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DEPOSITIONAL ENVIRONMENTS OF THE RESERVOIR SANDBODIES IN ‘’LAYO’’ FIELD EASTERN NIGER DELTA
TABLE OF CONTENT
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLE
CHAPTER ONE: INTRODUCTION
CHAPTER TWO: METHODOLOGY
3.1 TECTONIC SETTING/EVOLUTION
3.2 STRATIGRAPHY OF THE TERTIARY NIGER DELTA
CHAPTER FOUR: THE RESERVOIR SANDBODIES AT ‘’LAYO’’ FIELD
4.1 SANBODY ‘’A’’
4.1.1 STRATIGRAPHIC POSITION
4.1.4 WELL LOG (GAMMA RAY) RESPONSE
4.1.5 DEPOSITIONAL ENVIRONMENT
4.2 SANDBODY ‘’B’’
4.2.4 WELL LOG (GAMMA RAY) RESPONSE
4.2.5 DEPOSITIONAL HISTORY
4.2.6 DEPOSITIONAL ENVIRONMENT
4.3 SANDBODY ‘’C’’
4.3.2 STRATIGRAPHIC POSITION
4.3.5 DEPOSITIONAL HISTORY
4.3.6 DEPOSITIONAL ENVIRONMENT
4.4 SANDBODY ‘’D’’
4.4.1 STRATIGRAPHIC POSITION
4.4.4 WELL LOG (GAMMA RAY) RESPONSE
4.4.5 DEPOSITIONAL HISTORY
4.4.6 DEPOSITIONAL ENVIRONMENT
TECTONIC AND GEOLOGIC HISTORY OF RESERVOIR SANDSTONE BODIES AT ‘’LAYO’’ FIELD
5.1 TECTONIC HISTORY
5.2 GEOLOGIC HISTORY
‘’Layo’’ field consists of thick sandstone bodies that are interbeded with laterally continuous shale.
Four major sandstone bodies spanning an interval of 300m (1999ft) were identified and studied using mainly gamma ray logs and sidewall sample descriptions from sic and two wells, respectively.
The sandobides which consist mostly of quartz arenite (85% – 95% quartz) are dominantly thick (37m to 61m), elongate, lenticular, and tabular shaped, fine – to very coarse-grained and are to a large extent upward coarsening sequences. Average sand thickness varies from 43.7m to 54.6m. Maximum sand development occur towards the southern part of the field.
The thick sandstone bodies ‘A’ and ‘B’ which seem to be laterally continuous in the field, are well-to poorly sorted, fine to very coarse-grained, silty, clayey and with lignite streaks are interpreted as delta-marine fringe sands. The bodies are thought to have prograded seaward in the southern direction. Log shapes vary from highly serrated hybrid-though blocky-to funnel-shaped.
Sandstone bodies ‘C’ and ‘D’ are fine to gravelly in grain size, well-to poorly-sorted, siltry, clayey, lignitic and contain plant remains. Geophysical well log indicates serrated blocky shapes with abrupt upper and lower contacts for the sands. They are interpreted as distributary channel sands deposited in deltaic environment. Thus, the sandbodies reveal stacked deltaic sequence which is due to cyclic regressive/ transgressive phases typical of this environment.
Structural hydrocarbon traps are present in the field. This is created at the up-dip margin of the fault that trends approximately in the east-west direction.
‘’Layo’’ field is one of the numerous petrolific fields in the Niger Delta. Most of the well drilled in the filed penetrated between 20 and 40 sandstone units which are separated by sahle breaks whose thickness increases downwards.
There is considerable controversy over the origin and development of sedimentary basins have been prosed in the past, but most of them had limited predictive powers or required unobserved processes operating in the crust and upper mantle.
Intensive petroleum exploration and exploitation activities in the delta region during the last two decades have led to the accumulation of vast amount of data from which it has been possible to establish the historical reconstruction and evolution of the Niger Delta basin (Short and Stauble, 1967; Webber and Daukoru, 1975; Evamy et al, 1978).
A key to exploring from hydrocarbons is an understanding of stratigraphy, structural and pale environmental relationships in which the sandstones accumulated.
Therefore, the need to sediment logically characterise reservoir rocks is to obtain better recovery of oil and gas from existing producing reservoirs and find new stratigraphic traps. A better understanding of the reservoir also will increase the efficiency of interpreting well logs and result in fewer unnecessary tests and completions.
According to Hilchie (1984), reservoir description by the use of well logs may be divided into three areas: (i) volumetric (ii) pore description and (iii) matrix description. In this study, attention has been focused on the matric description to arrive at a suitable environment of deposition from each of the sandstone reservoirs.
The objectives of this study are to characterise the reservoir sandbodies at ‘’Layo’’ field and to use these to interpret their depositional environments. It is hoped that the depositional model will serve as workable predictive sedimentological model for the reservoir rocks. These models will hopefully be useful in the exploration and exploitation of oil and gas in the Niger Delta.
To achieve the set objectives, gamma ray logs and sidewall samples description from six and two wells, respectively, were integrated in this study.
The area of study is ‘’Layo’’ field, eastern Niger Delta (fig.1.1) it is situated towards the south western part of Port Harcourt in Rivers State and is delimited by Longitudes 6015’E and 6055’E and Latitudes 40N and 4030’N. The field covers an area of about 26.25 square kilometres. In the geological map of Nigeria the study area is located within the Niger Delta Mangrove swamps and coastal plain sands.
