This research work is aimed at usingacousticimpedance as means of predicting lithology and hydrocarbon away from well control of “Ovi” Field hence providing a detailed evaluation of the hydrocarbon potential of the area. The methodology used involves identification of hydrocarbon bearing reservoirs from well logs using Gamma ray and resistivity logs, wells correlation, petrophysical analysis, well to seismic tie, horizon and fault mapping, generation of structural maps, acousticimpedance crossplot analysis and seismic inversion using model based approach. Three reservoir sand were mapped within the Agbada Formation. From the crossplot of acousticimpedance against gamma ray, porosity and water saturation, the acousticimpedance ranges from 24500-27500 (ft/s)*(g/cc) for shale and 17500-24500 (ft/s)*(g/cc) for sand based on the saturating fluids, the results also shows that acousticimpedance have a linear relationship with water saturation, while porosity have an inverse relationship with acousticimpedance for the study area. Average acousticimpedance maps for reservoir tops generated from the inverted seismicdata indicated areas of low acousticimpedance corresponding to hydrocarbon bearing zones that were not detected on the time maps. The result provided detailed information about the subsurface lithology and hydrocarbon saturation away from well control of the study area.
Seismic and welllogdata are widely used in hydrocarbon exploration to map the subsurface and to evaluate the hydrocarbon potential in the reservoir. The two data sources are complementary; seismic profiles provide an almost continuous lateral view of the subsurface by defining its geometry and providing an estimate of the acousticimpedance which is related to the formation densities and velocities whereas well logs yield fine vertic- al 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 band- width of seismicdata constrains the vertical resolution of the subsurface. High frequency data are essential for delineating subtle traps. Also, the seismic expression of anomalies cannot be interpreted uniquely in terms of the geologic variables. Well logs can be helpful in the interpretation of seismic profiles.
The investigation of wells/boreholes, using various instruments and techniques (depending on the well /borehole environment) and specific parameters being sought for, is known as Geophysical Well Logging or Borehole Geophysics. The subsurface geologic investigation with the use of wireline geophysical well logs has progressed over the years and has thus become a standard of operation in petroleum exploration. With the integration of exploration results from gravity, magnetic and seismic geophysical prospecting methods, favorable geological conditions for hydrocarbon accumulation may be identified. Exploratory wells are drilled into the prospective structure to evaluate the prospect. This is called Formation Evaluation. It is the process of using information obtained from borehole to determine the physical and chemical properties of subsurface rocks and their fluid content along the borehole (Figure 1.1). It involves the analysis and interpretation of well-logdata, drill-stem tests, cores, drill cuttings, etc. Petrophysics is a term used to express the physical and chemical properties of rocks which are related to pore and fluid distributions, particularly as they pertain to detection and evaluation of hydrocarbon bearing layers, (Archie, 1950). Petrophysics pertains to the science of measuring rock properties and establishing the relationships between these properties. It is related to petrology as much as geophysics is related to geology. Petrophysics is an important tool in hydrocarbon exploration. Its use in hydrocarbon prospecting involves well drilling and formation evaluation. The measurements are displayed as a set of continuous curves called Log, from which hydrocarbon reservoirs can be identified and reservoir parameters such as porosity, water saturation, hydrocarbon saturation and reservoir thickness can be estimated. These parameters will help in the estimation of hydrocarbon in place.
