Palynology, palynofacies and paleoenvironmental studies of the Late Devonian sediments of the Atiavi-1 well, Keta Basin, southeastern Ghana

Accepted Manuscript Palynology, palynofacies and paleoenvironmental studies of the Late Devonian sediments of the Atiavi-1 well, Keta Basin, southeastern Ghana David Atta-Peters, Christopher A. Achaegakwo PII:

S1464-343X(16)30377-6

DOI:

10.1016/j.jafrearsci.2016.11.018

Reference:

AES 2731

To appear in:

Journal of African Earth Sciences

Received Date: 17 December 2015 Revised Date:

14 November 2016

Accepted Date: 18 November 2016

Please cite this article as: Atta-Peters, D., Achaegakwo, C.A., Palynology, palynofacies and paleoenvironmental studies of the Late Devonian sediments of the Atiavi-1 well, Keta Basin, southeastern Ghana, Journal of African Earth Sciences (2016), doi: 10.1016/j.jafrearsci.2016.11.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Palynology, palynofacies and paleoenvironmental studies of the Late Devonian sediments of the Atiavi-1 well, Keta Basin, southeastern Ghana.

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*David Atta-Peters, and Christopher A. Achaegakwo Department of Earth Science, University of Ghana, P. O. Box LG 58, Legon, Accra, Ghana

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*Corresponding author e-mail: [email protected]; [email protected]

ABSTRACT

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Palynomorph and palynofacies analysis have been performed on 34 cutting samples from the Late Devonian sediments of the Atiavi-1 well in the Keta Basin. Five (5) palynofacies associations linked to different depositional environments have been identified based on the kerogen content in the samples. Palynofacies types Pf A, Pf B and Pf E reflect deposition under

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mud-dominated oxic shelf conditions (distal shelf), characterized by kerogen type III >IV which is gas prone. Palynofacies type C (Pf C) and D (Pf D) reflect heterolithic oxic shelf (proximal) conditions and are also characterized by type III or IV kerogen.

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The palynomorph associations (miospores, acritarchs and chitinozoans) indicate an offshore distal or outer shelf environment for the lower section (1490 m – 1244 m) with the

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upper sections (1244 m – 936 m) showing intermixing of land - derived elements (miospores, phytoclasts and cuticles) with minor amounts of acritarchs, which strongly suggests deposition in shallow marine/fluvio-deltaic environment.

The abundance and diversity of spores is an

indication of humid climatic condition which existed at the time of deposition. Palynomorphs with marker species based mainly on miospores assemblages and some diagonistic chitinozoans and acritarchs association from the Atiavi-1 well indicate an age of upper lower Devonian (Emsian) to latest Devonian (Strunian).

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Keywords: Palynomorphs, palynofacies, acritarchs, chitinozoans, Keta Basin, Emsian, Strunian,

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Ghana.

1. Introduction

The Atiavi – 1 well is located in the Keta Basin in the southeastern corner of Ghana (Fig.

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1). The basin is a coastal wrench pull-apart Mesozoic basin that extended from continental to

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marine depositional environment (Atta-Peters et al., 2012). The basin development has experienced three important tectonic periods that influenced the structure and the sedimentation rate; the Pre-Rift stage (Precambrian – late Jurassic), Syn-Rift stage (late Jurassic – early Cretaceous) and Post-Rift stage (late Cretaceous – Recent) (Abu et al., 2010). The rift phases reflect the progressive separation of the African plate and South American plates, resulting in

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marine ingression in basins of the continental margin, with variations in the sea levels resulting in deposition of sediments in varied local palaeoenvironments. concentrated

Earlier workers have

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on the use of palynology for biostratigraphic and palaeonenvironmental analysis of the

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exploratory wells from the Keta Basin (Anan-Yorke, 1994; Atta-Peters et al. 2012; Apaalse and Atta-Peters, 2013). This study is aimed at (1) identifying the different types of sedimentary organic matter (kerogen) and thus hydrocarbon type expelled, (2) characterize the particulate organic matter identified, with a view to identifying the palynofacies types and to infer depositional palaeoenvironments ascribed to these facies. Palynofacies analysis involves the integrated study of all aspects of the palynological organic matter assemblage, which include the identification of the individual particulate

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components, assessment of their absolute and relative proportions, particle sizes, and their preservation state (Tyson, 1995). The palynofacies classification as presented by Tyson (1995), identified four main groups of particulate organic matter. These are amorphous organic matter

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(AOM), palynomorphs, phytoclasts, and opaques. It can be applied in the determination of kerogen types and abundance, providing clues concerning depositional environments and

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hydrocarbon generation potential.

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2. Geology and Geological Setting

The Keta basin is a southward open monocline, controlled by basement flexures and faults (Akpati, 1975). It is characterized by two sub parallel faults, the Fenyi-Yakoe and Adina faults which are generally regarded as the northeastern extensions of the Romanche Fracture

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Zone. The Keta Basin is one of the chains of Mesozoic and Tertiary sedimentary basins along the Gulf of Guinea Province which also includes Ivory Coast, Tano, Saltpond, and Benin Basins and the Dahomey Embayment, due to similarities in structural geology and stratigraphic history. The

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province experienced complex tectonic activity (late Jurassic – early Cretaceous) that was typified by block and transform faulting superimposed on extensive Palaeozoic basin in the

2006).

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course of the breakup of the African and South American plates (Brownfield and Charpentier, The developments of the basins experienced three important tectonic periods, that

influenced the structure and sedimentation rate; the pre-rift stage ((Precambrian to late Jurassic), syn-rift (late Jurassic to early Cretaceous), and post-rift (late Cretaceous to Recent) (Brownfield and Charpentier, 2006; Abu et al. 2010) (Fig. 2). The occurrence of these basins are attributed to the subsidence along the line of the Late Precambrian orogenic belt, that later marked the

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separation of the Africa and South America (Sutton, 1968).

