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Murphy F.E.,ARKeX | Schodt N.H.,Maersk Oil | Andersen J.,Maersk Oil
73rd European Association of Geoscientists and Engineers Conference and Exhibition 2011: Unconventional Resources and the Role of Technology. Incorporating SPE EUROPEC 2011 | Year: 2011

The Kwanza basin is part of the greater Aptian salt basin of West Africa. The recent discovery of giant fields in the sub-salt section of the conjugate Brazilian margin has focussed interest on the essentially unexplored deep water Kwanza basin. The presence of a functioning petroleum system is known from onshore wells. However, the deep water and complex geology associated with poor seismic imaging makes conventional exploration challenging. Due to the high density contrast between the sediments, salt and basement, the area is an ideal case for application of potential field methods in an integrated interpretation workflow. Modelling of interpreted seismic sections constrained by potential field data exploits the complementary nature of the datasets. This study uses a subset of the WesternGeco multi-client dataset covering much of the Angolan passive margin. Gravity models were constructed along 10 regional seismic lines. The initial models were based on the pre-existing seismic interpretation. The basement was modified to produce a model consistent with both the seismic and potential field data. A regional basement surface was derived utilizing the 2D models as a constraint. Significant changes to the seismic interpretation of salt were also required to ensure a consistent fit to the gravity data.

Houghton P.,ARKeX
Hart's E and P | Year: 2012

Identifying, mapping, and staying within sweet spots determining well locations and spacing so drilling can be optimized and designing the most effective production strategy, while taking heed of environmental sensitivities, are all key drivers for shale gas operators today. While information generated from seismic is crucial in guiding drilling and completion programs, gravity gradiometry imaging (GGI) also is playing an important role alongside 2-D seismic in optimizing exploration strategy in shale gas plays. These benefits are principally seen in two crucial areas - firstly, in indentifying zones where there is a high probability of structural complexity, which can subsequently have a negative effect on production and fracing programs and secondly, in reducing cost and risks surrounding shale gas exploration.

Brazil, the Gulf of Mexico, and Africa enclose significant discoveries in the pre-salt, much of them exhibiting complex overburdens. As a result, seismic technologies are of importance in drilling and to cover large areas. In Brazil, for example, the focus is now on the plays of the pre-salt in the hydrographic basins of Santos, Campos, and Espiritu Santo, covering over 800 km. Capture of high quality seismic images of pre salt prospects becomes exigent because the drilling costs in deep waters per well reach $(US)100 million. The discussion covers gravimetric gradiometry and the pre-salt; solution to the problem of the structure of the salt in the K-2 field; and data analysis. Gravity Gradiometry Imaging (GGI) maps small variations in density in underlying rocks. The K-2 field is in Block 562 of Green Canyon, in the deep waters of the Gulf of Mexico, 290 km south of New Orleans. It has a solid body of salt over 3,048 m thick. The GGI contributes to the outlining of the salt structure by obtaining images using the algorithm of migration.

Houghton P.,ARKeX
Hart's E and P | Year: 2011

Gravity gradiometry imaging (GGI), a technique with an ability to qualify vast regions quickly, accurately, and cost-effectively and optimize the design of future seismic surveys, is proving particularly attractive to today's independents oil and gas companies. Forent Energy Ltd., a Calgary-based oil and natural gas producer, has used GGI as a cost-effective method to image the subsurface of its Nova Scotia prospect area, leading to the more efficient placement of its 2-D seismic lines and also having minimal landowner impact. Tower Resources, a UK-based independent oil and gas exploration company, has been using GGI to improve structural definition within a proposed license area in northwest Uganda along the East African Rift System. In Tower's case, the GGI data are helping to improve the planning for a 91- to 122-mile 2-D seismic program, to take place in the next few months - by mapping structures in the deeper part of the basin to improve structural definition and assist in identifying a better developed reservoir section.

