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Reservoir Characteristics and Palaeo-Depositional Environment of Duski Field, Onshore, Niger- Delta, Nigeria

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Reservoir Characteristics and Palaeo-Depositional Environment of Duski Field, Onshore, Niger- Delta, Nigeria DOI: http://dx.doi.org/10.4314/gjgs.v14i1.5 GLOBAL JOURNAL OF GEOLOGICAL SCIENCES VOL. 14, 2016: 49-68 COPYRIGHT© BACHUDO SCIENCE CO. LTD PRINTED IN NIGERIA ISSN 1596-6798 49 www.globaljournalseries.com, Email: [email protected] RESERVOIR CHARACTERISTICS AND PALAEO-DEPOSITIONAL ENVIRONMENT OF DUSKI FIELD, ONSHORE, NIGER- DELTA, NIGERIA MATTHEW E. NTON AND SALAMI ROTIMI (Received 25 July 2015; Revision Accepted 26 October 2015) ABSTRACT The Niger Delta is a prolific hydrocarbon producing belt in the southern Nigeria sedimentary basin on the continental margin of the Gulf of Guinea. This study used well log suites to delineate the hydrocarbon reservoirs, depositional environments and lithostratigraphy of the Duski Field, Onshore Niger Delta, Nigeria. A comprehensive interpretation of the three wells revealed five (5) reservoir units with low volume of shale and thickness variations between 24m and 60.20m. The average porosity values ranged from 12% to 34%, with high hydrocarbon saturation in all the reservoir sands. Generally, porosity and permeability values decrease with depth in all the wells. Cross-plots of water saturation (Sw) and porosity ( ) (Buckles plot) revealed that some reservoirs were at irreducible water saturation; hence producing water-free hydrocarbons. Therefore the hydrocarbon accumulation of this field is commercially viable and promising. This study revealed that the reservoir sand units were deposited within marginal marine depositional environment which include fluvial channel, transgressive marine, progradational and deltaic settings. KEYWORDS: Reservoir characteristics, depositional environment, Niger Delta. INTRODUCTION sandstones and shales with rock units varying in thickness from 30m to 4600m (Short and Stauble, 1967). The The Niger Delta Basin occupies the Gulf of sandstones in this Formation are the main hydrocarbon Guinea continental margin in equatorial West Africa reservoirs with shale providing lateral and vertical seals. between Latitudes 30 and 60 N and Longitudes 50 and 80 Petrophysical interpretation of logs plays an important role E. It ranks among the world’s most prolific petroleum in the discovery and development of petroleum and producing Tertiary Deltas (Kulke, 1995). The stratigraphy, natural gas reserves. It also helps to correlate zones, sedimentology, structural configuration and palaeo- assist in structural mapping, identification of productive environment of the Niger Delta have been discussed by zones, determination of depth and thickness of zones to various workers (Short and Stauble, 1967; Weber, 1971; distinguish between oil and gas or water in a reservoir and Weber and Daukoru, 1975; Evamy et al, 1978; Nton and to estimate hydrocarbon reserves (Darwin and Singer, Adesina, 2009) among others. Doust and Omatsola 2008). (1990) noted that from the Eocene to the present, the This study provides a better understanding of the delta has prograded southwestwards, forming depobelts, reservoir properties (porosity, permeability) and related which represents the most active portion of each stage of sedimentological features likely to impact on fluid flow. the development of the delta. According to Kulke (1995), The fluid types and their contacts were determined as well the Niger Delta contains only one identified petroleum as the palaeodepositional environment. Such findings will system referred to as the Tertiary Niger Delta (Akata- assist in future exploration and exploitation activities within Agbada) petroleum system. The Niger Delta province is the Field and can help locate new targets. ranked the twelfth richest petroleum province with 2.2% of the world’s discovered oil and 1.4% of world’s discovered Location of Study Area and Geology gas by the US Geological Survey’s World Energy The study area is located within the Duski field, Assessment (Klett et al., 1977). onshore Niger Delta and belongs to Addax Petroleum In the Niger Delta, petroleum is produced in Development Company, Nigeria Concession (Figure 1). sandstone and unconsolidated sands of the Agbada The three wells within the field are all located around the Formation. This formation is characterized by alternating centre of the field as shown in the base map (Figure 2). Matthew E. Nton, Department of Geology, University of Ibadan, Ibadan, Nigeria Salami Rotimi, Department of Geology, University of Ibadan, Ibadan, Nigeria 50 MATTHEW E. NTON AND SALAMI ROTIMI Fig.1: Map of southern Nigeria showing location of study area. Map of Africa inset. (Modified from Doust and Omatsola, 1990) RESERVOIR CHARACTERISTICS AND PALAEO-DEPOSITIONAL ENVIRONMENT OF DUSKI FIELD 51 Figure 2: Base map showing the location of the three wells A1, A2, and A3 of DUSKI field. The Tertiary Niger Delta is a prograding The Niger Delta sedimentary complex is characterized by sedimentary complex, deposited under transitional marine, coarsening upward regressive sequences. The overall deltaic and continental environments since Eocene in the regressive sequence of clastic sediments was deposited north to Pliocene in the south. It is located in the southern in a series of offlap cycles that were interrupted by periods Nigeria margin of the Gulf of Guinea and consists of over of sea level changes (Etu-Efeotor, 1997; Bouvier et al, 12 km thick sediments (Short and Stauble, 1967; 1989). Avbovbo, 1978; Doust and Omatsola, 1990; Kulke, 1995). 