It is a basin in which its

development has Stratigraphically, the Keta Basin consists of Cretaceous and Tertiary sediments overlying Palaeozoic rocks of Devonian age (Table 1). These Palaeozoic rocks correlate with

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rocks of the Sekondian Group of the Saltpond or Central Basin. The Cretaceous-Tertiary rocks within the basin are separated from the Palaeozoic unit by a dolerite sill which serves as a good marker for local stratigraphy (Anan-Yorke, 1974; Apaalse and Atta-Peters, 2013). The evolution

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of the basin started in the early Cretaceous period, with the opening of the South Atlantic Ocean when Eastern Ghana was a shallow seaway in the gradually subsiding basin that had experienced

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block and extensional faulting (Wright et al., 2009). The grabens were filled with significant clastic input derived from the Volta drainage system (Fig. 1). As the opening of the South Atlantic progressed, accommodation space was created due to the rapid deepening of the Keta basin and it was further filled with a Cenomanian clastic wedge that delineates the transition

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from a shallow water seaway between Africa and South America to a deepwater environment with a very narrow shelf. The Cenomanian - Turonian marine transgression is attributed to deposition of organic rich shales in the basin (Wright et al., 2009). Uplift in the late Cretaceous

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caused an erosion that was very extensive and responsible for removing large amount of the late Cretaceous rocks, thus creating a peneplain in the basin (Brownfield and Charpentier, 2006). The

1992).

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Oligocene unconformity also led to significant erosion of Tertiary rocks (Kjemperud et al.,

The prospective source rock intervals include the Devonian shales (Apesegah and Hotor,

2013), lower Cretaceous shales (early Cretaceous Lacustrine shales and Barremian - Aptian shales), and late Cretaceous shales (Abu et al., 2010). The Albian shales and Devonian shales are

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also present in the Tano Basin and Saltpond Basin respectively where they serve as effective

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source rocks (Adda, 2013; Bansah et al., 2014).

3. Materials and methods

A total of thirty four (34) cutting samples slides were obtained from the Core Laboratory

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of the Ghana National Petroleum Corporation (GNPC) from the Atiavi - 1 well onshore Keta Basin. The slides were prepared by standard palynological maceration techniques that involved

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cold hydrochloric acid (HCl) and hydrofluoric acid (HF) (Phipps and Playford, 1984) to dissolve carbonates and silicates. The residue was neutralized and centrifuged in zinc bromide (Zn Br2) with specific gravity 2.0, sieved through a nylon mesh of 20 µm, and mounted on the microscope slides using polyvinyl chloride (PVC) and then cured under ultra violet light. The microscope

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slides were all studied using the Leica DMEP 750 microscope fitted with an AmScope Toup View 3.2 digital camera for photomicrography. Palynofacies analysis were also performed on these microscope slides. A total of 400 particulate organic matter (POM) were counted for each

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(Appendix 1-3).

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sample to determine the relative abundance in percentage of POM at each studied depth

All the slides used in this study are deposited in the palynological collections in the Core Lab of GNPC.

Four main constituents of kerogen have been recognized in the study.

Palynomorphs

refers to organic walled microfossils that remain after maceration using HCl and HF for carbonate and silicate digestion respectively. Amorphous organic matter (AOM) refers to particulate organic matter (kerogen) that appear structureless at the scale of microscopy, whether

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of marine or non-marine origin. Phytoclasts refers to all structured, yellow to brown dispersed particles of plant–derived kerogen including terrestrial plant fragments such as cuticles, wood and tracheid, but excluding palynomorphs. Opaque phytoclasts (black debris) include all

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structured brownish – black to black oxidized woody tissues, or carbonized particles of plantderived kerogen.

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The quantitative and qualitative variations in the kerogen constituents separated the respective samples into clusters which refer to five (5) palynofacies types (Fig. 3)

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4. Palynostratigraphy

Atta-Peters et al. (2012) assigned early Devonian to late Devonian (Emsian to Strunian) age to sediments from the Atiavi – 1well. Atta-Peters et al. (op. cit) identified stratigraphically significant spore forms and species indicative of Emsian to Frasnian age between the depth

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intervals in the lower sections of the well (1524 – 1224 m). The Emsian – Eifelian sections (1524 – 1357 m) are made up of the proximally radial forms Emphanisporites rotatus (Plate 1, Fig. J), E annulatus, E. erraticus, rare zonate spores and Umbonatisporites distinctus (Plate 1,

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Fig. H), Dictyotriletes trivialis (Plate 1, Fig. F)., Hystricosporites porcatus (Plate II, Fig. C), Verrucosisporites scurrus, Cristatisporites orcadensis (Plate, I, Fig. I), and Samarisporites sp;

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grapnel-tipped spores, Ancyrospora ancyrea, A. grandispora, A. longispinosa); large camerate spores including Auroraspora macra, Grandispora spp, Calyptosporites triangulatus (Plate I, Fig. A), Rhabdosporites langii (Plate II, Fig. G), and Densosporites devonicus. It is instructive to note that the absence of Geminospora lemurata at this interval indicates pre-Givetian age, since its first appearance is documented at the Eifelian/Givetian boundary (Loboziak et al., 1991; Streel and Loboziak, 1994; Marshall, 1996). These forms within these intervals according to

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Atta-Peters et al. (2012) compare very well with similar Emsian - Eifelian assemblages identified by other works (Richardson, 1974; Richardson and McGregor, 1986; Marshall and Fletcher, 2002). The Givetian – Frasnian section (1355 – 1192 m) was characterized by Geminospora

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lemurata, Rhabdosporites langii, Ancyrospora spp, Densosporites devonicus, Emphanisporites spp, Dibolisporites echinatus, Cristatisporites triangulatus, Grandispora spp, Auroraspora spp. According to Playford (1983), G. lemurata is very abundant at the Givetian/Frasnian boundary

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and the possible vertical range can be stated no more precisely than Givetian – late Frasnian. Similar species have been reported from Givetian – Frasnian rocks (Marshall and Allen, 1981;

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Marshall et al, 1996; Marshall, 2000; Ville de Goyet et al. 2007).

