Geological Survey of Denmark and Greenland Bulletin 28, 2013, 61-64 61 Calibration of spectral gamma-ray logs to deltaic sedimentary facies from the Cretaceous Atane Formation, Nuussuaq Basin, West Greenland Gunver Krarup Pedersen, Niels H. Schovsbo and Henrik Nøhr-Hansen Gamma-ray logs are widely used as a lithology indicator in wells as part of standard petrophysical interpretations. In cored wells, gamma-ray logs should always be calibrated to the lithology in order to correct the petrophysical model. Gamma radiation is emitted from three elements, K, Th and U (potassium, thorium and uranium) which occur in miner- als such as feldspar, mica, glauconite, clay minerals, zircon, titanite and apatite as well as in organic complexes. Organic- rich mudstones usually have high gamma-radiation values and quartz-rich sandstones low values. In many places, up- ward-coarsening successions are recognisable from the gam- ma log. The gamma log records the sum of radiation from K, Th and U, and their relative contributions are measured in a spectral gamma-ray log. The present case study focuses on spectral gamma-ray characterisation of the deltaic Atane Formation which shows well-developed, upward-coarsening delta-front deposits in outcrops (Fig. 1C). Geological setting The Nuussuaq Basin is a rift basin, which contains the only exposures of Cretaceous and Paleocene sediments along the west coast of Greenland. The siliciclastic sediments are over- lain by a thick pile of volcanic rocks (Chalmers et al. 1999; Dam et al. 2009). During Late Cretaceous, Greenland was characterised by a warm and probably humid climate. The sediments range from alluvial fans overlying deeply weath- ered Precambrian basement through marginally marine deposits to marine deep-water deposits, all referred to the Nuussuaq Group (Dam et al. 2009). The f loodplains and delta plains had a rich f lora, recorded in well-preserved plant Fig. 1. A: Geological map of central West Greenland showing the location of Ataata Kuua on the south coast of Nuussuaq. B: The Atane Formation is erosionally overlain by the Kangilia Formation in the western slope of Ataata Kuua. The yellow dot marks the drilling site of borehole 247801, d: dyke. Height of section c. 500 m. The frame shows the position of Fig. 1C. C: The Atane Formation with depositional environments indicated. Note the distinct coarsening-upward successions (triangles). Height of section c. 100 m. © 2013 GEUS. Geological Survey of Denmark and Greenland Bulletin 28, 61–64. Open access: www.geus.dk/publications/bull Greenland 70° Disko 50 km Nuussuaq 53° Volcanics Ataata Kuua Precambrian Cretaceous sediments Atane FormationAtane Formation A C B C Kangilia Formation 13 0 m d d d 247801 Delta front Delta plain Shoreface Channel 60 m 6262 Fig. 2. Data from borehole 247801: a simplified sedimentological log of the entire core (566 m), a gamma log measured in the borehole to a depth of 320 m, and the new range chart for the dinof lagellate cysts in core samples. API: American Petroleum Institute units. GGU 247801 is located at 70°19.87´N, 52°55.8´W. The framed interval is shown in Fig. 3. The drill site is shown in Fig. 1B. D ep th (m ) 25 50 75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 C hr on o. La te C re ta ce ou s Pe ri od /E po ch ea rl y C on ia ci an – e ar ly S an to ni an A ge Sa m pl es 23.70 43.90 54.35 91.50 118.05 138.55 149.70 182.40 219.00 239.60 256.65 290.85 336.20 360.45 391.00 405.30 433.60 464.00 485.70 519.60 545.70 Cl ei st os ph ae rid iu m m ul tif ur ca tu m Isa be lid in iu m s pp . O do nt oc hi tin a sp p. Ra ph id od in iu m fu ca tu m Tr ith yr od in iu m s us pe ct um Ch at an gi el la d iti ss im a Ch at an gi el la g ra nu lif er a Ci rc ul od in iu m d ist in ct um H et er os ph ae rid iu m d iff ici le La cin ia di ni um a rc tic um O do nt oc hi tin a op er cu la ta O lig os