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Collaborating Authors
Western Australia
Marine controlled-source electromagnetic of the Scarborough gas field — Part 3: Multicomponent 2D magnetotelluric/controlled-source electromagnetic inversions
Constable, Steven (Scripps Institution of Oceanography) | Orange, Arnold (Scripps Institution of Oceanography) | Myer, David (BlueGreen Geophysics)
ABSTRACT We carried out a multicomponent electromagnetic (EM) survey of the Scarborough gas field in 950 m water on the northwest Australian shelf. Magnetotelluric (MT) data, along with transmitter inline horizontal electric (Ey), vertical electric (Ez), and horizontal magnetic (Bx) field controlled-source electromagnetic (CSEM) data were collected. The Scarborough reservoir is a challenging EM target because it lies between a resistive overlying siltstone and a resistive basement. We carried out 2D inversions of various data combinations to determine how well they recover the expected geology. In particular, we examined the value of the vertical electric CSEM fields. Individual inversions of the Ey and Bx components generate almost identical models, suggesting that these two data sets do not carry independent data, although model studies suggest that this may not be the case in shallower water. Both models smear the siltstone, reservoir, and basement resistors together. The Ez-only inversion includes a resistor with a clear lateral extent at reservoir depths that is separated from basement, but when combined with other CSEM components, Ez provides only marginal improvements in resolution. Not surprisingly, an MT-only inversion is blind to the thin reservoir resistor but combined with CSEM data produces a clear separation of the reservoir from the basement. The combination of Ey, MT, and Ez also separates the siltstone horizon from the reservoir. The sensitivity of MT to horizontal conductivity makes it a powerful complement to the standard Ey CSEM data. The Ez CSEM component adds some value, but perhaps not commensurate with the logistical costs of data collection. The horizontal magnetic CSEM field appears to add little value at these water depths, but if simultaneous MT data are being collected, this component will be available at little cost.
- North America > United States (0.68)
- Oceania > Australia > Western Australia > North West Shelf (0.61)
- Oceania > Australia > Western Australia > North West Shelf > Carnarvon Basin > Exmouth Plateau > WA-1-R > Scarborough Field (0.99)
- Europe > Norway > North Sea > Central North Sea > Utsira High > PL 338C > Block 16/1 > Gemini Prospect > Hugin Formation (0.99)
- Oceania > Australia > Western Australia > Carnarvon Basin > Exmouth Basin (0.89)
- (2 more...)
ABSTRACT We explored the application of 2D inversion of marine controlled-source electromagnetic and marine magnetotelluric data to image an ambiguous target. The Scarborough gas reservoir off the west coast of Australia lies in close repose to a layer in the overburden of similar resistivity-thickness product and also is not far above the resistive basement, making it a difficult electric target. We found that the standard 2D smooth-inversion method yielded models that were unable to resolve this ambiguous structural configuration. We solved this problem by developing a two-step workflow, in which we first invert for a coarse background resistivity model (e.g., anisotropic layers), then invert for the minimum deviation from this background using a much finer model discretization. The main purpose of our two-stage workflow is to inject the knowledge into the inversion that the subsurface is composed of self-similar geologic domains. Though the resulting models did not resolve fine-scale structural details, they might still be used to map the overall extent and bulk qualities of a target in an otherwise confounding setting.
- Oceania > Australia > Western Australia > North West Shelf > Carnarvon Basin > Exmouth Plateau > WA-1-R > Scarborough Field (0.99)
- Africa > South Africa > Western Cape Province > Indian Ocean > Bredasdorp Basin > Block 9 > EM Field (0.99)
- Oceania > Australia > Western Australia > Carnarvon Basin > Exmouth Basin (0.89)
- Europe > Norway > Norwegian Sea > Vøring Basin (0.89)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.66)
Summary Uncertainties in marine controlled source electromagnetic (CSEM) data consist of two independent parts: measurement noise and position uncertainties. Measurement noise can be readily determined using stacking statistics in the Fourier domain. The uncertainties due to errors in position can be estimated using perturbation analysis given estimates of the uncertainties in transmitter-receiver geometries. However, the various geometric parameters are not independent (e.g. change in antenna dip affects antenna altitude, etc.) so how uncertainties derived from perturbation analysis can be combined to derive error-bars on CSEM data is not obvious. In this study, we use data from the 2009 survey of the Scarborough gas field to demonstrate that (a) a repeat tow may be used to quantify uncertainties from geometry, (b) perturbation analysis also yields a good estimate of data uncertainties as a function of range and frequency so long as the components are added arithmetically rather than in quadrature, and (c) lack of a complex error structure in inversion yields model results which are unrealistic and leads to "over-selling" of the capabilities of CSEM at any particular prospect.
