Palynology, palynofacies and palaeoenvironments of sedimentary organic matter from Bonyere – 1 Well, Tano basin, western Ghana

Palynofacies analyses from Bonyere Well No. 1 in the Tano basin, western Ghana has revealed five palynofacies associations (I – V) based on the percentage relative abundances of the sedimentary organic matter (SOM). The palynofacies associations reflect deposition in a fluvio-deltaic (oxic) environment, a distal dysoxic-anoxic shelf environment, a proximal dysoxic-suboxic environment, nearshore (oxic) and a fluvio-deltaic/nearshore environment with high oxygen levels and low preservation rates respectively. Based on marker palynomorphs, Campanian – Maastrichtian age has been assigned to sediments within the interval (1800-10 ft) – (90-100 ft), Turonian – Lower Senonian (Santonian) age between (3160-70 ft) – (1980-90 ft) and Aptian age between the interval (8140-50 ft) – (3340-50 ft). The absence of the elater-bearing pollen which are typical Africa-South America (ASA) elements for the Albian – Cenomanian age is indicative of an unconformity between the Aptian and Turonian sediments.


INTRODUCTION
The Tano Basin in the southwestern Ghana has been a major target for oil and gas exploration since 1896 and palynology and geophysical data has been used to subdivide the fine grained sediments of the basin.Palynofacies analysis is an interdisciplinary approach that pulls together the study of palynomorphs as well as the entire organic content of sediments which reflects the original conditions in the source area and the depositional environment.
The aim of this paper is to describe the palynostratigraphic and palynofacies characteristics observed in the well section of Bonyere -1 well onshore Tano Basin (Fig. 1) to determine the geological age of the formations and their environment of deposition.(Modified from GNPC Report, 2010).

GEOLOGICAL SETTING
The Tano basin is the eastern part of the Cote d'Ivoire (Ivorian) basin that stretches for about 500 kilometers along the coastal shelf area of Cote d'Ivoire and south-western Ghana.The basin is bounded by the African main land to the north; the St Paul and Romanche transform zones to the west and east respectively and the ocean-continent boundary to the southwest.The Tano basin was formed by extension and subsidence of thinned continental crust in a rift system between diverging continental plates of West Africa and Northern Brazil (Fig. 2).The Ivory Coast basin is a wrench-modified segment of a dislocated interior fracture basin formed prior to continental break-up and opening of the Atlantic Ocean which is referred to as rifting.A marginal sag basin then developed by thermal contraction [cooling] as International Letters of Natural Sciences Vol. 5 plate separation continued along major east-west oceanic transform faults including the Romanche fracture and this sag now overlies the interior fracture basin.Thinning of the crust during stretching did not only cause initial syn-rift subsidence, but the decay (cooling) of the consequent thermal anomaly caused by the thinning of the lithosphere resulted in post-rift thermal subsidence.The wrenching component is a product of oblique strike-slip transfer-transform movement along the St. Paul, Chain fracture and International Letters of Natural Sciences Vol. 5 Romanche Fracture systems of the Equatorial Mega shear zone.Faulting and tilting which were mainly extensional in type and erosion affected the boundaries of these units.

STAGE I THERMAL UPLIFT
The North Tano fault-bounded tilted structural block is one product of Early Albian tectonics.The creation of the South Tano Structural trend, faulting, uplift and erosion along the outer margin of the Tano basin resulted from the final separation of the West Africa and Northern Brazilian continental plates (Davies, 1989).The end of Cretaceous time of the Tano basin was marked by widespread deposition of very porous shelf and shoreline sands.
The initial rift phase of the basin has sediment thickness of more than 4000 meters.The sediments are of lower Cretaceous (Aptian -lower Albian) age and mainly non-marine in the lower section but increasingly marine-influenced in the upper part where thick sandstones form oil and gas reservoirs in wells offshore Tano area.