The Fammenian- Strunian section (1190 – 936 m) is characterized by the appearance and abundance of notable forms including Retispora lepidophyta (Plate I, Fig. L), Indotriradites explanatus, Verrucosisporites nitidus (Plate I, Fig. G), Dictyotriletes trivialis, Corbulispora

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cancellata, Grandispora senticosa, Vallatisporites pusillites (Plate II, Fig. F), V. vallatus, V. verucosus, V. hystricosus, Retusotriletes incohatus (Plate I, Fig. D) and Punctatisporites irrasus. There is low representation of R. lepidophyta towards the top of the section which is an

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indication of the approach to the Devonian- Carboniferous boundary as defined by the upper

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stratigraphic limit of this species (Playford, 1976; Richardson and McGregor, 1986; Avchimovitch et al. 1988; Higgs et al.1992). The lower sections (1490 m – 1244 m) of the well also contain abundant and diverse

Devonian chitinozoans and acritarchs. Anan-Yorke (1974) identified various species of chitinozoan/acritarch from the well and assigned an upper lower Devonian (Emisian) to lower upper Devonian (Frasnian) age, after comparison with similar species from North Africa, North and South America. The chitinozoans recovered from the samples include Ancyrochitina spinosa

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(Plate III, Fig. K), A. ancyrea (Plate III, Fig. G), A. langei (Plate III, Fig. H), Angochitina devonica (Plate III, Fig. I), Alpenachitina eisenacki, Cladochitina varispinosa (Plate III, Fig. J,

(Plate II, Fig. E), Conochitina spp, Eisenachitina sp.

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L), Fungochitina spp., Lagenochitina amottensis, Sphaerochitina spp., Hoegisphaera glabra

The acritarchs include Evittia remota (Plate II, Fig K; Plate III, Fig. E),

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Multiplicisphaeridium spp., Navifusa eisenacki (Plate II, Fig. I), Stellinium octoaster, Viliferites tenuimaginatus (Plate III, Fig. D), Tunispharidium spp, Umbellasphaeridium saharicum (Plate

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III, Fig. F), Veryhachium stelligerum ( Plate III, Fig. C), V. lairdi (Plate III, Fig. B), V. pastoris (Plate II, Fig. J), Triangulina alargada, Maranhites braziliensis, Leiofusa spp, Ammonidium spp., Pterospermella spp, Gorgonisphaeridium cf. ohioense (Plate II, Fig. L) 5. Results and Discussions

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5.1. Palynofacies Analysis

Combaz (1964) introduced the term palynofacies to describe the ‘total microscopic image of the organic component’. However, different definitions have been ascribed by different

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authors (Hughes and Moody-Stuart, 1967; Quadros, 1975; Boulter and Riddick, 1986; Traverse,

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1988; Powell et al., 1990). The modern concept of palynofacies was introduced by Tyson (1995). Tyson (op. cit) defined palynofacies as ‘a body of sediment containing a distinctive assemblage of palynological organic matter thought to reflect a specific set of environmental conditions or to be associated with a characteristic range of hydrocarbon generating potential’. He further defined palynofacies analysis as ‘the palynological study of depositional environments and hydrocarbon source rock potential based upon the total assemblage of particulate organic matter’.

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5.2. Palynofacies assemblages Palynofacies type A (Pf A) (Palynomorphs abundance with AOM) Plate IV, Fig. A, B This occurs at the depth intervals between 936-978 m (Fig. 3) and is characterized by the high

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abundance of palynomorphs (up to about 50% of total POM). Palynomorphs of terrestrial origin are the dominant forms (up to 70%) with marine palynomorphs about 30%. The AOM constitute about 26% of the POM with phytoclasts of about 17%. Opaques constitute as little as 7% of the

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total POM.

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Palynofacies type B (Pf B) (Palynomorphs dominant) Plate IV, Fig. C, D

It straddles the depth range from 980-994 m (Fig. 3) and is characterized by the highest proportion of palynomorphs (up to about 67% of the total POM) of all studied palynofacies. Palynomorphs of terrestrial origin are the dominant forms (up to 80%) of total palynomorphs.

7% respectively.

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The phytoclasts make up 16% of total POM with the AOM and black debris forming 10% and

Fig. E, F.

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Palynofacies type C (Pf C). (Equal abundance of black debris and palynomorphs). Plate IV,

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This is found in depths between 998-1038 m and 1118-1214 m (Fig. 3).

It is

characterized by equal proportions of black debris and palynomorphs (up to 37% each) with phytoclasts and AOM represented by 14% and 12% of the total POM respectively. Palynomorphs are mainly terrestrial forms (up to 98% of total palynomorphs).

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Palynofacies type D (Pf D) (palynomorphs dominant with relatively equal abundance of phytoclasts and black debris) Plate IV, Fig. G, H. This palynofacies occurs at samples depth interval 1062-1097 m (Fig. 3). Pf D is

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characterized by high proportion of palynomorphs (up to 42% of POM) of which 90% are of terrestrial origin. The phytoclasts and black debris are of relatively equal abundance (20% and

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22% respectively), with the AOM forming 16% of the total POM.

Palynofacies type E (Pf E) (palynomorphs dominant with phytoclasts) Plate IV, Fig. I, J.

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This palynofacies occurs between the depth intervals of 1215-1490 m (Fig. 3). It is characterized by the high proportion of palynomorphs (up to about 54% of total POM). The palynomorphs of marine origin are the dominant forms (up to 75%) with terrestrial forms of about 25%. The AOM constitutes about 15% with phytoclasts representing 25% of the POM.

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The black debris form as little as 6% of the total POM.

6. Palaeoenvironmental Interpretation

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6.1. Palynofacies implications

Palynological and palynofacies data plotted on the APP ternary diagram of Tyson (1993)

types.

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enabled us to discriminate the respective samples into clusters which refer to the palynofacies Pf A, Pf B and Pf E samples plots in the field V which reflect deposition in a mud-

dominated oxic shelf conditions (distal shelf) (Fig.4). The high content of palynomorph (terrestrial) dilutes all other organic particles, and thus can be classified as kerogen type III>IV (Tyson, 1995) which is gas prone (Fig. 4).

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The high percentages of miospore in relation to other palynomorphs (microplankton, etc) indicate the proximity to terrestrial sources. Miospore concentrations capable of diluting all other forms of palynomorphs are generally confined to the vicinity of active fluvio-deltaic sources,

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where low salinities suppress the productivity of fossilizing phytoplanktons (Tyson, 1993). As a result a small percentage of AOM is observed due to low preservation rate. Pf A and Pf B have more than 70% of their palynomorphs being of terrestrial origin with few marine forms

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(acritarchs). The high occurrences of miospores and lower abundances of acrirtarchs and AOM, indicate deposition in a shallow marine (inner/proximal) environment under prevailing mud-

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dominated oxic shelf conditions. Palynofacies type Pf E is dominated by marine palynomorphs (mainly chitinozoans) constituting 75% of the total palynomorphs (54% of POM). In instances where marine forms (microplanktons, etc) dominate the palynomorph group, the environment may be the distal shelf, with the adjacent land areas being generally arid, oxygenated and with

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low AOM preservation but with high productivity (Tyson, 1993). The relative abundance of marine forms is inversely proportional to the miospores and this ratio increase offshore and the environment may be a distal shelf (Carvalho et al., 2013). This palynofacies with its abundant

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and diverse marine forms (chitinozoans and acritarchs) over terrestrial spores, reflect a distal offshore marine environment of deposition under mud-dominated oxic shelf conditions.