Rippington S.J.,ARKeX | Mazur S.,ARKeX | Anderson C.,ARKeX
76th European Association of Geoscientists and Engineers Conference and Exhibition 2014: Experience the Energy - Incorporating SPE EUROPEC 2014 | Year: 2014

Thick basalts are a major challenge for conventional seismic imaging on the Faroe-Shetland Margin. However, gravity and magnetic data record variations in the density and susceptibility of the entire crust. Consequently, the thick basalt piles that are shallow in the section do not hinder the ability to detect deeper features. The principal aim of this study was to investigate the crustal architecture of the Faroe-Shetland Margin, using a combination of high resolution gravity and magnetic data, seismic data and horizons and published information. By combining seismic data with gravity and magnetic data, the strengths of all three data types were exploited; seismic data were used to constrain the shallow geology so that the deeper structure of the margin could be investigated by gravity and magnetic modelling. A network of 2D gravity and magnetic models based on seismic lines provided by PGS, are used to determine the configuration of the basement beneath the Faroe-Shetland Margin. Greater insight into the depth of basins and the timing of rifting is gained, and a model of asymmetric simple shear is proposed for the area.

The interpolative power of FTG measurements is playing a key role in helping oil and gas explorations achieve a better representation of the anomaly field and a clearer picture of the subsurface. Applications range from frontier exploration to augmenting existing data sets such as 2D seismic to produce more accurate 3D models of the geology. By measuring all the components of the gravity gradient tensor, the ability to interpolate or predict the variation of the field in between the survey lines can be greatly improved. This gives rise to an increased effective resolution of an airborne survey which is significantly better than that suggested by the line spacing. The key to the improvement is through joint processing where the information from the tensor components is combined to give a more complete picture of the gravity field. Our preferred method of achieving this is through an equivalent source density inversion which can accommodate any number of components measured at arbitrary locations. The resulting density distribution can subsequently be used to predict (Figure presented) components of the gravity and gravity gradient fields in between the original measurement locations by means of forward calculations. In the field example, it was shown that collecting full tensor data along flight lines with 5 km spacing could be processed to yield a data set with a useable average radial resolution down to 2 km wavelengths. This Figure is much less than the Nyquist limit of 9 km suggested by the line spacing (Pedersen and Rasmussen, 1990). The increased level of effective resolution was sufficient to image the signal from salt structures that had a width of approximately 2.5 km. The signal from the salt structures had a large amplitude and therefore provided ample signal to noise ratio even when only the data along the widely spaced survey lines were used. In other cases, the target signals can be much smaller and the line spacing is limited by the need to accurately measure the anomalous field in the presence of noise. This could be referred to as a detectability requirement and drives the line spacing down to produce a useable resolution that has sufficient bandwidth where the signal is above the noise. The enhanced effective spatial resolution resulting from multiple tensor measurements will then be superfluous as a high sampling resolution will already be provided by the survey pattern. In these cases, measuring a single component with a greater signal to noise ratio would be more beneficial. When signal to noise ratio is not the limiting factor, FTG measurements can be used to increase the line spacing without sacrificing resolution and therefore ultimately reduce the cost of a survey. © 2012 EAGE.

Barraud J.,ARKeX | Assouline F.,Maurel et Prom MEP | Dyer N.,ARKeX | Watson J.,ARKeX
72nd European Association of Geoscientists and Engineers Conference and Exhibition Incorporating SPE EUROPEC 2010, Workshops | Year: 2010