MATERIALS AND METHODS OF STUDY
Study was carried out on ninety one (91) samples within intervals 90-100 ft -8140-50 ft from the Bonyere -1 well obtained from the Core Laboratory of Ghana National Petroleum Corporation (GNPC).Standard methods for processing and concentration of organic matter were employed.These involved the use of Hydrochloric (HCl) (35-38 %) and hydrofluoric (HF) acids (40 %) to digest the carbonates and silicate content of the sediments respectively and the release the organic matter from the rock martrix.
The residues were sieved through a 10 μ mesh and washed using ultrasonic cleaning for preparing slides.Polyvinyl Alcohol (PVA) was used as a mounting medium.For discriminating among the different palynofacies characteristics, 500 particulate organic matter and palynomorph particles were counted and used to calculate relative abundances.Photomicrographs of specific SOM and palynomorphs were taken for comparisons and variations between individual palynofacies associations.

1. Palynofacies definition and classification
The word palynofacies was first used by Combaz (1964) to describe the floral complement of particulate organic matter in a sediment sample recovered by palynological processing techniques using HCl and HF as observed under a microscope.Although the original definition of palynofacies is broadly acceptable, the term has been used by various authors though with some modifications (Powell et al., 1990;Batten, 1973Batten, (a), 1981Batten, , 1982;;Hughes and Moody-Stuart, 1967;Boulter and Riddick, 1986).
The palynofacies classification terms used here follows Tyson (1995): amorphous organic matter (AOM), phytoclasts, opaque phytoclasts (black debris) and palynomorphs (spore and pollen, dinoflagellates).Amorphous organic matter is a term used to describe particulate organic matter that lacks a distinct shape, outline and structure under light microscopy.These include phytoplankton and bacterially-derived AOM, higher plant resins, and amorphous products of the diagenesis of macrophyte (Tyson, 1993(Tyson, , 1995)).
Phytoclasts refers to microscopic particles of plant -derived kerogen but excluding palynomorphs.It includes all structured terrestrial plant fragments such as cuticles, wood and tracheid.The structure may be displayed, only faintly discernible or suggested, sometimes merely by the fact that it is a membrane or filament that has a clearly defined, non-amorphous outline (Batten, 1996).Opaque phytoclasts (black debris) refers to oxidized brownish -black to black woody tissues, including charcoal.International Letters of Natural Sciences Vol. 5 They are produced as a result of oxidation and natural pyrolysis on terrestrial plant tissues.Palynomorphs refers to all discrete acid-resistant, organic walled microfossils that may be encountered in a palynological preparation (Tschudy, 1961).

RESULTS
The relative abundances of sedimentary organic matter (SOM) and palynofacies composition in the samples from Bonyere -1 presented in Fig. 5 and 6, and plotted on the APP ternary diagram (Fig. 4) show five palynofacies types.It is characterized by high percentage (up to 72 % of the total particulate organic matter) of well -preserved and poorly preserved terrestrial plant fragments (mainly brown wood, tracheids and cuticles).

Phytoclasts
It also contains opaque phytoclasts (up to 20 % of the total particulate organic matter).Palynomorphs dominated by peridinoids contribute about 8 % of the total particulate organic matter.

Palynofacies IV: Opaque phytoclasts with few palynomorphs (Plate III, Fig 1, 2)
This palynofacies occur at sample depth interval 5340-5350 ft where there is a predominance of black debris (up to 75 %) of the total sedimentary organic matter.It also contains fair amounts of palynomorphs (15 %) and AOM (5 %).The palynomorphs are dominated by pollens and spores.

5. Palynofacies V: Palynomorphs with Amorphous organic matter (Plate IV, Fig 5)
This palynofacies occur at sample depth 5430-5440 in which there is a predominance of Palynomorph (fungal spores up to 55 %) of the total organic matter.It contains also fair amounts of AOM (35 %) and some phytoclasts (10 %).Palynofacies II is characterized by amorphous organic matter comprising resins, structured and non -structured amorphous organic matter.The high percentage of AOM is the result of a good preservation rates and low -energy environments.The preservation of the amorphous organic matter is indicative of a dysoxic -anoxic condition within the sample depths (Tyson, 1993).