This study examined the application of 3-D seismic and welllogdata for proper optimization and development of hydrocarbon potential in “ NANA ” field of Niger-Delta Province. The data were conditioned for interpretation using Petrel software. These delineated reservoirs found in each well were then correlated across the field. A well-to-seismic tie was done usingSeismic attributes (Frequency, RMS amplitude, AcousticImpedance and Variance) and property modeling (Facies, net-to-gross, porosity, water saturation, hydrocarbon saturation and permeability) were distributed stochastically within the constructed 3-D grid. The reservoir structural model shows a system of differently oriented growth faults are quite extensive). The trapping mechanism is a fault assisted anticlinal closure.. This study demonstrated the effectiveness of 3-D static modeling technique as a tool for better understanding of spatial distribution of
The cross plotting of rock properties for fluid and lithology discrimination was carried out in a NigerDelta oil fieldusingwelldata X-26 from a given oil field in the coastal swamp depobelt. The data used for the analysis consisted of suites of logs, including gamma ray, resistivity, sonic and density logs only. The reservoir of interest Horizon 1, was identified using the available suite of logs on the interval where we have low gamma ray, high resistivity, and low acousticimpedance specifically at depths 10,424 ft (3177.24 m) to 10 724 ft (3268 m). We first obtained other rock attributes from the available logs before cross plotting. The inverse of the interval transit times of the sonic logs were used to generate the compressional velocities and the S-wave data was generated from Castagna´s relation. Employing rock physics algorithm on Hampson Russell software (HRS), rock attributes including Vp/Vs ratio, Lambda-Rho and Mu-Rho were also extracted from the welldata. Cross-plotting was carried out and Lambda Rho (λρ ρ) cross-plots proved to be more robust for lithology identification than Vp versus Vs crossplots, while λρ Versus Poisson impedance was more robust than Vp/Vs versus Acousticimpedance for fluid discrimination, as well as identification of gas sands. The crossplots were consistent with Rock Physics Templates (RPTs). This implies the possibility of further using the technique on data points of inverted sections of various AVO attributes within the field in areas not penetrated by wells within the area covered by the seismic.
simulation models of prospects, by accurately defining both the static and dynamic properties of reservoirs, through the integration of petrophysics in the interpretation of seismicdata. The use of inversion algorithms based on the approximations of Zoeppritz’s equations has been studied by various researchers  , for the inversion of prestack seismicdata into acoustic and shear impedance volumes. Their results demonstrated the use of seismically derived attributes such as acousticimpedance, lambda-rho, Poisson impedance and Murho as effective tools for lithology and fluid prediction in a hydrocarbon reservoir.
A post stack acousticimpedanceseismic inversion study was carried out by integrating well logs and 3D seismicdata obtained in a NigerDelta offshore field. The aim was to delineate lateral variations in subsurface rock properties especially lithology and density, which could aid in petrophysical and facies modeling to constrain the extent of hydrocarbon zones in the development of the field. Logdata comprising GR, density and compressional sonic logs from one well, and a 5 – 18 degree angle stack of approximately 43.05 sq.km 3D PTSM dataset were used for the study. A deterministic model-driven inversion workflow that includes forward modeling of reflection coefficients from a low frequency impedance model derived from the well logs and convolution of the reflection coefficients with a source wavelet derived from the seismic and welllogdata was adopted for the study. Density and acousticimpedance volumes obtained from the inversion reveal the reservoir top at the well location, and show lateral variations in these properties away from well control. The results obtained are important for accurate structural and stratigrahic interpretation to reduce the risk of exploratory and development well locations in the field.
The earth subsurface consists of several layers that contribute reflections to a single seismogram. Seismic reflection surveying is the most widely used geophysical technique. This is because, seismic reflection data permits mapping of many horizons or layers with each shot. The basic principle of seismic reflection technique is to generate seismic waves and measure the time taken for the waves to travel from the source, reflect off an interface and be detected by an array of receivers (geophones) at the surface . The knowledge of travel times from the source to various receivers and the velocity of the seismic waves, the pathways of the waves can be constructed to build an image of the subsurface. Multiples from the bottom of a body of water and the air-water interface are common in marine seismicdata (which is not desirable), and are suppressed by seismic
