Hydrocarbon reservoir beds have been delineated using direct hydrocarbon indicator on seismic sections as well as well logs data in X field, Onshore Niger Delta. The research methodology involved horizon interpretation to produce sub-surface structure map. Geophysical well log signatures were employed in identifying hydrocarbon bearing sand. The well-to-seismic tie revealed that the reservoir tied directly with hydrocarbon indicator (bright spot) on the seismic sections. The major structure responsible for the hydrocarbon entrapment is anticline. The crest of the anticline from the depth structural map occurs at 3450 metres.
The Niger Delta is a proven prolific hydrocarbon system with a number of world-class oil discoveries. Hydrocarbon in the delta is mainly produced from sandstones and unconsolidated sands in the Agbada Formation. The objective of hydrocarbon exploration is to identify and delineate structural and stratigraphic traps suitable for economically exploitable accumulations and delineate the extent of discoveries in field appraisals and development. These traps could be very subtle and complex and are therefore difficult to be mapped accurately [
Seismic and well log data are widely used in petroleum exploration to map the subsurface. The two data sources are complementary: seismic profiles provide an almost continuous lateral view of subsurface, whereas well logs yield fine vertical resolution of the geology at the borehole. Seismic profiles can resolve, with relatively high precision, the structural and stratigraphic changes from the arrival times and amplitudes of the reflection events.
The bandwidth of seismic data constrains the vertical resolution of the subsurface. High frequency data are essential for delineating subtle traps. Also, the seismic expressions of anomalies cannot be interpreted uniquely in terms of the geologic variables. Well logs can be helpful in the interpretation of seismic profiles in both respects, viz., they can unambiguously provide, at borehole, a high-resolution estimate of many essential geologic variables. Therefore, the integration of well log and seismic data would provide a high degree of reliability in mapping subsurface structural and stratigraphic plays. It will also provide insight to reservoir hydrocarbon volume which may be utilized in exploration evaluations and in well bore planning [2,3].
The primary geophysical seismic data are often recorded in time. However, meaningful interpretation need be displayed in depth. The primary objective of geophysical seismic interpretation is to prepare contour seismic maps showing the two way time to a reflector as picked on the seismic sections. This time map must be converted to depth map through seismic time-depth conversion process. The depth conversion process is usually carried out using average velocity information obtained from well logs and check shot data. The integration of geological data with 3D seismic may lead to solving some of the problems of identifying facies and structural details [
Exploration and Asset development teams are composed of multiple disciplines which use different tools to analyse different data types in different disciplines. This means seismic data has not been readily available to petrophysicists and that geologic cross sections have been unavailable to seismic interpreters. Display of seismic and well data together can strengthen the interpretation of both. This research is part of the effort to use seismic and well logs data to indicate favourable areas in which exploration can be concentrated especially at deeper levels. Therefore the aims of this study are to pick key horizons, identify and map prospects and to ascertain if the structures and their closures are favourable for hydrocarbon accumulation in this onshore block using integrated geophysical approach.
Geology of the Niger DeltaThe study area is located within the Niger Delta, Nigeria (
The geology of the Tertiary section of the Niger Delta is divided into three formations, representing prograding depositional facies distinguished mostly on the basis of sand-shale ratio [
The Agbada Formation consists of fluvio-marine sands, siltstones and shales. The sandy parts constitute the main hydrocarbon reservoirs. The grain sizes of these reservoirs range from very coarse to fine. The Niger Delta province is generally adjudged to contain only one identified petroleum system referred to as the tertiary Niger delta (Akata-Agbada) petroleum system [
Most of the traps in Niger delta fields are structural although stratigraphic traps are not uncommon. The structural traps developed during synsedimentary deformation of the Agbada paralic sequence [9,10]. Structural complexity increases from the north (earlier formed depobelts) to the south in response to increasing instability of the under-compacted, over-pressured shale. [
The data used for this study are composite geophysical well logs, seismic sections, check shot data and base map of the seismic lines which were obtained from Nigeria Agip Oil Company Limited. The geophysical logs include gamma ray, resistivity and sonic logs from four oil wells. Two seismic profiles, an arbitrary line in NW-SE direction and the other, an inline in E-W direction were interpreted. The base map of the seismic lines and the well locations are shown in
Preliminary study on the well logs revealed one major hydrocarbon bearing reservoir. The results of the well logs correlation connecting the four wells in the survey area is shown in
good agreement was observed of their continuity within the extent of the well location. Hydrocarbon reservoir was delineated on the well logs with the aid of the gamma-ray and deep resistivity logs. The top of the identified reservoir is shown in the correlation panel.
Quantitative evaluation of the well logs was not carried out because of insufficient information.
The seismic sections extend to 4.0 seconds two way travel time (Figures 4(a) and (b)). The reflection continuity below 3.2 seconds is generally poor. The character of the seismic record changes with depth. The study focuses on reflections between 1.0 second and 3.5 seconds. Reflection within this interval has good continuity and high to moderate amplitude variation. Reflections in the shallowest part of the seismic sections are parallel and nearly horizontal.
The well-to-seismic tie revealed that the hydrocarbon bearing reservoir is associated with direct hydrocarbon indicator (Bright spot) on the seismic sections. The horizon picking was initiated on the bright spot. The top of the reservoir in the wells was delineated and mapped using adequate seismic to well correlation and their seismic continuity. The depth structure map of the top of the reservoir is shown in
The integration of seismic data and well logs proved to be a useful and valid tool in structure and stratigraphic mapping. From the well logs analysis one major reservoir was identified. The result of the qualitative interpretation of the gamma ray and resistivity logs shows that the reservoir contains hydrocarbon of appreciable thickness. The well-to-seismic tie revealed that the hydrocarbon bearing reservoir is associated with direct hydrocarbon indicator on the seismic sections. The major structure responsible for the hydrocarbon entrapment in the field is an anticlinal structure. The crest of the anticline from the depth structural map occurs at 3450 metres. Information extracted from the integration of the seismic and well logs data have resulted in more understanding of the structure, stratigraphy and hydrocarbon potentials of the “X” field, onshore Niger delta. The result suggests more development opportunities at deeper levels.
The authors are grateful to the Department of Petroleum Resources and Nigeria Agip Oil Company Limited for granting our request for data with which the research was accomplished.