ABSTRACT

Veryhachium is an acritarch genus that occurs commonly in marine shales from the Ordovician to the Tertiary, with morphologically closely related forms appearing in the Cambrian. Its simple morphology together with its long stratigraphic range and abundance makes it attractive as a potential thermal maturity indicator. The colour of Veryhachium is characterised in terms of red, green and blue intensity of transmitted white light. With increasing thermal maturity, Veryhachium changes from almost colourless to black, with the most rapid change taking place in rocks that are marginally postmature with respect to the oil window. The cutoff point for Veryhachium fluorescence lies between 1.5% and 2.0% in terms of vitrinite reflectance (Rr), providing another useful tie-point in terms of thermal maturity. This technique is simple, inexpensive and has potential for establishing the thermal maturity of both marine Lower Palaeozoic rocks and younger marine sections deficient in vitrinite.

INTRODUCTION

Mean random vitrinite reflectance (VR) is the industry standard for assessing thermal maturity. However, vitrinite is absent from most Lower Palaeozoic successions, only appearing from the Upper Silurian onwards (Tricker et al., 1992). Because vitrinite precursors are derived from terrestrial sources, it is also sometimes rare or absent in post Silurian marine successions.

The assessment of microfossil colour is perhaps the most common tool used to assess thermal maturity of sediments after VR. Thermal alteration of microscopic organic material within sediments affects both its chemical and physical properties, typically resulting in colour change. Conodonts, spores and pollen are the most frequently used microfossils for assessing thermal maturity based on colour (Marshall, 1990).

The five point Conodont Alteration Index (CAI) of Epstein et al. (1977) was correlated with specific temperature ranges based on experimental heating. CAI is determined by visual estimation of colour and comparison with a representative series of photographs. This technique is widely used because of the range of temperatures that can be estimated and the relatively long (Cambrian to Triassic) fossil record of conodonts (Marshall, 1990). However, it is rarely applicable to lithologies other than limestones.

Many workers have explored techniques for describing palynomorph colour. Two different approaches have been attempted, the first based on spectral analysis of light transmitted through palynomorphs (Gutjahr, 1966; Grayson; 1975; Smith, 1983) and the second utilising image analysis to characterise colour in terms of red, green and blue intensity values (RGB) (Yule et al., 1998; Ujiié, 2001; McCaughan, 2002).

Early attempts to utilise spectral analysis included those of Gutjahr (1966) and Grayson (1975) who demonstrated a systematic decrease in spore transmittance with increasing depth of burial. Smith (1983) used spectral analysis data to correlate ‘standard’ palynomorph material from Staplin’s TAI scale (1977), the Robertson Research International SCI scale and Batten’s seven-point scale (1981). Lo (1988) measured the transmittance spectra of spores at a wavelength of 546 nm and compared the results with visually estimated TAI and vitrinite reflectance data. Marshall (1991) also used spectral analysis to describe spore colour change and was the first to use the Commission Internationale de l’Eclairage (CIE) colour system. This system defines colour in terms of three variables; X and Y, the chromaticity co-ordinates, and L, luminance; the total amount of light as defined by the transmittance or absorbance of a substance. The advantage of this technique is that inter-laboratory variation can be reduced significantly but the disadvantage is that micro-spectrophotometers required for this work are expensive and not readily available in most laboratories.

An alternative approach for the determination of palynomorph colour utilises colour image analysis (CIA). This involves digitising images of palynomorphs, which are usually displayed on a VDU and consist of an array of pixels each having specific RGB values (Ujiié, 2001). Using the RGB system, colours are described in terms of three variables; red, green and blue intensities. The intensity of each colour can vary between 0 and 255, and 16.7 million colours can be described using this scheme. The equipment necessary to make RGB colour determinations is inexpensive and readily available, comprising only a transmitted light microscope, a colour video or digital camera, and an image analysis system. However, unlike the CIE colour system, determinations of RGB intensities are device-dependant with the choice of microscope, camera and software all influencing the RGB values determined.

We discuss herein the determination of the colour of specimens of a single common and long-ranging acritarch genus (Plate 1), Veryhachium, using image analysis techniques. These results are correlated with VR determined from the same samples, enabling the relationship between the two variables to be defined.

ACRITARCHS AND ACRITARCH COLOUR ALTERATION

Acritarchs are considered to be the resting cysts of single-celled or apparently single-celled eukaryotic, predominantly marine, organic walled microfossils (Martin, 1993; Wicander, 2002). They vary in size from <10 μm to more than 1 mm, but the majority of species range from c. 15 μm to 80 μm (Wicander, 2002). First appearing in the Proterozoic, they are most abundant and diverse between the early Cambrian and late Devonian (Martin, 1993; Wicander, 2002; Playford, 2003).