FTG data were acquired over an onshore Gabon Block. The objectives of this survey were to delineate accurately the salt structures; to derive a base salt structure map and to map basement structures associated with rifting. The survey area is situated along a clear trend of oil & gas subsalt fields that runs roughly north-south from offshore fields in south Gabon to the Lambaréné horst in the north. As a proven reservoir rock, the primary objective is the Gamba Sandstone. It is overlain by the Gamba Formation Vembo Shale which in turn is overlain by the Ezanga Salt formation. The trap is formed by reactivation of Cocobeach Formation faults due to the sediment load of the overlying Gamba and Ezanga Formations and the Madiela Group limestone and dolomite. The Ezanga salt also provides the top seal. Prior to the FTG survey three wells were drilled to target the Cretaceous Gamba sandstones. Although one of the wells appeared to be gas-bearing, the other two wells were dry. In addition, the significant discrepancies between predicted and actual depths demonstrated the need for new and independent geophysical data that would shed a new light on the "salt problem". Acquisition of seismic data in this area is difficult given the environment, described as coastal, marshy, tree covered areas with small rolling hills. Processing and depth migrating the 2D seismic data is also difficult for several reasons, including a thick and variable weathering layer, 3D nature of salt structure and uncertainties in velocities. These problems resulted in misinterpretations of the seismic data and significant errors on the predicted depths of formations tops. The excellent quality of the FTG data and the large density contrast between salt and the surrounding sediments (clastics and carbonates) ensured that the workflows employed returned a successful result. Compared to traditional land gravity techniques, airborne FTG data has the advantage of offering fast and complete horizontal coverage, and providing the high resolution and bandwidth that is necessary to image the shallow salt structures with confidence. The workflow involved a back-stripping approach in which the model is constructed from top to bottom. Independent data (seismic and magnetic data) was also used to constrain the results. Following on from this an area of interest was defined to focus the interpretation effort on the most promising targets. In this core area, eleven seismic lines were selected for reprocessing and depth migration. The new PSDM seismic data were then used as constraints, together with well data, to build a model in 3D. Manual 3D forward modelling was used to build detailed surfaces with a maximum control on the geometry.

Protacio J.A.,ARKeX | Watson J.,ARKeX | Van Kleef F.,Dubai Supply Authority DUSUP | Jackson D.,ARKeX
72nd European Association of Geoscientists and Engineers Conference and Exhibition Incorporating SPE EUROPEC 2010, Workshops | Year: 2010

The east of the Emirate of Dubai is dominated by the geologically complex western thrust front of the northern Omani Mountains. This deformation front is the boundary between the western foredeep basin and the eastern Omani fold-and-thrust belt. Complex geology makes conventional exploration challenging. The reservoir (Thamama Group) structures are thrusted anticlines with the overlying Tertiary units showing large-scale thrusting as well. The Lower Cretaceous Thamama Group limestone is one of the main hydrocarbon reservoirs in the Middle East. It forms a major hydrocarbon-producing reservoir in the U.A.E., Iraq, Bahrain and Oman and has a high hydrocarbon potential in southeast Iraq, offshore Oman and offshore northeast Saudi Arabia. Due to the significant density contrast between the reservoir and the overlying sediments, Margham Dubai Establishment commissioned an airborne gravity gradiometry (GG) survey to improve the confidence in top reservoir location and to aid in ongoing exploration activity. GG, magnetic and LiDAR data were acquired and used in an integrated interpretation with existing seismic and well data. The integration of these data allowed a better understanding of the thrust linkages at different levels, and a better insight into the interaction of thrusts, backthrusts, detachment levels, imbricated zones, and lateral ramps. The survey is designed around the airborne GG technology known as the Full Tensor Gravity Gradiometer (FTG). GG measures the rate of change of the Earth's gravitational field while conventional gravity (CG) measures the vertical acceleration. Acquired from an aircraft, GG has a strategic advantage over CG due to the resolution limitation imposed by the Differential Global Positioning System (DGPS). The DGPS limits airborne gravity resolution to > ∼4000m wavelengths while GG can resolve wavelengths of > ∼300m. The shorter wavelengths are crucial to accurately model the geology above the reservoir. In complex geology, multiple lithological units contribute to the GG signal. To map the reservoir, it is vital to isolate its response from the overlying geology. The high resolution GG data facilitated an accurate investigation of the 3D Shallow Earth Model (SEM). Modelling of seismic sections constrained by GG and magnetic data exploits their complementary nature. Seismic data respond well to horizontal discontinuities while potential field data respond to vertical discontinuities. This produces a geologically realistic SEM. Forward calculation of the GG signal from the 3D SEM was performed and then subtracted from the observed signal. The SEM corrected data were then used to interpret the Thamama reservoir structure.

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