EXPLANATION TO PLATES
According to Kholeif and Ibrahim (2010), the high AOM sediments indicate reducing conditions and increase water column resulting in dysoxic or anoxic bottom conditions.The palynomorphs are mostly spores and pollen grains of terrestrial origin with a few dinoflagellates.Also present are a few phytoclasts and black debris (opaque phytoclasts).Unlike Palynofacies I, AOM association in Palynofacies II indicate that sediments must have been transported farther away from land and into nearshore/marine environment (transitional environment).A brackish/lagoonal environment of deposition has been suggested by Carvalho et al., (2006), to a similar palynofacies assemblage from their unit A-17 and A-24 from wells GTP-17-SE and GTP-24-SE respectively from the Aptian-Albian of the Sergipe Basin in NE Brazil.Quattrochio et al., ( 2006) International Letters of Natural Sciences Vol. 5 also ascribe a nearshore environment of deposition to a similar palynofaces assemblage in their Palynofacies Type 3 from well sections in NE Tierra del Fuego, Argentina.
Palynofacies III contains a mixture of both amorphous organic matter and phytoclasts, with high amounts of AOM and few opaque phytoclasts (black debris), and palynomorphs.This palynofacies is comparable with palynofacies association observed in interval B (Samples 6947 -6951) of Zavattieri et al., (2008) from the Neuquen Basin in Argentina.In oxygen deficient environments with high amounts of AOM preservation, allochthonous terrestrial material is dominant in the immediate vicinity of fluvio-deltaic sources or turbidites (Tyson, 1989(Tyson, , 1993)).Description of the palynofacies assemblage is also consistent that of Palynofacies type-5 (P-5) of Martinez et al., (2008) from the Neuquen Basin in Argentina, which was interpreted as indicative of shoreline and brackish/freshwater environment (marginal marine) under a dysoxic condition.
Palynofacies IV contains predominantly opaque phytoclasts (black debris) with a few spores and pollen grains.The opaque phytoclasts are derived from the oxidation of woody materials either during prolonged transport or post -depositional alteration.The high values of the black debris indicate oxidizing conditions and either proximity to terrestrial sources or redeposition of organic matter from fluvio-deltaic environment of deposition (Tyson, 1989).This palynofacies is similar to Units B17 and D24 of Carvalho et al., (2006), interpreted as non-marine to lagoonal coastal plain in transitional environment with an arid hinterland environment.Al-Ameri et al., (2001) also interpreted their palynofacies PF 2 which is dominated by opaque phytoclast with a few or no palynomorhs as sedimentation in well-sorted nearshore sandstone and siltstone.A similar SOM association described by Zavattieri et al., (2008) has been interpreted as fluvio-deltaic environment with high oxidation conditions.A similar palynofacies association, Palynofacies cluster C of Makled and Baioumi (2013) from the Aptian -Cenomanian sediments in Egypt, has been attributed to shallow near shore environment under oxic conditions.
Palynofacies V.According to Traverse (1992), Oboh (1992), van Waveren (1993) the presence of fungal spores appears to correlate with abundances of land plants debris and, therefore, with both modern and ancient swamp, fluvial-lacustrine, lagoonal, delta-top and near-shore marine facies.
The indication is that this facies was deposited in a fluvio-deltaic/ nearshore (proximal shelf) where there is a high level of oxidizing conditions and low preservation rates.

CONCLUSIONS
Five palynofacies have been identified in the sediments from Bonyere -1 well, and these are indicative of five paleoenvironments.

Fig 2 .
Fig 2. The evolution of the Tano Basin and its structural and stratigraphic development related to several tectonic stages (after GNPC, 2010).
Figures A, B. Yellowish to dark brown/ black amorphous organic matter with some palynomorphs.Fig.A shows structured AOM; Fig. B shows unstructured AOM (palynofacies II) C, D. Amorphous organic matter with some phytocalsts.Fig. C is spongy and Fig. D shows relicts of the original cuticular organic matter.(Palynofacies III).