52 MATTHEW E. NTON AND SALAMI ROTIMI Figure 3: Stratigraphic column showing the three formations of the Niger Delta. Modified from Shannon and Naylor (1989) and Doust and Omatsola (1990). The Niger Delta is divided into three formations 1999). The type sections of these formations are (Doust and Omatsola, 1990; Fig.3), representing described in Short and Stauble (1967) and summarized in prograding depositional facies that are distinguished a variety of papers (Doust and Omatsola, 1990; Kulke, mostly on the basis of sand-shale ratios (Tuttle et al., 1995). RESERVOIR CHARACTERISTICS AND PALAEO-DEPOSITIONAL ENVIRONMENT OF DUSKI FIELD 53 DATA SET AND METHODOLOGY include; gamma ray log (lithology identification), volume of The data set used in this study was provided by Addax shale log (porosity correction), density and neutron logs Petroleum Development Company, Nigeria. These include (delineating fluid contacts), resistivity and water saturation Base map, showing location of three wells (A1-A3); logs (identifying pore fluid type). In addition, gamma ray electronic copy of composite well logs consisting of and resistivity logs were used in determining the reservoir Gamma ray, Resistivity, Neutron and sonic logs present in zones and the presence of hydrocarbon. The gamma ray LAS format. Density log was generated from sonic log and log shapes were used for characterisation of various other relevant logs were also created as continuous logs depositional environments. The methodologies adopted in using Petrel calculator. Relevant wireline log signatures this study are summarized in Figure 4 and details are were employed to identify hydrocarbon-bearing reservoirs documented in Salami (2015). and computing reservoir petrophysical parameters. Such Lithological Identification log was used to delineate the different lithologies in the A gamma ray (GR) cut-off was set to distinguish the sand three wells. The GR log reflects the proportion of shale and shale units by creating a facies template on Petrel and can be used quantitatively as a shale indicator software for lithofacies identification. The gamma ray (GR) (Schlumberger, 1989). Sand bodies were identified by the 54 MATTHEW E. NTON AND SALAMI ROTIMI deflection to the left of the GR log due to the low BVW=Sw*Фe………(Asquith and Krygowski, 2004)…... (3) concentration of radioactive minerals in sands while deflection to the right signifies shale unit, with high Where: concentration of radioactive minerals (Schlumberger, BVW = bulk volume water; 1989). Conventionally, GR log is set to a scale of 0-150 Sw = water saturation; API unit and a central cut off point of 65 API was inferred Фe =effective porosity based on the sand-shale discrimination. This implies that API value greater than 65, is interpreted as shale unit If values for bulk volume of water (BVW), calculated at while lesser indicates sandstone (Schlumberger,1989). several depths within a formation are consistent, then the zone is considered to be homogeneous and at irreducible Correlation water saturation. Therefore, hydrocarbon production from The three wells A1, A2, and A3, located within the Duski such zone should be water - free (Morris and Biggs, Field, were correlated using gamma ray and resistivity 1967). logs responses. According to Schlumberger, (1989), the gamma ray and resistivity logs are good correlation tools Identification of fluid type in both open and cased holes. The wells were correlated A general indication of fluid type was inferred from the from bottom to top using lithological log responses that resistivity readings. High deep resistivity readings may be product of similar depositional processes and corresponding to sand units indicated hydrocarbon environment. Sand units of interest were carefully picked bearing or freshwater zones while low deep resistivity and correlated across the wells to give an idea of the readings, showed water bearing zones (Schlumberger, continuity of the reservoirs at different depths across the 1989).Usually, a definite identification of fluid type whole Field. This is based on the concept of electric log contained within the pore spaces of a formation is correlation
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