Taking into cognizance all these analyses as regards the crisis in the Western NigerDelta Region particularly in Warri, we would be quick to argue that the root of the Warri crisis is the Kaiama Declaration of the Ijaw youths which was essentially a declaration that the entire NigerDelta Region of Nigeria to some extent Belonged to them. This of course is not a truism as it is a common knowledge that several other ancient ethnic groups like the Ibibios, the Itsekiri, the Andonis, the Urhobos, the Efiks, etc. alongside the Ijaws inhabit the area. The Warri crisis was the beginning of the implementation of the Kaiama declaration by the Ijaw youths. The Itsekiris, a tiny minority occupying lands containing some 20% to 40% of the total oil wealth of Nigeria, were a prime target of the Ijaws and they were to be quick and gain as the Ijaws envisaged that they could easily be run over, considering their status as a minority of minorities and the fact that they, unlike the Ijaws were unprepared for war. Simultaneously, in the hope of making quick work of the Itsekiri, Ijaws took their expansionist agenda also to Edo State as well as to Ondo State where they were brutally halted by the majority ethnic groups in those States as claimed by some scholars. The trigger or opportunity for the commencement of the implementation of the Kiama Declaration and thus of the so called Warri crisis was when the then Military administrator of Delta State, Colonel David Dungs, was seen to have unilaterally announced in a broadcast to the state that the Headquarters of the newly created Warri South-West Local Government Area was Ogbe-Ijoh, an Ijaw settlement, contrary to Ogidigben, an Itsekiri town, as was dully gazzetted by the Federal Government of Nigeria. The Colonel Dungs maneuver is believed to have been sponsored by the Ijaws, he being not from the area but from far away Plateau State in North Central Nigeria and therefore aught not to have had any vested interest in the matter and considering that the action was solely to the benefit of the Ijaws. That the Itsekiris put up no resistance initially buttresses this point. That the ijaws commenced a war rather than a protest and even went on to “capture” and retain known traditional Itsekiri territories like Kantu and Okenrengigho (which they said to have re-christened “Okenrenkoko” contrary to what is on every map of the Area) also buttressed this point. The so-called Warri crisis therefore was not a crisis but a war of attrition waged by the Ijaws against their Itsekiri brothers with whom they had co-existed peacefully before advent of Ijaw nationalism. 15
In this paper, seismic evidences of hydrocarbon seepage are mapped using a seabed bathymetry generated from a dense grid of 3D seismicdata acquired in an area of overlapping maritime boundary between Nigeria and Sao Tome and Principe (Figure 1). With increasing oil and gas exploration activities in the deep-water NigerDelta, this paper demonstrates that conventional 3D seismicdata can provide invaluable data for petroleum system analysis in offshore NigerDelta.
The joint interpretation of the structural, velocity, and resis- tivity model contributes to a more robust understanding of the subsurface, better characterizing the existing faults – a key aspect for the risk assessment of the Hontomín pilot stor- age site. The combined interpretation of the geoelectrical and the structural model derived from the seismicdata allowed estimating the fluid flow characteristics of the major faults in the area. The methodology used to derive velocity from re- sistivity has been successfully applied at the Hontomín site, pointing out the importance of employing more than one em- pirical relationship between the properties to have a better control of the uncertainties inherent to the approach. The major differences observed between the velocity models de- rived from the resistivity model and the predicted and logged velocity in the injection and monitoring wells are attributed to the lower resolution of the magnetotelluric method. The joint interpretation for depths shallower than 40 m allowed extracting information about the characteristics of the shal- low sediments, suggesting variations in clay and water con- tent. The seismic and magnetotelluric methods have shown their compatibility across scales, pointing out the importance of joint interpretation in characterization surveys. This work provides a more complete picture of the Hontomín site sub- surface, essential in the monitoring of the site, and estab- lishes the basis for a potential joint inversion of the two geo- physical datasets.
The data used for this research include suites of digital wireline logs (e.g. gamma-ray, resistivity and density logs from three wells, checkshot, and seismic lines. The avail- able 3D seismic reflection data cover 102 km 2 area with 637 inlines and 595 crosslines in the interval of 25. The sampling interval is 4 ms with sampling intervals per trace of 1251. The resolution of the seismic section in term of quality was further enhanced using post-processing tech- niques such as seismicdata cropping and data smoothening with structural preserving algorithms. Figure 3 shows the basemap and the location of the drilled wells in the study area. The well correlation to map out various delineated lithologies and reservoir zones was conducted using gamma-ray and resistivity logs. The correlation was done using the stacking patterns such as fining upward, coarsing upward and blocky patterns. Despite the unavailability of the biostratigraphic data, the reservoir correlation was done with respect to a mappable candidate stratigraphic surface (cMFS) across the wells using gamma-ray logs. Four delineated and correlated reservoir sand units were code named R1, R2, R3 and R4 (Fig. 4). The step of 10 with manual horizon tracking tool was used to map out the delineated reservoir horizons across inlines and crosslines on seismic section. Seismic-to-well tie of the reservoir units on both 3D seismic and well logs data was conducted using the derived velocity model from the available checkshot data of well 0555. The time-to-depth conversion technique was used to generate synthetic seismogram which was later employed for seismic coloured inversion.