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Phytoclasts on average of 25% supports the suggested offshore settings, where the irrelatively low concentrations were equated to weak terrestrial influx and deposition in distal settings that were located far from land vegetation (Müller, 1959; Pocklington and Leonard, 1979; Tyson, 1993).

The APP ternary plot of Tyson (1993) hosts palynofacies type C (Pf C) and type D (Pf D) on the field III reflecting heterolithic oxic shelf (proximal) conditions classified as kerogen type

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III or IV which is gas-prone (Fig. 4). Pf C contains the highest percentage of black debris in all samples (about 37% of the total PM) with very high palynomorphs in the presence of common to moderate phytoclasts. The opaques are as a result of oxidizing conditions and either in proximity

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to terrestrial sources or redeposition of organic matter from fluvio-deltaic environment (Tyson, 1989; Kholeif and Ibrahim, 2010; Carvalho et al., 2013). Ninety-eight percent (98 %) of the palynomorphs recovered in Pf C are of terrestrial origin indicating deposition in a terrestrial

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environment. Pf D also has relatively high proportions of black debris with terrestrial

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palynomorphs forming 90% of total palynomorphs recovered.

6.2. Implications from miospores, acritarchs and chitinozoans

Palynomorphs distributed within the well are dominated by pteridophytic spores,

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acritarchs and chitinozoans. (Fig. 5). The spores to acritarch/chitinozoan ratio is about 62% to 38% in the upper terrestrial/nearshore section of the well (1244 m – 936 m) and about 21% to 79% for the lower marine section (1490 m – 1244 m). The lower marine section is

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characterized by abundant and diverse marine forms (acritarchs/chitinozoans) and the upper unit contain abundant and diverse terrestrial miospores (Fig 6).

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The chitinozoans and acritarchs inhabited Palaeozoic shallow water deposits as well as in

shelf and slope sediments. Distal deposits (slope and outer shelf), however, yield abundant chitinozoans with few spores and other planktonic elements (Paris, 1996). According to Grahn and Paris (2011), the disappearance of chitinozoans and therefore the extinction of the chitinozoophorans coincided with a regression and fall in sea level in connection with the glaciation in western Gondwana at the end of the Fammenian (lepidophyta zones). Grahn and

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Paris (2011) further reported that the extinctions of the chitinozoans resulted from a combination of factors, i.e. the chitinozoophorans probably no longer had the genetic potential for successful adaptations to successive drastic environmental changes, their usual niches were invaded by

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more efficient groups, and their usual food supply disappeared or was no longer sufficient.

Acritarchs distribution and occurrence in a range of lithologies and facies suggest planktonic mode of life. Staplin (1961), Wall (1965) and Smith and Saunders (1970) showed that acritarch

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abundance and diversity is at maximum in open marine facies, whilst marginal marine facies

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contain no or very few acritarchs.

Jacobson (1979) recognized sphaeromorph assemblages as characteristic of shallow water and a baltisphaerid-veryhachid-polygonium assemblage for open marine environments of the Ordovician of North America. Dorning (1981) documented nearshore and deep water assemblages to be of low diversity and variable, sometimes low abundance and are dominated by

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sphaeromorphs, whilst the offshore assemblages are of greater diversity and abundance with a variety of acanthomorphs and polygonomorphs and prasinophyte genera. Al-Almeri (1983) studied Siluran sediments from Libya and also recognized similar general trends, with

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sphaeromorphs dominating nearshore environments with the more diverse assemblages Wall (1965) concluded that

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(polygonomorphs and netromorphs) inhabiting offshore facies.

distal assemblages contain more complex morphotypes (polygonomorphs and netromorphs). The marine environment is characterized by abundant and diverse forms of

acanthomorph (Evittia, Ammonidium, Multiplicisphaeridium), polygonomorph (Veryhachium, Stellinium),

netromorph

(Navifusa,

Leiofusa)

and

prasinophyta

(Pterospermella,

Cymatiosphaera) groups but with rare sphaeromorph group. This fits the documented environment of deposition of an offshore, distal or outer shelf interpretation by Smith and

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Saunders (1970), Jacobson (1979), Dorning, (1981) and Al-Almeri (1983). Above the marine unit the samples contain abundant well preserved and diverse pteridophytic spores with some acritarchs, but with the absence of chitinozoans (Fig. 5). The fairly high abundance and diverse

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pteridophytic spores indicate deposition close to source vegetation in wet lands or a swamp under humid climatic conditions. The environment was also subjected to occasionally flooding or incursions by marine or brackish waters, thereby incorporating some marine organic walled

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microplanktons (acritarchs). These fluctuations between acritarchs and miospores represent changes within the depositional environment and are of stratigraphic and /or palaeoecological

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significance. The observed intermixing of land –derived elements (miospores, phytoclasts and cuticles) with minor amounts acritarchs, strongly suggests that the sediments were deposited in shallow marine/fluvio-deltaic environment.

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7. CONCLUSIONS

Palynomorphs with marker species based on miospores assemblages and to some extent from chitinozoans and acritarchs from the Ataivi-1 well indicates ages of upper lower Devonian

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(Emsian) to latest Devonian (Strunian).

