bay F-34 Skate C-59 Skybattle Bay C-15 Skybattle Bay M-11 Southwest Hecla C-58 Sutherland O-23 Talemen J-34 West Hecla P-62 Whitefish 2h-63 Whitefish A-26 Wilkins E-60

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5. Conclusions A large dataset of thermal maturity measurements was compiled from numerous sources for the Canadian Arctic Islands. Several steps were taken to verify the data. Firstly, the Rock-Eval Tmax parameter was assessed for repeatability by comparing closely spaced samples. Samples with S2 greater than 0.35 mg HC/g rock were found to be reliable. Secondly, Tmax was converted to an equivalent vitrinite reflectance so that it could be plotted with measured vitrinite reflectance. Depth vs maturity plots were made for each borehole. Common problems identified in the dataset include caved material from higher in the borehole, misidentified macerals, reworked material from older stratigraphic successions, suppression of Tmax and vitrinite reflectance due to high hydrogen content, and poor correspondence between different thermal maturity parameters. An independent parameter against which the thermal maturity data can be compared is recommended as a way to ensure data quality. In the Arctic Islands, the sonic interval transit time (SITT) was compared to the vitrinite reflectance. Both parameters should respond to burial; the sonic velocity increases due to compaction and vitrinite reflectance increases due to heating. Samples from widespread shale units in the Arctic Islands generally have good correlation between SITT and reflectance, up to a reflectance of about 1.0%. Correlation between reflectance and SITT may differ between sedimentary basins due to differences in heat flow, rate of burial, and chemistry of the shales. Two ways in which the entire dataset can be used in the interpretation of the basin history are illustrated. Correlation between SITT and reflectance is generally good for the Arctic Islands. However, samples from one area in the western Arctic fall below the SITT–reflectance curve. These are inferred to have undergone anomalous heating. Comparison of the thermal maturity of a single horizon vs its depth shows that samples from structurally-undeformed areas define a normal burial curve for the Sverdrup Basin. Samples from salt-cored anticlines have much greater reflectance than their current depth of burial would indicate. From this, it can be inferred that the structures post-date maximum burial, and the amount of uplift can be estimated graphically. References Beauchamp, B., Harrison, J.C., Henderson, C.M., 1989. Upper Paleozoic stratigraphy and basin analysis of the Sverdrup Basin, Canadian Arctic Archipelago: Part 1, time frame and tectonic evolution. Geological Survey of Canada Paper 89-1G, pp. 105–113. Bordenave, M.L., Espitalie, J., Leplat, P., Oudin, J.L., Vandenbroucke, M., 1993. Screening techniques for source rock evaluation. In: Bordenave, M.L. (Ed.), Applied Petroleum Geochemistry. Editions Technip, Paris, pp. 246–250. Brooks, P.W., Embry, A.F., Goodarzi, F., Stewart, R., 1992. Organic geochemistry and biological marker geochemistry of Schei Point Group (Triassic) and recovered oils from the Sverdrup Basin (Arctic Islands, Canada). Bulletin of Canadian Petroleum Geology 40, 173–187. Buchardt, B., Lewan, M.D., 1990. Reflectance of vitrinite-like macerals as a thermal maturity index for Cambrian–Ordovician Alum Shale, southern Scandinavia. American Association of Petroleum Geologists Bulletin 74, 394–406. Bustin, R.M., 1986. Organic maturity of Late Cretaceous and Tertiary coal measures, Canadian Arctic Archipelago. International Journal of Coal Geology 6, 71–106. Carr, A.D., 2000a. Suppression and retardation of vitrinite reflectance: Part 1. Formation and significance for hydrocarbon generation. Journal of Petroleum Geology 23, 313–343. Carr, A.D., 2000b. Suppression and retardation of vitrinite reflectance: Part 2. Derivation and testing of a kinetic model for suppression. Journal of Petroleum Geology 23, 475–496. Dewing, K., Harrison, J.C., Pratt, B.R., Mayr, U., 2004. A probable Late Neoproterozoic age for the Kennedy Channel and Ella Bay formations, northeastern Ellesmere Island and its implications for passive margin history of the Canadian Arctic. Canadian Journal of Earth Sciences 41, 1013–1025. Dewing, K., Obermajer, M., Goodarzi, F., 2007a. Geological and Geochemical Data from the Canadian Arctic Islands. Part III: Organic matter reflectance data. Geological Survey of Canada Open File 5476, CD-Rom. Dewing, K., Obermajer, M., Snowdon, L., 2007b. Geological and Geochemical Data from the Canadian Arctic Islands. Part VII: Composition of gas from petroleum exploration borehole cuttings. Geological Survey of Canada Open File 5561, CD-Rom. Embry, A.F., 1991. Mesozoic history of the Arctic Islands. In: Trettin, H.P. (Ed.), Geology of the Innuitian Orogen and Arctic Platform of Canada and Greenland. Geological Survey of Canada, Geology of Canada no. 3 (also Geological Society of America, Geology of North America v. E), 370–433.

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