Four crude oils were used for this study. The crude oil samples were collected from the NigerDelta region, Nigeria, by field technicians from the wellheads of producing wells. Two oil samples were collected from Akwa Ibom State, one from Rivers State and the fourth from Delta State and were labeled as ND-A3, ND-A6, ND-B7 and ND-E5 respectively. The light hydrocarbons were analyzed using the Hewlett Packard (HP) 6890 gas chromatography (GC) fitted to a fused silica capillary column (30m x 0.25μm) and equipped with a flame ionization detector (FID). Ultra high resolution gas chromatography oven temperature
The Luhais oil field is situated in southeast Iraq, approximately 105 Km west of the city of Basra and 450 Km south east of Baghdad as shown in the Fig. (1a) as well as shows the base map of the study area Fig (1b). The Nahr-Umr Formation was considered as the most important oil reservoir in Luhais oil field, therefore our research forces on it. Its age is of Albain deposit and identified in southern Iraq. It comprises black shale interbedded with medium to fine grained sandstone with pyrite (Bellen et al. 1959). The depositional environment represents a mixture of ground, shallow marine, tidal and deltaic environment with interspersed natural marine area (Al-Sayyab et al. 1982). According to many previous studies, the Nahr-Umr reservoir subdivided into three main members, which are upper member (comprises of shale with low rate of sand), middle member (comprises of sand with low rate of shale) and lower member (comprises of sand), these results were discussed rely on stratigraphic analyses and well correlation. In Luhais oil field 49 wells were drilled, which shows that the given stratigraphic variation in the field is similar to the stratigraphic variation of adjusting area such as Subba, north and south Rumaila oil fields. The drilling results showed that the oil is available in Nahr-Umr Formation except Lu-21 shows oil for several meters within the Nahr-Umr Formation. The Luhais oil field lies within Zubair subzone of the Mesopotamian zone in the unstable shelf according to (AL-Khadhimi et al. 1996). The Luhais structure is asymmetrical antiform with dip of less than one degree on both flanks. Large scale faulting cannot be recognized on seismic section at Luhais, but regional maps suggest structural strike NE- SW, NW-SE and N-S (Anadarko 2004). The present research aimed to use the acousticimpedance inversion as a tool and a diagnostic parameter to evaluate the petrophysical properties and delineate the lateral and vertical facies heterogeneity as well as provide insight to porosity and lithology variations away from limited well control of Nahr-Umr Formation in Luhais oil field. Many of seismic studies depend to seismic inversion in their interpretation. The reason is obvious due to there is many of problems- which is noise, losses and other mistakes- in the interpretation of this seismic information and no equation directly relates these multiple information of seismicdata can resolve with unique answers. Therefore, the interpreter was resorted to inversion, which is a mathematical approach of evaluating and assessing an answer, examining it verses observation and adjusting it until when the final answer is more reasonable.
main target reservoir in Ness County is the Mississippian carbonates. In this study the welllog and facies analysis indicate the presence of carbonates while the dominant mineral is silica near the top of the Mississippian interval (Appendix D). As stated in the “Background” section, the carbonates in the Mississippian interval in western Kansas have a high silica content due to diagenesis that resulted in the formation of chert. Therefore, the welllog and facies interpretation are accurate. The high silica facies also are suspected to be the producing facies within the Mississippian interval because they exhibit higher porosity values and show density-neutron cross-over associated with the presence of hydrocarbons. Although an increase in resistivity within Facies 4 may be interpreted as a potential reservoir, its resistivity values are being skewed due to the low porosity. Structurally, the main anticlinal features present the most probable area for accumulation of hydrocarbons, while the reverse fault on the western flank of the structure seals the fluids from traveling further west. This is confirmed from the number of producing wells drilled in structural high areas (Figure 22).