Five palynofacies types (Pf A – Pf E) have been identified in the different sample levels

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in the Atiavi 1 well. These are linked to different palaeoenvironments based on quantitative and qualitative variations and distribution in the kerogen content. Palynofacies types Pf A, Pf B and Pf E reflect deposition under mud-dominated oxic shelf conditions (distal shelf). Palynofacies type C (Pf C) and type D (Pf D) reflect a heterolithic oxic shelf (proximal) conditions. Kerogen types for the samples in the identified depositional environment are classified as types III and IV and are gas prone. The amorphous organic matter is low with the majority of the

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samples being composed of woody and coaly fragments with a low proportion of exinous fragments suggesting a low source-rock potential for liquid hydrocarbons. Palynomorph associations (miospores, acritarchs and chitinozoans) indicate an offshore

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distal or outer shelf environment of deposition for the lower section of the well. The upper sections shows intermixing of land - derived elements (miospores, phytoclasts and cuticles) with minor amounts of acritarchs, which strongly suggests deposition in shallow marine/fluvio-deltaic

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environment under a humid climatic condition.

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Acknowledgement: The authors are grateful to the management of Ghana National Petroleum Corporation (GNPC) for providing samples for the study.

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(CG2007_M0/11), 68 – 73. Wall, D., 1965.

Microplankton, pollen, and spores from the Lower Jurassic in Britain.

Micropalaeontology 11, 151 – 190.

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paleoenvironmental association. Mar. Pet. Geol. 28, 1475– 1482.

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Appendix 1. Relative percentage abundance of SOM

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PALYNOMORPHS 46.3 57.4 39.4 29.5 75.4 56.2 70.8 45.4 32.0 62.0 36.6 38.9 54.2 45.7 54.8 31.2 31.8 35.4 40.2 35.4 48.6 48.3 56.0 52.5 65.5 56.0 57.8 66.3 68.0 60.0 64.5 64.9 59.3 62.3

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BLACK DEBRIS 4.30 4.90 9.10 10.0 3.70 6.90 8.90 34.3 45.7 6.00 14.3 22.6 14.3 32.9 10.9 48.0 37.1 25.4 42.3 29.4 18.6 3.70 10.0 6.60 5.70 5.10 5.40 4.90 5.40 4.30 4.60 5.40 6.90 4.30

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PHYTOCLAST 22.3 18.0 18.6 21.4 12.3 20.6 14.3 11.7 16.6 12.0 25.7 19.1 24.9 10.0 15.7 9.10 13.7 14.3 8.60 12.3 17.7 39.1 15.7 26.6 17.1 18.9 25.7 17.1 16.6 20.0 18.6 18.3 12.9 13.4

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AOM 27.1 19.7 32.9 39.1 8.60 16.3 6.00 8.60 5.70 20.0 23.4 19.4 6.60 11.4 18.6 11.7 17.4 24.9 8.90 22.9 15.1 8.90 18.3 14.3 11.7 20.0 11.1 11.7 10.0 15.7 12.3 11.4 20.9 20.0

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DEPTH/M 936-938 944-946 960-962 976-978 980-981 984-986 992-994 998-1000 1016-1018 1036-1038 1062-1064 1076-1078 1080-1081 1090-1092 1096-1097 1118-1120 1134-1136 1150-1152 1152-1154 1168-1170 1212-1214 1215-1217 1224-1226 1242-1244 1266-1268 1286-1288 1298-1300 1318-1320 1336-1338 1350-1352 1450-1452 1454-1456 1460-1462 1488-1490

ACCEPTED MANUSCRIPT Appendix 2. Relative percentage abundance used for ternary plot

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PALYNOMORPHS 46.3 57.4 39.4 29.5 75.4 56.2 70.8 45.4 32-0 62.0 36.6 38.9 54.2 45.7 54.8 31.2 31.8 35.4 40.2 35.4 48.6 48.3 56.0 52.5 65.5 56.0 57.8 66.3 68.0 60.0 64.5 64.9 59.3 62.3

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PHYTOCLASTS 26.6 22.9 27.7 31.4 16.0 27.5 23.2 46.0 62.3 18.0 40.0 41.7 39.2 42.9 26.6 57.1 50.8 39.7 50.9 41.7 36.3 42.8 25.7 33.2 22.8 24.0 31.1 22.0 22.0 24.3 23.2 23.7 19.8 17.7

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AOM 27.1 19.7 32.9 39.1 8.60 16.3 6.00 8.60 5.70 20.0 23.4 19.4 6.60 11.4 18.6 11.7 17.4 24.9 8.90 22.9 15.1 8.90 18.3 14.3 11.7 20.0 11.1 11.7 10.0 15.7 12.3 11.4 20.9 20.0

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DEPTH/M 936-938 944-946 960-962 976-978 980-981 984-986 992-994 998-1000 1016-1018 1036-1038 1062-1064 1076-1078 1080-1081 1090-1092 1096-1097 1118-1120 1134-1136 1150-1152 1152-1154 1168-1170 1212-1214 1215-1217 1224-1226 1242-1244 1266-1268 1286-1288 1298-1300 1318-1320 1336-1338 1350-1352 1450-1452 1454-1456 1460-1462 1488-1490

ACCEPTED MANUSCRIPT Appendix 3. Relative percentage abundance of terrestrial palynomorphs and marine palynomorphs

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MARINE PALYNOMORPHS 6.30 18.8 1.40 0.90 30.8 5.60 16.5 2.00 0.30 1.40 2.30 1.80 3.10 4.30 3.90 2.60 5.20 9.70 6.50 7.40 8.30 2.30 16.3 9.10 28.6 35.7 37.4 40.7 37.9 33.6 37.2 38.8 38.9 42.2

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TERRESTRIAL PALYNOMORPHS 40.0 38.6 38.0 28.6 44.6 50.6 54.3 43.4 31.7 60.6 34.3 37.1 51.1 41.4 50.9 28.6 26.6 25.7 33.7 28.0 40.3 46.0 39.7 43.4 36.9 20.3 20.4 25.6 30.1 26.4 27.3 26.1 20.4 20.1

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DEPTH/M 936-938 944-946 960-962 976-978 980-981 984-986 992-994 998-1000 1016-1018 1036-1038 1062-1064 1076-1078 1080-1081 1090-1092 1096-1097 1118-1120 1134-1136 1150-1152 1152-1154 1168-1170 1212-1214 1215-1217 1224-1226 1242-1244 1266-1268 1286-1288 1298-1300 1318-1320 1336-1338 1350-1352 1450-1452 1454-1456 1460-1462 1488-1490

ACCEPTED MANUSCRIPT Appendix 4. Alphabetical list of palynomorphs recorded in the Atiavi 1 well.