The reconstruction of the depositional history of ancient sandstone units have been widely documented.
Bloomer (1977) establishment the depositional environments of a reservoir sandstone in west-central Texas with the aid of a combination of surface and subsurface information. He interpreted the cook sandstone to be an essentially continuous depositional systems from fluvial channels through deltas and submarine canyons, to deep marine sands.
Berg (1979) demonstrated that the delta-margin facies of Dakota sandstone are made up of essentially fluvial-channel and delta-front sandstones.
Berg (1986) using core and dip logs suggested that the Yegua sandstone represents a prograding delta-front sequence.
Downing and Walker (1988) worked on the long and narrow Viking formation sandstones at Joffre, using essentially cores, resistivity log and gamma-ray logs. They established that the formation was deposited during low stand and reworked by tidal and/or wave-generated currents to form long and narrow sand body which consists of Cross-bedded glauconitic sandstones encased in bioturbated marine mudstones and siltstones formed during the ensuing transgression.
Jordan and Tillman (1987) in their subsurface studies of the Bartlesville sandstone made use of five oriented cores to decipher its environments of deposition. They identified five sedimentary facies which were interpreted as (i) delta plain shale and siltstone, (2) distributary channel sandstone, (3) overbank shale and siltstone, (4) interdistributary bay sukstune, shale and sandstone and (5) crevasse splay sandstone siltstone.
Weightman et al. (1987) worked on the depositional modelling of the Upper Manville (lower cretaceous), East central Alberta utilizing several cross-sections, core descriptions, palynology, x-ray diffraction for clay mineralogy, pyrite and siderite, oil saturation and permeability’s. They interpret them which sandstones in the Upper manbille to have been formed in several ways. Including: stacked flubial deposits, stacked crevasse splays and amalgamated channel and Maine shoreline facies, which was proposed to have changed from a predominantly continental nature to a magical marine setting.
Flint et al (1988) correlation 23 wells coupled with examination of core data and established the sirikit deltas as a constructive lobate type with shetlike mouth-bar reservoir sandstone geometries.
Fruit and Elmore (1988) conducted the subsurface studies of the cottage Grove Sandstone, north western Oklahoma using cores and electric logs. They established that the cottage grove sandstones were deposited in a shelf environment with deposits consisting of linear sand ridges encased in shale.
Reinson el al (1988) in their work utilized an extensive high-quality log and core data base to arrive at a reasonable conclusion on the reservoir geology of crystal Viking filed. With the detailed log cross-sections across the sandbody and core examination, they interpreted the crystal deposits as tidal channel and estuary bayfill, whose combination was referred to as estuarine tidal channel-bay complex.
Pozzobon and walker (1990) regarded the sand ridge in viking formation at Eureka ‘’as having formed from a plume of and in a shallow sea, with its western end close to a deltaic distributary channel’’.
In Nigeria, Amajor (1986) using outcrop studies described the campano-masstricchtain Ajali Sandstone in Okigwe, south-eastern Nigeria as an intertidal felt with tidal channels, all of which graded land-wards into continental deposits.
Amajor (1987) in his surface studies of the paleocurrent, petrography and provenance analyses, of the Ajali sandstone in south-eastern Nigeria, reached a conclusion that the sandstone is of multicycle (second) origin derived from the santonian Okigwe-Abakaliki anticlinorium which is, in itself, first cycle sedimentary rocks.
In the Niger Delta petroleum province, the sedimentological aspects of the sandstones have been summarised by weber (1971) and Evamy el al (1978).
Aqeousxun (1981) using thin section studies, electric log analyses and data from modern deltas established the depositional environments of the reservoir sandstones of Ossu-Izombe oilfield as distributary mouth bars, point bar and distributary channel sandstones, braided stream deposits and shallow marine deposits.
By a process-response method of prognosis of environments of offshore marine bar for the sandbodies of Apara and Akpor oil fields in the eastern Niger Delta.
Previous workers have used either outcrop studies, subsurface studies or both in the prediction of the environments of deposition of sandstone reservoir. The depositional environment determination using the data base derived from outcrop studies the sandstone can be carried out particularly where there are good exposures which reveal sedimentary structures, fossil content, etc. it gives room for insitu measurements of environmental parameters. However, where we need to know the environments of deposition of reservoir sandstones that are deeply buried in the earth’s scrust, we have to rely on subsurface methods of study because of lack of direct observation of the sandstones. However reservoir geologists, the world over, depend soley on cores, ditch cuttings, sidewall samples and wireline logs to decipher environments of deposition. The better the core recovery and handling the better the results of interpretation of environments of deposition.
In the present study, I have chosen to determine the environments of deposition of the reservoir sand bodies in ‘’Layo’’ filed using subsurface studies due to lack of exposure of the sandstone units.
The uniqueness of the study area relative to the regional picture however, have not been recognised. This is not unconnected with the react that no sedimentological work of the area was available to the author and partly with the confidential nature of some of the detailed data in this country. Unfortunately, much of this data was not available to the author for the present study. Therefore, on the basis of available data base, the author have interpreted the environments of deposition of the sandstones (sandbodies) in ‘’Layo’’ field using the constructed depositional models of Krueger (1968), selley (1978), Busch and Link (1985), ketch et al. (1990) and the method employed by Amajor and Agbaire (1989).
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