ph ae rid iu m a lb er te ns e O lig os ph ae rid iu m s pp . Pa la eo hy st ric ho ph or a in fu so rio id es Sp in id in iu m c f. ec hi no id eu m Sp in id in iu m s pp . Sp in ife rit es s pp . Ta ny os ph ae rid iu m s pp . Isa be lid in iu m m ag nu m Fr om ea fr ag ilis Isa be lid in iu m c oo ks on ia e Pa la eo pe rid in iu m p yr op ho ru m Ch at an gi el la s pp . Tr ith yr od in iu m s pp . Ex oc ho sp ha er id iu m s pp . Su rc ul os ph ae rid iu m lo ng ifu rc at um H et er os ph ae rid iu m h et er ac an th um Pa ra le ca ni el la in de nt at a Al te rb id in iu m s pp . Ch at an gi el la m cin ty re i Se ne ga lin iu m s pp . Pa la eo cy st od in iu m s pp . ? Dinoflagellate cysts Ru gu bi ve sc icu lit es ru go su s Ru gu bi ve sic ul at es re du ct us Ap pe nd ici sp or ite s cf . in sig ni s ? ? ? ? ? ? ? ???? ?? ?? ? ?? ? ? ? ? ? ? SP Events Chatangiella mcintyrei Heterosphaeridium difficile Palaeoperidinium pyrophorum Spinidinium cf. echinoideum, Chatangiella ditissima, Chatangiella granulifera, Heterosphaeridium difficile Trithyrodinium suspectum, Rugubivesciculites rugosus, Isabelidinium spp. 0 100 200 API Base Top 200 250 300 350 400 450 500 550 Tr Tr Tr Tr Tr Tr 50 100 20 m 150 Tr Tr Tr Core from borehole GGU 247801 A ta ne F or m at io n Q ila ki ts oq M em be r Delta front Channel Delta plain Coal Shoreface 63 macrofossils and abundant comminuted plant debris. A huge volume of non-marine to shallow marine deposits consti- tutes the Cretaceous Atane Formation, which is well exposed along 65 km of the south coast of Nuussuaq from sea level to altitudes of 500–800 m. Seismic data indicate a minimum thickness of 3000 m for the formation (Dam et al. 2009). Ataata Kuua – A narrow f luvial valley at Ataata Kuua, on the south coast of Nuussuaq (Fig. 1), shows the deltaic Atane Formation erosively truncated and overlain by the turbid- itic, Paleocene Kangilia Formation (Dam et al. 2009). In 1980, the Geological Survey of Greenland drilled a 566 m deep borehole (GGU 247801) at Ataata Kuua as part of a regional study of the composition and distribution of coal in the Atane Formation. The entire succession was cored, with 100% recovery, and a gamma log was measured in the upper 320 m of the borehole. This gamma log as well as a simplified sedimentological log of the entire core are shown adjacent to the new biostratigraphical range chart in Fig. 2. Biostratigraphy – In recent studies of 21 delta-front mud- stone samples from core 247801, palynomorphs have been examined (Fig. 2). The diversity and density of dinof lagellate cysts, spores and pollen are very low, but the presence of Cha- tangiella granulifera, Heterosphaeridium difficile and Spini- dinium cf. echinoideum in the lower part and Chatangiella mcintyrei and Spinidinium cf. echinoideum in the middle to upper part indicates an early Coniacian age or younger. An early Santonian minimum age of the upper part of the core is indicated by the presence of Rugubivesciculites spp., the absence of Campanian marker species and by the pres- ence of Heterosphaeridium difficile, Laciniadinium arcticum and Spinidinium cf. echinoideum in a sample from the Ataata Kuua 2004-3 section situated immediately above the site of borehole 247801. The relatively uniform dinof lagellate as- semblage recorded throughout the core (Fig. 2) supports the interpretation of a relatively high sedimentation rate. Sedimentology – The delta deposits of the Atane Formation represent four depositional environments: delta front, distrib- utary channel, delta plain and shoreface (Figs 1C, 2, 3). The delta-front deposits include mudstones, heterolithic sand- stones with wave-generated sedimentary structures and well- sorted sandstones, all with comminuted plant debris. The distributary channel deposits are mostly cross-bedded, medi- um- to coarse-grained sandstones with some feldspar grains. The