Miospores Ancyrospora grandispinosa Richardson 1960 A. longispinosa Richardson, 1962

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A. ancyrea (Eisenack) Richardson, 1962 Emphanisporites erraticus (Eisenack) McGregor 1961

Corbulispora cancellata (Waltz) Bharadwaj and Venkatachala, 1961

Dictyotriletes trivialis (Naumova) Kedo, 1963

E. rotatus McGregor, 1961

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E. annulatus McGregor, 1961

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Densosporites devonicus Richardson 1960

Rhabdosporites langii (Eisenack) Marshall & Allen 1982 Geminispora lemurata Balme, 1962

Grandispora velata (Eisenak) Playford, 1971

G. megaformis (Richardson) McGregor, 1973

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G. protea (Naumova) Moreau-Benoit, 1980 G. senticosa (Ishchenko) Byvsheva, 1985

Hystricosporites porrectus (Winslow) Allen, 1965

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Indotriradites explanatus (Luber) Playford, 1991 Knoxisporites literatus (Waltz) Playford 1963

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Retispora lepidophyta (Kedo) Playford 1976 Retusotriletes rotundus (Streel) Lele and Streel 1969 R. incohatus Sullivan, 1968 Samarisporites orcadensis (Richardson) Richardson, 1965 1965 Umbonatisporites distinctus (Clayton) Playford 1976. Verrucosisporites premnus Richardson 1965 V. scurrus (Naumova) Mc. Gregor and Camfield, 1982 V. nitidus (Naumova) Playford, 1964 Vallatisporites hystricosus (Winslow) Byvsheva, 1985

ACCEPTED MANUSCRIPT V. pusillites (Kedo) Dolby and Neves, 1970 V. vallatus Hacquebard, 1957 V. verrucosus Hacquebard, 1957

Chitinozoans

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Alpenachitina eisenacki Dunn and Miller 1964 Ancyrochitina langei Sommer and van Boekel 1964 A. tomentosa Taugourdeau and Jekhowsky 1960

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A. striata Taugourdeau 1963 A. ancyrea (Eisenack) Eisenack 1955

A. devonica Eisenack 1955 A. magnifica Lange 1967

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Angochitina mourai Lange 1952

A. callawayensis Urban and Kliene 1970 A. ramosi Sommer and van Boekel 1964

Conochitina sp Eisenachitina sp Fungochitina sp

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Cladochitina varispinosa Lange 1967

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Hoegisphaera glabra Staplin, 1961

Lagenochitina amottensis Grignani and Mantovani 1964

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Sphaerochitina pilosa Collinson and Scott 1958 S. sahwalbi Collinson and Scott 1959

Acritarchs

Evittia remota (Deunff) Lister 1970 E. granulatispinosa (Downie) Lister 1970 Maranhites braziliensis Brito 1965 Multiplicispahaeridium ramusculusum (Deflandre) Lister 1970 M. arbusculiferum (Downie) Staplin, Jansonius and Pocock 1965

ACCEPTED MANUSCRIPT Navifusa eisenacki (Brito and Santos) Combaz et al. 1967 N. braziliensis (Brito and Santos) Combaz et al. 1967 Stellinium octoaster Staplin 1961 Umbellaspaeridium saharicum Jardine et al. 1972 Veryhachium europaeum (Stockmans and Williere) Cramer 1964

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V. lairdi (Deflandrea) Deunff 1958 V. pastoris Deunff 1966 V. trispinosum (Eisenack) Cramer 1964

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Viliferites tenuimarginatus Brito 1967

ACCEPTED MANUSCRIPT EXPLANATION OF PLATES PLATE I All Figures Х 500 unless otherwise stated Fig. A. Calyptosporites triangulatus Higgs 1976. A-1/ 992-994 ft.,140.6/10.8 Grandispora echinata Hacquebard 1957. A-1/998-1000 ft., 125.5/17.7

C.

Grandispora velatus (Eisenack) Richardson 1962. A-1/980-981 ft., 133.1/16.2

D.

Retusotriletes incohatus Sullivan, 1968. A-1/1016-1018 ft., 150.2/10.4

E.

Grandispora sp. A-1/1036-1038 ft., 131.5/12.3

F.

Dictyotriletes trivialis (Naumova) Kedo 1963. A-1/1062-1064 ft., 129.8/23.4

G.

Verrucosisporites nitidus (Naumova) Playford 1964. A-1/944-946 ft., 142.7/19.8.

H.

Umbonatisporites distinctus (Clayton) Playford 1976. A-1/1076-1078 ft., 148.5/30.3.

I.

Samarisporites orcadensis Richardson 1962. A-1/960-962 ft., 132.8/13.9

J.

Emphanisporites rotatus (McGregor) McGregor 1973.

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B.

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146.7/25.5.

A-1/1212-1214 ft.,

Raistrickia cf. spathulata (Winslow) Higgs 1954. A-1/1215-1217 ft., 146.4/19.6

L.

Retispora lepidophyta (Kedo) Playford 1976. A-1/1454-1456 ft., 148.2/19.3

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PLATE II

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All Figures Х 500 unless otherwise stated Fig. A. Apiculiretusispora cf. brandtii Streel 1964. A-1/1118-1120 ft., 149.8/15.8 B.

Baltisphaeridium cf. distentum Playford 1977. A-1/1298-1300 ft., 135.5/19.7

C.

Hystricosporites porrectus sp. A-1/1318-1320 ft., 144.1/26.2

D.

Grandispora megaformis (Richardson) McGregor 1973.

A-1/1168-1170 ft.,

140.2/15.9 E.

Hoeglisphaera glabra Staplin 1961. A-1/1152-1154 ft., 151.6/22.3

ACCEPTED MANUSCRIPT F.

Vallasporites pusillites (Kedo) Dolby & Neves 1970. A-1/1212-1214 ft., 139.5/27.4.

G.

Rhabdosporites langii (Eisenack) Marshall & Allen 1982.

A-1/936-938 ft.,

140.5/29.5. H.

Multiplicisphaeridium ramusculosum (Deflandre) Lister 1970. A-1/1336-1338 ft.,

I.

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138.7/29.3.

Navifusa eisenacki (Brito and Santos) Combaz et al. 1967. A-1/1350-1352 ft., 130.8/25.5

Veryhachium pastoris Deunff 1966. A-1/1460-1462 ft., 135.5/15.5

K.

