Magnetic anomaly patterns, fault-block tectonism and hydrocarbon related structural features in the Niger Delta basin
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Gridded airborne magnetic data, 3D magnetic forward models in combination with filtered maps and derivative techniques such as analytic signal and directional derivatives were used to reveal basement structure and architecture of the study area in offshore Niger Delta.The generating mechanisms of the magnetic anomalies are boundaries (fault) of basement blocks of different compositions having different physical properties (susceptibilities and remanence) and horst/graben structures with associated normal faults and extensions.The intrabasement sources exhibit anisotropic susceptibilities and depth to basement (sedimentary thickness) is between 4000m to over 10,000m indicating deep depocenters.All these factors combined to influence magnetic anomaly patterns which have implications in hydrocarbon prospect target.The regional structural characteristics of the basement interpreted from the regional magnetic data sets show clear relationship between inferred rift-related basement structures and oil fields.Oil and gas discoveries appear to be located on the flanks of steep/faulted basement and on top of basement structural highs and lows.This study, therefore, is significant in understanding the petroleum system, contributes to basin modelling and can unravel areas of identical structural development in less well-explored sectors of the basin.Keywords:
Niger delta
Anomaly (physics)
Hydrocarbon exploration
Fault block
Dongpu sag is a sophisticated oilfield which has been explored and developed for 30 years.At present,complex fault-block reservoirs are the major target for exploration and development,of which the fault blocks are small and cracked,the fault plane is steep,and oil column thickness is small.All the features require more accurate seismic exploration.To meet the requirements of exploration and exploitation,combined with the trends of geophysical technology,we shall learn from the relevant advanced techniques both at home and abroad and focus on high precision 3D seismic techniques to tackle the imaging problems in complex fault-block reservoirs.Finally,a series of techniques are developed to implement high precision 3D seismic exploration.The application of the techniques in the areas such as Machang and Qiaokou shows that they are effective in solving the exploration and development problems in complex fault-block areas,and are valuable to be more widely applied in the same problematic area.
Fault block
Hydrocarbon exploration
Seismic exploration
Petroleum exploration
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Estuaries in the Niger Delta-Nigeria are influenced by tidal currents and wave actions due to their proximity to the Atlantic Ocean.Tidal current provides the energy required to move sediments in and out of the estuaries from the seashores, while wave actions curtails the deposition of sediments at the bottom of the estuaries as the bed get shallower, resulting in the modification and long-term regulation of siltation through erosion or accretion.The interaction between estuaries bathymetric shapes, tidal currents and wave actions can only be fully understood through analytical or mathematical relationships.Analytical methods are less accurate in predicting future tides, while none of the existing mathematical relationships can accurately predict tidal behavior in the Niger Delta region due to the fact that parameters governing tidal actions vary from region to region, hence the need for this study.This study shows the behavior of the estuaries in response to variation in tidal heights, currents and wave actions through mathematical modeling, a knowledge which is useful in planning and timing of marine activities that requires pre-knowledge of tidal levels, direction and current velocities.The models were formulated and calibrated using parameters generated from the hydrographic, hydraulics and geotechnical investigation, including local field observations and measurements conducted within the study area.Soil samples taken from the area are composed of peat, organic clays, silty clays and sand.Peat constitutes the dominant soil which is locally known as 'chikoko' with high compressibility and color ranging from dark brown to dark gray and texture from soft to firm.Typical bed material size (D 50 ) is approximately 0.2mm.Tides in the region are mostly semidiurnal with tidal prism ranging from 0.4 to 1.5m.The tidal strength is strongest at the inlets and decreases with distance inland.The depth of the estuaries is controlled by the strength of the tidal currents.Areas very close to the ocean with stronger tidal effect are very deep; while shallow canals and creeks predominates the hub of the estuaries.The results obtained using the present models compared favorably with the field measurements.Average correlation coefficients of 0.9 were obtained in some cases.The results showed that the estuaries erodes, accretes, or remain stable, depending on the rate at which sediment is supplied or removed from them.
Siltation
Deposition
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Anomalies of the earth's magnetic field have been detected over producing oil fields and have generated great interest in the exploration community because they may be direct indicators of hydrocarbon reservoirs. These anomalies are of low amplitude (typically 1 to 5 nT). Their horizontal dimensions vary from linear zones to equidimensional areas. These enhanced magnetic fields may be produced by diagenetic magnetite created from hematite by the reducing action of vertically migrating hydrocarbons. To measure such small anomalies in the magnetic field, the airborne magnetic survey must be flown at low altitudes. Much cultural noise, created by farm buildings, power lines, oil well casings, and other structures, is recorded with the data. From the research carried out to date, it may be concluded that anomalous near-surface magnetic bodies associated with hydrocarbon reservoirs exist and can be detected.
Hydrocarbon exploration
Magnetic survey
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