Although much work has been carried out on spore colour and its use as a thermal maturity indicator, acritarchs have, for the most part, been neglected. Legall et al. (1981) first published an ‘Acritarch Alteration Index’ (AAI) (Figure 1) based on the thermal maturation of Palaeozoic strata in Southern Ontario, Canada. In this study, they calibrated the colour of the sphaeromorph (leiosphere) acritarch genus Leiosphaeridia Eisenack 1958 emend Downie and Sarjeant, 1963 against CAI.

Legall et al. (1981) considered Leiosphaeridia to be the most suitable acritarch for colour determinations because of its simple morphology and large size. They visually estimated the colour of leiospheres and, to ensure consistency, compared them to colour photographs representing each point on their scale. Williams et al. (1998) studied Palaeozoic strata in western Newfoundland, Canada. Unlike Legall et al. (1981), they used acritarch assemblages rather than a single genus as a thermal maturity indicator. Their acritarch assemblage colours from Cambro-Ordovician rocks were correlated with TAI based on miospores extracted from nearby Carboniferous rocks (Figure 1).

Veryhachium Deunff 1954 ex Downie emend Sarjeant and Stancliffe is a common, long ranging (Ordovician-Tertiary, with morphologically closely related forms appearing in the Cambrian) acritarch with a simple and smooth-walled morphology. Numerous species have been erected but they are all very similar to each other in terms of size and wall thickness so that inter-specific variation in colour is negligible. Only the triangular form of Veryhachium with one process at each apex was used. Veryhachium was selected for this study because it occurs commonly in muddy marine Lower Palaeozoic strata whose thermal maturity has often been extremely difficult to establish.

MATERIALS AND METHODS

Sampling

Silurian, Devonian and Lower Carboniferous samples were investigated in order to obtain residues that contained both acritarchs and vitrinite. Outcrop samples, cuttings and cores were investigated from several regions, including Jordan, Belgium, the United Kingdom, Venezuela, Canada and the USA, details of which are given in Table 1. The Jordanian samples contained chitinozoans but no vitrinite, so chitinozoan reflectance was measured and the equivalent VR was calculated using the relationship derived by Tricker et al. (1992).

Sample preparation

All samples were processed using standard HF extraction techniques (Wood et al., 1996) with no oxidation. Once processed, sieved organic residue was pippetted onto No. 1 thickness 22 x 40 mm coverslips and allowed to dry overnight. These were then mounted onto 1–1.2 mm thick, 76 x 26 mm slides using Elvacite mounting medium.

Acritarch colour determination

Slides were studied using a Leitz Dialux 20 transmitted light microscope with a Leitz NPL Fluotar L25/0.55 objective. Images of the selected acritarchs were captured using a JVC TK-C1380 colour video camera attached to the microscope’s phototube. Illumination was provided by a Leitz Wild Heerbrugg power supply set to 9 V and a 100 W halogen light bulb. Equipment settings were not altered from one session to the next and the camera’s white balance remained the same. Images of suitable palynomorphs were captured by the colour video camera and converted to a digital image by an analogue-to-digital converter (ADC) within the Leica Q500IW imaging workstation.

In making RGB colour determinations, pixels were only selected where the wall (eilyma) of Veryhachium was intact; thinned portions of the wall associated with the excystment structure or where the vesicle had been damaged were avoided, as were surface blemishes, pyrite inclusions, and the yellow ‘halo’ around the pyrite. Furthermore, the three processes of Veryhachium were excluded from measurement because of their limited surface area.

Vitrinite reflectance determination

Polished palynological sections offer an alternative to resin blocks. Polished slides were used in this study, based on a technique adapted from that described by Hillier and Marshall (1988). Vitrinite and chitinozoan reflectance measurements were made using standard techniques (Tricker et al., 1992; Taylor et al., 1998).

Fluorescence

Qualitative fluorescence properties were determined using the same palynological slides and microscope used for colour determinations. The microscope was fitted with a Leitz PLOEMOPAK 2.4 high-powered mercury vapour fluorescence illuminator and a Leitz H2 filter block (BP 390–490 excitation filter, RKP 510 dichroic mirror, LP 515 barrier filter). Using this system, violet-blue radiation (390–490 nm) was produced. Palynomorphs were viewed using a Leitz NPL Fluotar L25/0.55 objective in a darkened room. After approximately 1-minute excitation of the organic matter, the presence or absence of fluorescence was recorded.

RESULTS

Mean RGB intensities and VR values from the same samples are shown in Table 1 and in Figure 2. Obviously caved or reworked Veryhachium specimens were excluded from the calculation of mean values for each sample.