Evittia remota (Deunff, 1955) Lister 1970. A-1/1298-1300 ft., 126.7/29.6.

L.

Gorgonisphaeridium cf. ohioense (Winslow) Wicander 1974.

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138.7/19.8

PLATE III

A-1/1134-1136 ft,

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All Figures Х 500 unless otherwise stated

Fig. A. Multiplicisphaeridium arbusculiferum (Downie) Staplin, Jansonius and Pocock 1965. A-1/1454-1456 ft., 125.5/25.5

Veryhachium lairdi (Deflandrea) Deunff 1958. A-1/1242-1244 ft., 145.8/26.7

C.

Veryhachium stelligerum Deunff 1957. A-1/1224-1226 ft., 154.1/28.2

E. F. G.

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D.

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B.

Viliferites tenumarginatus Brito 1967. 1080-1081 ft., 124.8/25.2 Evittia remota (Deunff) Lister 1970. A-1/1488-1490 ft., 141.8/26.8

Umbellasphaeridium saharicum Jardine et al. 1972. A-1/1460-1462 ft., 150.4/27.8. Ancyrochitina ancyrea (Eisenack) Eisenack 1955. A-1/1454-1456 ft., 148.2/26.5. (X 250)

H.

Ancyrochitina ?langei Sommer and Van Boekel 1964. A-1/1450-1452 ft., 129.7/19.7. (X 250)

ACCEPTED MANUSCRIPT I.

Angochitina devonica Eisenack 1955.A-1/1454-1456 ft., 140.6/24.5. (X 250)

J.

Cladochitina varispinosa Lange 1967. A-1/1488-1490 ft., 138.1/25.4. (X 250)

K.

Ancyrochitina spinosa (Eisenack) Eisenack 1957. A-1/1450-1452 ft., 146.3/14.4. (X 250) Cladochitina varispinosa Lange 1967. A-1/1488-1490 ft., 148.7/29.5. (X 250)

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L.

Plate IV

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All figures X 400

Palynofacies type A (Pf A). Plate IV (Fig. A, B). Palynomorphs abundance with AOM

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Palynofacies type B (Pf B). Plate IV (Fig. C, D). Palynomorphs dominant

Palynofacies type C (Pf C). Plate IV (Fig. E, F). Equal abundance of black debris and palynomorphs.

Palynofacies type D (Pf D). Plate IV (Fig. G, H). Palynomorphs dominant with relatively equal abundance of phytoclasts and black debris.

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Palynofacies type E (Pf E). Plate IV (Fig. I, J). Palynomorphs dominant with phytoclasts

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Table 1. Stratigraphy of the Keta Basin as described from exploratory oil well in the basin AGES AND DESCRIPTIVE REMARKS FROM ROCK UNITS

TERTIARY

Pliocene - Recent Formation This unit is composed of dark grey claystones with calcareous patches, grey to dark grey argillaceous and sandstones with unconsolidated coarse grained quartz, very poorly sorted, and good inferred porosity. Presence of relatively abundant traces of shell debris are found throughout this unit. Brownfield and Charpentier (2006) inferred an unconformity between the Miocene and Pliocene rocks.

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Miocene Formation: This unit is marine, with sandstones, shales and limestone. The shales are dark grey in the upper part, becoming greenish grey at the base of the interval. The sandstones are fairly clean in the upper part but become increasingly silty and argillaceous with depth. The sandstone is friable, very fine grained and well sorted. This formation overlies unconformably the Paleocene-Eocene formation due to the major Oligocene to Miocene unconformity that eroded a significant amount of Tertiary rocks (Kjemperud et al., 1992) Paleocene - Eocene Formation: The Paleocene formation consists of mainly marine shales due to regional Paleocene transgression. The Eocene sequence comprise siltstone, sandstone, fossiliferous and glauconitic clays with frequent foraminiferal, and bioclastic limestones

Upper Cretaceous Unit Cenomanian - Maastrichtian Formation: This is a relatively thin sedimentation interval that comprise predominantly marine shales, continental rift type sandstones and limestones. The sandstones are fine to coarse grained, occasionally conglomeratic. The topmost Cretaceous interval is marked by the appearance of a very soft, pale coloured claystone which becomes varicoloured with depth. The Maastrichtian formation which was deposited in a marine environment grades into continental clastics eastward toward the Benin Basin (Brownfield and Charpentier, 2006).

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MESOZOIC

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Lower Cretaceous Unit Aptian-Albian Formation: This is represented by marine sandstone and shales with some organic-rich black shales, coarse sandstone and conglomerates, and minor limestone (Kjemperud et al., 1992). The marginal marine sand present in the Albian unit indicates deposition in a transitional depositional environment. The marginal marine sand is reservoir prone with its porosity between 8-28% (Apesegah and Hotor, 2013). Barremian - Aptian Formation: The Barremian beds are non-marine although they are interpreted to have been deposited in deltaic/lacustrine depositional environment due to the observation of the continuous behaviour of the seismic reflectors on seismic section which indicate deposition under water and this is can be linked to the pull-apart basin structure caused by the rifting. This sequence comprises sandstones with thin, shaly interbeds. According to Apesegah and Hotor (2013), the lacustrine shales were are potential hydrocarbon source rocks.

ACCEPTED MANUSCRIPT PALAEOZOIC

Sekondian Group The Palaeozoic sedimentary rock units in the Keta Basin can be correlated with that in the Saltpond Basin (Sekondian Group). These are intruded by a dolerite sill of Jurassic age, which was formed by magmatic activity generated during the break of the South American and African plates. It is also an excellent marker that separates the prerift and rift stages.

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Carboniferous - Jurassic Unit Sekondi Sandstone Formation: This rock unit is of the Triassic-Jurassic age and is made up of a lower and upper sequence unit. This formation is composed of an interbedded sequence of varicoloured sandstones, claystones, and also minor chert. The lower unit of this formation is intruded by a dolerite sill which serves as a very good horizon marker for dating. Effia-Nkwanta Beds: These beds are of the ?Carboniferous to Permian age and are predominantly brown to reddish brown, fine-grained argillaceous cherty sandstones with progressively increasing intercalations of reddish brown hard silty shales. There are also minor occurrences of microcrystalline dolomitic limestones within the rock unit.

