The compositional characteristics of shales are often determined by the effects of surface processes (weathering, erosion, transportation) on their source rock(s) and subsequent diagenesis [1] [2] . Weathering, erosion, sediment recycling and diagenesis often affect the alkalis (Na₂O and K₂O) and the alkali-earth elements (CaO and MgO); whereas Al₂O₃, TiO₂, the high field strength elements (HFSE) (Co, Sc, Hf, Ta, Nb, Ti and Y) and some other trace elements are geochemically immobile in nature and are widely known as provenance diagnostic [3] . Very fine grained sediments such as clays or shales are often analyzed for their geochemical attributes with the assumption that they match closely average composition of the upper continental crust [4] . This assumption is based on the fact that thorough mixing occurs in very fine grained sediments, which often host Rare Earth Elements (REE) and Th that are relatively immobile; as such preserve the source rock composition.

In this study, the geochemistry of shale samples from Gbekebo-1 well, southwestern, Nigeria was applied to characterize provenance, paleogeography and source area weathering/tectonic setting of the study area (Figure 1). Geochemical studies aimed at characterizing provenance, inferring sediment source area(s) weathering/ tectonic setting and palaeogeography of the Dahomey Embayment that are based on subsurface data sets are rare (e.g. [5] ), probably due to inadequate data availability or poor exposures [6] . Since the discovery of oil seeps and tar sands in the Okitipupa area, much effort has been put into exploring the hydrocarbon potentials of the Dahomey embayment [7] - [11] . A number of exploration wells (such as Gbekebo-1, Afowo-1, Araromi-1, Benin west-1, Ife-1, Ore-1, Ise-1, Ojo-1, Lekki-1, etc.) were drilled by Shell d’Arcy (1936-1960), Tennessee Inc (1960-1966) and other firms/agencies. These exploration wells were all dry holes with minor showings of tarry oil and oil sands at varied depths up to the basement [12] . However, large reserves of bitumen have been proven in tar sands deposits of the Abeokuta Group [13] . Furthermore, although no commercial oil discovery was made onshore, the offshore section of the embayment offered a ray of successful discoveries such as the Aje Field.

The Dahomey Embayment is part of the Gulf of Guinea province that comprises Benin, Keta, Central, Saltpond,

Tano and Ivory Coast sub-basins [14] . The province is bounded to the east by the Niger Delta province [15] and in the west by the West African Coastal province. The east-west structurally aligned sub-basins in the Gulf of Guinea are divided into three transform fault zones (FZ): St Paul FZ (at northwestern side); Romanche FZ (demarcating Ivory Coast, saltpond and Tano from Keta and Benin sub-basins); and Chain FZ (along the eastern part of the province). The province is a wrench-modified basin, formed at the termination of the tectonic event that lasted from Late Jurassic to Early Cretaceous. This tectonic event marked by transform and block faulting was operational throughout the entire Paleozoic leading to the break-up of the American (north and South) from African palaeo-continents [16] . A three-stage development of the basin has thus been suggested, namely: i) Pre-rift or pre-transform or intra-cratonic stage (Late Proterozoic to Late Jurassic), ii) Syn-rift or syn-transform or rift stage (Late Jurassic to Early Cretaceous) and iii) Post-rift or post-transform or drift stage (Late Cretaceous to Holocene) [17] - [20] .

The stratigraphy of the Dahomey embayment has been discussed by a number of workers such as [21] - [25] , amongst others. The sedimentary successions of this basin ranged from Cretaceous to Recent. The stratigraphic package is broadly divided into the Cretaceous Abeokuta Group (Ise, Afowo, and Araromi Formations) and Cenozoic units (Ewekoro, Akinbo, Oshosun, Ilaro, Ogwashi-Asaba and Benin Formations) [23] [26] . Rocks of the pre-transform stage in the Benin and Dahomey Embayment constitute the lower part of Ise Formation (Figure 2) composed of conglomerates, sandstones, shales of fluvio-deltaic environment [14] [17] [23] . The Afowo Formation at its type section (Afowo-1 well, Figure 2) comprised medium to coarse sandstones with thick to thin intercalations of siltstones and claystones; while the Araromi Formation (Maastrichtian to Paleocene) (type section Araromi-1 well, Figure 1) is marked by shale units similar to Awgu and Nkporo shales. At the base, this formation exhibited shale thickening seaward with siltstone, limestones and marl. Overlying the Araromi Formation is the Ewekoro which consist of limestone, shales and mudstone/claystone. Akinbo Formation is shaly with some clay interbeds, nodules of calcareous and phosphatic materials, impure limestone and glauconite [27] . The Oshosun Formation is made up of greenish clays and glauconitic shales intercalated with sandstones and limestones [5] . The Oshosun Formation is overlain by Ilaro, Ogwashi-Asaba and Benin Formations all of which consist of medium to coarse sands, alternating sands and clays, coastal plain sands respectively. A NNW-SSE correlation sketch involving Ise-1, Gbekebo-1, and Benin West-1 (Figure 2) indicated that these wells penetrated Araromi, Ewekoro, Oshosun and Ogwashi-Asaba/Benin Formations.

Eight core samples of shale obtained at different depths from Gbekebor-1 well (Figure 1) were analyzed for fifty-five elemental components (10 major and 45 trace/REE) using the combined methods of Fusion Inductively Coupled Plasma (FUS-ICP) and Fusion Inductively Coupled Plasma emission/Mass Spectrometry (FUS-ICP/ MS). Pre-analytical sample preparation that included crushing with agate mortar and pestle were undertaken at the Geology laboratory, Delta State University, Abraka; following which the samples were packaged and sent to the Activation laboratories, Ontario, Canada for the FUS-ICP and FUS-ICP/MS analyses. Details of FUS-ICP and FUS-ICP/MS are as reported by [28] and [29] respectively. Prior to the laboratory analyses (ICP-MS) more than 95% of each sample was ground so as to pass through a 200 mesh for a complete fusion of even the resistant minerals that may be present. Next, the main analyses involved the use of lithium metaborate/tetraborate fusion, followed by ICP and ICP/MS. Quality control technique was ensured through mass balance and elemental totals of oxides is usually between 98% to 101%.

The provenance characteristics and palaeogeography of the shales from Gbekebo-1 well, southwestern Nigeria have been investigated with geochemical methods. The study revealed that the shales from the Dahomey Embayment were derived mostly from felsic continental sources with some mafic to Quartzose sedimentary input. The provenance characteristics indicated that the Gbekebo-1 shales were deposited on a passive margin setting that received most of its detritus from the nearby southwestern Nigeria Basement Complex rocks. Inferred protoliths (source rocks) from this study consist of tonalite to granodiorite composition subjected to prolong intense chemical weathering likely under a tropical palaeo-climatic condition.

The author wish to thank the Nigerian Geological Survey Agency (NGSA) and Activation Laboratory, Canada, for the release of the core samples and analyses respectively. He is also, grateful for the constructive reviews by Prof. I.A. Akpoborie.

O. Innocent Ejeh, (2016) Geochemical Discriminant for Provenance Characterization and Palaeogeography of Shales from Dahomey Embayment, Southwestern Nigeria. Journal of Geoscience and Environment Protection,04,56-68. doi: 10.4236/gep.2016.46005