Mean RGB values from samples containing degraded Veryhachium specimens plot off the trend defined by the well-preserved samples. The colour of well-preserved samples represents the thermal alteration of the original eilyma, which has essentially remained intact. The walls of degraded specimens appear to have been thinned in some cases, counteracting the darkening caused by thermal alteration of the vesicle. Therefore, the thermal alteration scale proposed herein is based on only well preserved specimens of Veryhachium. Degraded and poorly preserved specimens were identified but excluded from the correlation with VR.

The cut-off point for Veryhachium fluorescence lies between 1.5 and 2.0% in terms of VR (Rr). This is at a significantly higher level of maturity than the VR level of c. 1.35% for spores (Van Gijzel, 1982).

DISCUSSION

RGB intensities as Thermal Maturity Indicators

Samples were classified as either mature (Rr 0.6 - 1.3%) or postmature (Rr > 1.3%) with respect to the oil window (Selley, 1998). The relationship between R, G, and B intensities and VR are summarised in Figures 2a to 2c and Table 2. It is clear that two discrete populations can be identified. This is particularly obvious for R vs. G mean intensities (Figure 3c) where mature samples cluster tightly and postmature samples show a large amount of scatter. There is little change in the colour of Veryhachium specimens until the floor of the oil window is reached. The lack of significant colour change in Veryhachium specimens in mature samples means a simple linear equation is an inadequate method for describing the darkening of the palynomorph colour with increasing maturity.

Comparison with Previously Published Acritarch Alteration Indices

In their study of thermal maturation of Palaeozoic strata in Southern Ontario based on Leiosphaeridia, Legall et al. (1981) described degraded leiospheres from strata with CAI values of 2–2.5 (indicating peak temperatures of 50°C to 90°C). They proposed a maximum peak palaeotemperature of 90°C for their Acritarch Alteration Index and suggested that leiospheres could not survive much higher temperatures. They also inferred a 60°C minimum palaeotemperature for the strata studied, based on a range of data collected in their study area. Results from the present study suggest that Veryhachium is useful as a maturity indicator for much higher palaeotemperatures, with well preserved specimens observed in samples with VR values of 2.45% indicative of peak palaeotemperatures of c. 207°C (Barker and Pawlewicz, 1994).

Comparison of Veryhachium Colour and Spore Colour

Bertrand and Heroux (1987) stated that for samples of equal rank, acritarch colour was lighter than spore colour. They also found that acritarchs were better preserved than spores in samples of higher maturity. Observations made during this study concur with these results.

Using multi-taxon acritarch assemblages, Williams et al. (1998) determined that, while there were different maturation paths for spores and acritarchs in immature strata, there was little difference within the oil window. They also found that AAI values in the upper part of the oil window were lower than corresponding (spore-based) TAI but by the time the floor of the oil window was reached, spore and acritarch colours were essentially the same. Our results show that marked differences in colour between Veryhachium and miospores still persist at the floor of the oil window. However, in many samples examined for this study, specimens belonging to other acritarch genera such as Unellium were observed to be darker in colour than those of Veryhachium. This observed variation in colour between genera probably accounts for the different conclusions reached.

A crude comparison of the change in colour of spores and Veryhachium specimens with increasing thermal maturity can be made. It is clear that whereas most spore colour change occurs within the oil window, Veryhachium colour appears to remain almost constant through this interval (Figure 4). It is not until after the floor of the oil window has been reached that significant Veryhachium colour change occurs.

In their CIA study of spores, Yule et al. (1998) found that most spore colour change occurred rapidly over a small temperature interval and interpreted this event as being directly related to the chemical breakdown of the spore wall during hydrocarbon generation. This represented the ‘mature phase’ of their samples where spores changed from a stable yellow colour to a stable brown/black colour. Our Veryhachium specimens did not change significantly in colour until after the floor of the oil window had been reached, suggesting that the chemical breakdown of their walls occurs at a different stage relative to that of spores, probably reflecting differences in chemical composition.

Choice of R, G or B as the Preferred Parameter

Based on R2 value and calculation of 95% confidence intervals (Table 2), it appears that blue intensity (B) is more sensitive to changes in thermal maturity than red (R) or green (G) for our Veryhachium dataset. This contrasts with results obtained by Yule et al. (1998) who used CIE to quantify the colours of spores from a series of boreholes and from artificially matured samples and who found that average green intensity of spores within a sample was the most effective indicator of maturity. It also contrasts with the conclusion of Van de Laar and David (1998), that red intensity (R) was the most sensitive parameter in terms of changing thermal maturity, based on their study of Carboniferous miospores.