Petro physics, rock physics and multi-attribute analysis have been employed in an integrated approach to delineate porosity variation across Tamag Field of NigerDelta Basin. Gamma and resistivity logs were employed to identify sand bodies and correlated across the field. Petro physical analysis was undertaken. Rock physics modelling and multi-attribute analysis were carried out. Two hydrocarbon reservoir sands (A and B) were delineated across the field. Reservoir A is a relatively clean sand, characterized with high average porosity of 0.28 while Reservoir B is also a relatively clean sand with lower average porosity of 0.24. Reservoir A is a replica of Friable Sand Model while reservoir B mirrors the Constant Cement Model. Acousticimpedance attributes serve as good predictors of lateral changes in porosity across the reservoirs. The internal fabric of Reservoir sand A is that of a clean high porosity sands implying that there are few or no diagenetic cement and the stiffness of the rock is weakly affected. This reservoir is relatively good quality due to its good porosity and sorting even at deeper depths. This unconsolidated sandstone reservoir is associated with high permeability but highly susceptible to sand production, which causes severe operational problem for oil and gas explorers. Reservoir B has good porosity but relatively lower that of Reservoir A. This conforms to the results of the petro physical analysis which shows that reservoir sand A with average porosity 0.28 is more porous than reservoir sand B with average porosity 0.24.
Shale usually contains small quantity of radioactive elements such as uranium (U) potassium (k) and thorium (TH). This produces gamma ray radiation from which the source can be detected by spectrometry. The log thus, detects shale horizon and can provide an estimate of the clay content and other sedimentary rocks. Amongst, the sediments, shales have by far the strongest radiation. That is why the log is called “Shale Log”.
gravity) and outcrop sections. Generally, seismic method is used at different scales of investigation ranging from the mapping of sedimentary basins, mapping of fault patterns within producing fields; mapping depositional packages to ascertain sand and pore fluid distribution and more detailed actual depth- controlled seismicdata acquisition (Vertical seismic Profiling -VSP) from drilled wells.
degree to which pore system can conduct fluids. Reservoir permeability is a directional rock property. Cross bedding, ripple marks, cut and fill structures as well as variation in cementation, grain size, sorting and packing contribute to the variation in permeability with a certain depositional unit. Permeability in the direction of elongation of the component grains is considerably greater than in any other direction. In logging hydrocarbon bearing reservoir, many tools are used to make a wide variety of measurements. One uses the injection of an electrical current and the measurement of the voltage response to calculate the electrical resistance, which is multiplied by a geometric factor (determined by the electrode positions in space) to become the material property called electrical resistivity. Electrical resistivity is the property that describes the ability of a material to support the process of charge transport. Archie's Law (Archie, 1942) describes the relationship between electrical resistivity and porosity, fluid saturation, and fluid type in a rock. However, the results obtained from Archie's Law have built in assumptions. These include considerations of the rugosity of the borehole wall, properties of the drilling mud, invasion of the mud into the formation, morphology of the porosity, connectivity of the pores, wettability of the rock, presence or absence of clay minerals, and more. Depending upon the choices made about these assumptions, different interpretations result for porosity, saturation and fluid type. The location of petroleum reserves requires an understanding of the nature of the rocks in which these reserves may occur. Borehole geophysical measurements opposite the investigated unit is a fundamental method of the formation analysis since they measure the physical properties of the rock matrix and pore fluids. Utilizing log-derived measurements of such petrophysical properties makes it practical to determine, lithology, porosity, shale content, water saturation and hydrocarbon saturation and types, when oil and/or gas are present and to estimate permeability, to predict water cut, calculate residual oil saturation and detect over-pressured zones.
A visual estimate of percentage of shale was plotted in the description sheet and grain size analysis done using a comparator for grain size and thus plotted on the logging sheet. The diluted acid (HCI) was used to delineate the presence of carbon. Carbonate cementation was also determined and graded into different degrees as medium, hard or very hard. Other sedimentary structures like lenticular bedding, cross bedding, flaser bedding and ripple laminations were observed and plotted. Irrespective of intensity Bioturbation features was seen. Using the above parameters in conjunction with Reijers (1995), lithofacies scheme and with the help of grain size trends, the lithofacies were determined. Major erosional breaks/boundaries were recorded using upright and inverted triangles to indicate coarsening upward and fining upward sequences respectively.