delta-plain mudstones are interbedded with coal beds or thin sandstone beds. The thin shoreface sandstones contain abundant marine trace fossils and overlie erosive surfaces. The delta-front deposits and the overlying f luvial sandstones form distinctly upward-coarsening units (Figs 1C, 2, 3), in- terpreted as formed during delta progradation. The shoreface sandstones are interpreted as deposited during a transgression. A detailed log of the core is shown in Dam et al. 2009 (fig. 43). The dominant minerals in the mudstones are quartz and kaolinite, neither of which contain more than traces of K. Small amounts of K-feldspar and mica result in a K content about 1.3–1.8% K2O, significantly lower than the 2.7% K2O of average mudstones (Rider 1990). All mudstones and many sandstones in the Atane Formation contain comminuted debris from higher land plants. Total organic carbon (TOC) values of the non-marine delta-plain mudstones range from 3 to 15% and include thin coal beds with 50–65% TOC (Pedersen et al. 2006). The marine delta-front mudstones contain 6–14% TOC, with the highest values in the fine- grained, lower part, which includes a f looding surface and had a low sedimentation rate. Despite this, marine organic particles, such as dinof lagellate cysts, only form a small part compared to terrestrial organic matter. The gamma log obtained in the borehole shows, as ex- pected, low values for the f luvial sandstones whereas the mud-dominated delta-front and delta-plain deposits are dif- ficult to identify from the gamma log alone (Fig. 2). In order to document the contributions of K, Th and U to the total gamma radiation we measured the spectral gamma radiation (Fig. 3). K is mainly located in feldspar, mica and glauconite; Th and U are hosted in zircon, titanite, and apatite. Clay minerals may contain small amounts of Th, and organic complex compounds may contain U. Spectral gamma-ray characterisation Core scanning – The core interval was scanned at the core labor- atory at the Geological Survey of Denmark and Greenland using a set-up which allows simultaneous spectral gamma-ray and density measurements. The spectral gamma-ray analysis is carried out using two 15 cm NaI (Tl) crystals and the bulk density is determined using a caesium source. The scanning speed was 1 cm/min., corresponding to a vertical resolution of approximately 2 cm for the density log. The scanning data thus supply high resolution data to support sedimentary and geochemical data from the core, as exemplified in Fig. 3. Results – The upward-coarsening successions, which are clearly seen in the field and in the core (Figs 1C, 2, 3), are difficult to identify on the total readings of the spectral gam- ma log (Fig. 3). The grain-size trends are, however, ref lected in the Th and K logs, which are negatively correlated. The mudstones have high Th and low K contents, whereas the sandstones have high K and low Th contents. No distinct 6464 relationship between high U content and mudstone grain- size is seen. Ruffell et al. (2003) presented a model for the f lux of K, U and Th in different weathering systems at basin scale. One of their scenarios is a basin with a low-relief hinterland and a humid climate, which applies to the depositional setting of the Atane Formation. The model predicts that chemical weathering dominates and that K and U are removed in so- lution to sea water, while Th is concentrated in detrital clay. This model may explain the relatively high Th radiation in the detrital mudstones of the Atane Formation. K-feldspar is a minor constituent of the sandstones but contributes sig- nificantly to the K-radiation in the sandstones. The model further predicts that K and U are enriched in authigenic minerals in the basin. Such enrichment of U is not observed in the Atane Formation, possibly due to the relatively high sedimentation rate. The low