Calibration

Several spore and pollen colour scales have been published, with Staplin’s (1969, 1977) five point ‘Thermal Alteration Index’ (TAI) perhaps the most widely used. Other schemes include The ‘Spore Colouration Index’ (SCI), a ten-point scale produced by Robertson Research International (Barnard et al., 1976) and Batten’s (1976, 1996) seven-point spore colour scale. Attempts to ensure inter-laboratory consistency have included published visual references such as the Pollen/Spore Colour ‘Standard’ of Pearson (1984) and sets of representative spore and pollen specimens, though neither of these has proved particularly successful.

Calibration of any formal Veryhachium Alteration Index would ideally utilize several readily-available standards consisting of thin sections of readily-available synthetic materials with colours closely corresponding to actual Veryhachium colours. Each standard would be allocated a nominal RGB value against which a microscope and camera could be calibrated. Preliminary attempts to identify suitable standards have included investigation of photographic filters but these have proved unsuccessful due to large variations in thickness and colour. A ‘Veryhachium Colour Index’ based on relative R, G and B intensities would largely overcome the problem of device dependence. However, the darkening of Veryhachium with increasing thermal maturity represents broadly similar reductions in R, G and B intensities (Figure 3), so that an index based on a comparative measure such as B(R+G+B)-1 would not be a satisfactory discriminant.

CONCLUSIONS

Determination of the colour of Veryhachium offers a partial solution to determining the thermal maturity of Lower Palaeozoic rocks and certain younger marine rocks deficient in other organic maturity indicators. Unlike spore colour, significant change in the colour of Veryhachium due to thermal alteration takes place only after the oil window floor has been reached. Veryhachium colour is most useful in distinguishing between samples that are mature, marginally postmature, and postmature with respect to the oil window.

The cut-off point for Veryhachium fluorescence lies between 1.5 and 2.0% in terms of VR (Rr). This is at a significantly higher level of maturity than the VR level of c. 1.35% for spores (Van Gijzel, 1982). Further work is needed to devise a system of calibration so that comparable Veryhachium RGB results can be obtained from different laboratories.

ACKNOWLEDGMENTS

This study was funded by a Petroleum Technology Scholarship from the Petroleum Affairs Division of the Irish Government in association with the Occidental Petroleum Corporation, Marathon Oil Corporation and ConocoPhillips. Additional funding was provided by Petrel Resources plc. and Trinity College Dublin. The authors are grateful to the following for assistance in collecting samples and data for the investigation; Craig Harvey, Mike Howe, Dave Naylor, Aideen McNestry, Munim Al-Rawi, Ron Rea, Thomas Servais and Ann Wilson. Thanks also to Robbie Goodhue, James Killen, John Marshall and Reed Wicander for their advice and support. The Jordanian Natural Resources Authority (NRA), the British Geological Survey, the Ohio Geological Survey, the Ontario Geological Survey and Petrel Resources plc. kindly provided access to samples and data for the study and have granted permission to publish. The final design and drafting by Nestor Buhay is appreciated.

ABOUT THE AUTHORS

Catherine M. B. Duggan is currently employed as a Geoscientist by Tullow Oil plc, based in their office in Dublin, Ireland. She studied Natural Sciences in Trinity College Dublin, graduating with a BA in Geology from the University of Dublin in 2002. She gained her PhD from the same university in 2006 for a thesis on, ‘Acritarch and prasinophyte colours as thermal maturity indicators’. After completing her doctorate, she was employed by the Department of Communications, Marine and Natural Resources of the Irish Government in the Exploration and Mining Division, involved with the regulation of the mining industry in Ireland. Catherine is an active member of the Commission Internationale de Microflore du Paléozoique (CIMP) and in September 2006 was elected Secretary of its Acritarch Subcommission. Her professional interest lies in organic maturation techniques.

catherine.duggan@tullowoil.com

Geoff Clayton is Associate Professor of Geology in Trinity College Dublin (TCD). He graduated from the University of Sheffield with a BSc in 1968 and PhD in 1972, and was awarded his ScD from the University of Dublin in 2001. He lectured briefly in University College Dublin before joining Trinity College Dublin in 1979. From 1984-1989, he was Director of the TCD Applied Geology Unit and was Head of the Department of Geology from 1993-1996. He is currently Vice Chair of the Subcommission on Carboniferous Stratigraphy, Past President of the Commission Internationale de Microflore du Paléozoique, and is a member of AASP. His publications include several papers on Saudi Arabia and Libya. In 2000 he was elected a Member of the Royal Irish Academy and he was TCD Berkeley Research Fellow in 2002. His professional interests cover palynology, Upper Palaeozoic stratigraphy and palaeogeography, and hydrocarbon source rocks.

gclayton@tcd.ie