Dahomeyan Basement: This is the crystalline basement rock that underlies the sedimentary rock units. They consists of metamorphosed rocks including quartzites, schists, acidic and basic gneisses, with nepheline syenite intrusions (Kennedy, 1964).

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Devonian Unit Takoradi Beds: This formation includes a sequence of upper and lower dark grey, micaceous, shale with interbedded sandstones. The unit is highly fossiliferous with abundant plant microfossils. The sand formation ranges from very fine to very coarse grained with increase in depth. It is well sorted to poorly sorted and contains high feldspar at the lower portion of the formation and with traces of white kaolinitic clay matrix. The depositional environment is interpreted to be deltaic to shallow marine which extended from the Saltpond basin in the west to the Keta Basin in the east. The shaly unit is the oldest proven reservoir rocks in the Gulf of Guinea Province (Brownfield and Charpentier, 2006) and its reservoir quality in terms of porosity is between 3-20% (Apesegah and Hotor, 2013).

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Fig. 1. Map showing the location of Atiavi-1 well onshore Keta Basin (modified after

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GNPC, 2010).

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Fig. 2. General stratigraphic column of the major basins in Gulf of Guinea Province (After Brownfield and Charpentier, 2006)

Fig. 3.

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Percentage relative abundances of palynomorphs, phytoclasts, black debris and

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amorphous organic matter of Atiavi-1 well.

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Palynofacies fields

Environment of deposition

Kerogen type

I

Highly proximal shelf or basin

Type III (Gas-prone)

II

Marginal dysoxic-anoxic basin

Type III (Gas-prone)

III

Heterolithic oxic shelf (‘proximal shelf’) Shelf to basin transition (IVa = dysoxic; IVb = suboxic - anoxic) Mud-dominated oxic shelf (‘distal shelf’) Proximal suboxic-anoxic shelf

Type III or IV(Gas – prone) Type III or II (Gasprone) Type II I> IV (Gas – prone) Type II (Oil-prone)

Distal dysoxic-anoxic ‘shelf’

Type II (Gas-prone)

Distal dysoxic-oxic shelf

Type II>> I (Oilprone) Type II ≥ I (Highly oilprone)

VI VII VIII

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Distal suboxic-anoxic basin

Fig. 4. A ternary APP and kerogen plot of samples from Atiavi 1 well (After Tyson, 1993).

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LEGEND

L at e De vo nia n G ivetia n -Fra sni an

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Sa mp le p oint (m )

Emph an isporite s erraticus E. annulatus E. rota tus Rhabd osporit es langii Gemin ispora lemu rata Grandispora velata G. megaformis G. prot ea G. sen ticosa An cyrospora gra nd ispnosa A. longispinosa A. an cyrea Dibolispo rit es echina tus Hystricospo rites porrectus Crista tisporites trian gu latus Sa ma risporites orcadensis Verrucosisp orites premnu s V. scurrus V. nit id us Densosporit es devo nicus Retusot rilete s rotu nd us R. incohatus Corbulispora cancella ta Dictyotrilete s trivia lis Kn oxispo rit es litera tus Vallat isp orites vallatus V. pusillit es V. verruco sus V. hystricosus Retispora lepidophyta In dotriradites expla nat us

Acritarchs

Evitt ia remota E. granulat ispinosa M. reamusculusum M. arbusculiferum Veryh chium trispinosum V. europ ae um V. lairdi V. pastoris St ellinium oct oast er Navifusa eisenacki N. brazilie nsis Umbellaspaerid iu m saharicum Maranhite s braziliensis Viliferites tenuimarginatus

Chitinozoans

GNEISS (DAHOMEYAN)

An cyrochitina langei A. tomento sa A. striata A. ancyrea An gochitina mourai A. devonica A. ma gn ifica A. callawaye nsis A. ramosi A. c f. bif urcata Alpenachitina eise na cki Clado chitina varisp in osa Fungochitina sp Lageno chitina amo ttensis Sp haeroch it ina pilosa S. sahwa lbi S. brevisino sa Hoegisphaera glabra Conochit ina sp Eisenach itina sp

Spores

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936 - 938 944 - 946 960 - 962

976 - 978 980 - 981.5 984 - 986 992 -994 998 1000

1016 - 1018

1032 - 1034 1036 - 1038

SANDY SHALE

MISSING INTERVAL

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D ep th (m ) Lith olog y

1046 - 1048

1076 - 1078

1118 - 1120

1090 - 1902 1096 - 1098

1134 - 1136

1168 - 1170

1190 - 1192

1242 - 1244

1212 - 1224 1215- 1217.5 1224 - 1226

1266 - 1268

1286 - 1288

1305 - 1307

1318 - 1320

1298 - 1300

1336 - 1338

1350 - 1352 1355 - 1357

1404 - 1406

1442 - 1444 1450 - 1452 1460 - 1462

1488 - 1490

1522 - 1524

CALCAREOUSSANDSTONE SHALE

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12 0 0

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1 40 0

1 50 0

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Age

Fa mm en ian -Stru nia n

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E mi sian -E ifel ian SHALYSANDSTONE/ SILTSTONE

Fig. 5. Palynomorph (spores, chitinozoans, acritarch) distribution in the samples from Atiavi

1 well (Modified after Atta-Peters et al., 2012).

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Fig. 6. Area plots of marine palynomorphs and terrestrial palynomorphs of Atiavi-1 well.

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PLATE I

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A

B

D

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PLATE II

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E

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I

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PLATE III

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PLATE IV (CONT’D)

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PLATE IV

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ACCEPTED MANUSCRIPT Highlights of research from the studies of Upper Devonian sediments of the atiavi-1 well, Keta Basin, southeastern Ghana.

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Five palynofacies associations linked to different depositional environments have been identified in the Atiavi-1 well. Palynofacies types Pf A, Pf B and Pf E reflect deposition in a mud-dominated oxic

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shelf (distal shelf), characterized by kerogen type III >IV and palynofacies type Pf C and Pf D reflect deposition in a heterolithic oxic shelf (proximal), characterized by type III or IV kerogen.

Palynomorph association indicate shallow marine/fluvio-deltaic to offshore distal

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environment.

Palynomorph assemblage based on miospore marker species points to an upper lower

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Devonian (Emsian) to latest Devonian (Strunian) age.

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