U content may also ref lect the predominance of land plants (type III kerogen) that gener- ally contain small amounts of U in organic complexes com- pared to marine organic material. A comparison of sedimen- tological logs from delta-front successions with their total gamma radiation in a case study from Ireland also indicates that the delta-front successions are difficult to identify from the gamma log alone (Davies & Elliott 1996). Summary The present study demonstrates the importance of calibrat- ing petrophysical logs to core data. The cyclicity which char- acterises the Atane Formation in outcrops and cores (Figs 1, 2), and which would be a means of identifying the Atane For- mation in an un-cored well, is obscured in the total gamma- ray log. This fails to resolve the grain-size variation in the Atane Formation because the presence of K-poor kaolinite, despite enrichment by Th, provides a ‘cleaner’ signature of the mudstones while the presence of sand-sized K-feldspar gives a ‘dirtier’ signature of the sandstones. The Th log can, to some degree, resolve the lithological variation but the en- ergy is too small to be ref lected in the total gamma-ray sig- nal. The high sedimentation rate and the predominance of terrestrial organic material precluded the development of a characteristic U signature in the marine mudstones. The gamma log thus shows the variations in elements which occur in small amounts, because the bulk of the sediment (quartz, kaolinite and terrestrial coal debris) contributes very little to the gamma-ray radiation. References Chalmers, J.A., Pulvertaft, T.C.R., Marcussen, C. & Pedersen, A.K. 1999: New insight into the structure of the Nuussuaq Basin, central West Greenland. Marine and Petroleum Geolog y 16, 197–224. Dam, G., Pedersen, G.K., Sønderholm, M., Midtgaard, H.M., Larsen, L.M., Nøhr-Hansen, H. & Pedersen, A.K. 2009: Lithostratigraphy of the Cretaceous–Paleocene Nuussuaq Group, Nuussuaq Basin, West Greenland. Geological Survey of Denmark and Greenland Bulletin 19, 171 p. Davies, S.J. & Elliott, T. 1996: Spectral gamma ray characterisation of high resolution sequence stratigraphy: examples from Upper Carbon- iferous f luvio-deltaic systems, County Clare, Ireland. In: Howell. J.A. & Aitken, J.F. (eds): High resolution sequence stratigraphy: innovations and applications. Geological Society (London), Special Publications 104, 25–35. Pedersen, G.K., Andersen, L.A., Lundsteen, E.B., Petersen, H.I., Bojesen- Koefoed, J.A. & Nytoft, H.P. 2006: Depositional environments, or- ganic maturity and petroleum potential of the Cretaceous coal-bearing Atane Formation at Qullissat, Nuussuaq Basin, West Greenland. Jour- nal of Petroleum Geolog y 29, 3–26. Rider, M.H. 1990: Gamma-ray log shape used as a facies indicator: critical analysis of an oversimplified methodolog y. In: Hurst, A., Lovell, M.A. & Morton, A.C. (eds): Geological applications of wireline logs. Geo- logical Society Special Publications (London) 48, 27–37. Ruffell, A.H., Worden, R.H. & Evans, R. 2003: Palaeoclimate controls on spectral gamma-ray radiation from sandstones. International Associa- tion of Sedimentologists Special Publication 34, 93–108. Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: gkp@geus.dk Fig. 3. Spectral gamma logs from 40 m of the core from borehole 247801. Note the difficulty in interpreting the total gamma radiation log, the negative correlation between K and Th content, and the absence of maxima on the U log. The core section is located in Fig. 2. Yel- low: sandstone, grey: mudstone, black: coal or plant debris, black stars: pyrite. cps: counts per second. The total gamma radiation (GR) may be compared to the radiation measured in the borehole (Fig. 2). 145 140 120 125 110 115 130 135 105 0 1 2 3 4 0 0 5 10 151 2 3 4 GR total (cps) K (%) 0 5 10 15 20 25 Th (ppm)U (ppm) ★ ★ vf clay silt sand f m c mailto:tl@geus.dk