Marine CSEM of the Scarborough gas field, Part 1: Experimental design and data uncertainty
Myer, David (University of California at San Diego, BlueGreen Geophysics, LLC) | Constable, Steven (University of California at San Diego) | Key, Kerry (University of California at San Diego) | Glinsky, Michael E. (CSIRO Earth Science and Resource Engineering, University of Western Australia) | Liu, Guimin (BHP Billiton)
ABSTRACT We describe the planning, processing, and uncertainty analysis for a marine CSEM survey of the Scarborough gas field off the northwest coast of Australia, consisting of 20 transmitter tow lines and 144 deployments positioned along a dense 2D profile and a complex 3D grid. The purpose of this survey was to collect a high-quality data set over a known hydrocarbon prospect and use it to further the development of CSEM as a hydrocarbon mapping tool. Recent improvements in navigation and processing techniques yielded high-quality frequency domain data. Data pseudosections exhibit a significant anomaly that is laterally confined within the known reservoir location. Perturbation analysis of the uncertainties in the transmitter parameters yielded predicted uncertainties in amplitude and phase of just a few percent at close ranges. These uncertainties may, however, be underestimated. We introduce a method for more accurately deriving uncertainties using a line of receivers towed twice in opposite directions. Comparing the residuals for each line yields a Gaussian distribution directly related to the aggregate uncertainty of the transmitter parameters. Constraints on systematic error in the transmitter antenna dip and inline range can be calculated by perturbation analysis. Uncertainties are not equal in amplitude and phase, suggesting that inversion of these data would be better suited in these components rather than in real and imaginary components. One-dimensional inversion showed that the reservoir and a confounding resistive layer above it cannot be separately resolved even when the roughness constraint is modified to allow for jumps in resistivity and prejudices are provided, indicating that this level of detail is beyond the single-site CSEM data. Further, when range-dependent error bars are used, the resolution decreases at a shallower depth than when a fixed-error level is used.
- Overview (0.66)
- Research Report (0.40)
- Oceania > New Zealand > South Pacific Ocean > Lau Basin (0.99)
- Oceania > Fiji > South Pacific Ocean > Lau Basin (0.99)
- Oceania > Australia > Western Australia > North West Shelf > Carnarvon Basin > Exmouth Plateau > WA-1-R > Scarborough Field (0.99)
- (2 more...)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
- Data Science & Engineering Analytics > Information Management and Systems (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Geologic modeling (0.93)
Summar In May-June of 2009, we carried out a magnetotelluric (MT) and controlled source electromagnetic (CSEM) survey over the Scarborough gas field on the Exmouth plateau off the northwest coast of Australia. At 144 receiver deployments, this is the largest academically collected CSEM dataset to date. The main purpose of this study is to provide a demonstration dataset over a well-studied area and to drive future development in EM methods by placing this data in the public domain. In this paper, we present first results of CSEM data processing and 1D inversion from phase 1 of the survey. Introduction The Exmouth plateau is a passive-margin between continental and oceanic crust left over from the break-up of Australia and India, and is surrounded on three sides by oceanic crust at abyssal depths. Since the Mesozoic era, the plateau has undergone a complex sequence of fracture, extension, uplift, truncation, and subsidence (Exon et al., 1982, Mutter and Larson, 1989, Lorenzo et al., 1991, Driscoll and Karner, 1998). The present plateau is covered by a number of mostly horizontal sedimentary layers of resistivities varying between 1 and 2 Om. The gas reservoir is a 20-40 m layer residing between 1900 and 2000 m depth with a moderate resistivity of 25 Om and is overlain by several thin layers of lower gas saturation with resistivities of 5-10 Om. An interesting feature, and one which presents a challenge for CSEM hydrocarbon exploration, is the presence at ~1500 m of the resistive Gearle siltstone formation with a thickness of ~100 m in the survey area and a resistivity of 3 Om (Veevers and Johnstone, 1974). The resistivity-thickness product of the Gearle is similar in magnitude to the reservoir, so we expect this confounding resistor to present a nice challenge for CSEM modeling and inversion methods. The CSEM method is primarily sensitive to resistive structures (Cox, 1981, Cox et al., 1986) and over the past decade has found increasing use in hydrocarbon exploration (Constable and Srnka, 2007). However, the field is relatively new, so academic quality codes for modeling and inversion are still being developed; e.g. (Li and Key, 2007, Key, 2009). Many of the supposedly more advanced inversion codes presented in publications and presentations over the past decade are held as the intellectual property of for-profit corporations and are therefore not open to inspection or operation by the general public. We believe that such private advancement deters the field as a whole. So our primary goal for this survey is to collect a high quality dataset which can be used to expand the CSEM method. It is our intention to release the data into the public domain in conjunction with inversion studies using academically available codes. We hope that the availability of a standard dataset with inversion results from open-source codes will provide a measure of quality by which the private codes can be evaluated and will push the development of the CSEM method as a whole back into the public domain.
- Oceania > Australia > Western Australia > North West Shelf > Carnarvon Basin > Exmouth Plateau > WA-1-R > Scarborough Field (0.99)
- Oceania > Australia > Western Australia > Carnarvon Basin (0.99)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.49)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (0.47)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Cross-well tomography (0.35)
Marine controlled-source electromagnetic (CSEM) surveying has been in commercial use for predrill reservoir appraisal and hydrocarbon exploration for . Although a recent decrease has occurred in the number of surveys and publications associated with this technique, the method has become firmly established as an important geophysical tool in the offshore environment. This is a consequence of two important aspects associated with the physics of the method: First, it is sensitive to high electrical resistivity, which, although not an unambiguous indicator of hydrocarbons, is an important property of economically viable reservoirs. Second, although the method lacks the resolution of seismic wave propagation, it has a much better intrinsic resolution than potential-field methods such as gravity and magnetic surveying, which until now have been the primary nonseismic data sets used in offshore exploration. Although by many measures marine CSEM is still in its infancy, the reliability and noise floors of the instrument systems have improved significantly over the last decade, and interpretation methodology has progressed from simple anomaly detection to 3D anisotropic inversion of multicomponent data using some of the world’s fastest supercomputers. Research directions presently include tackling the airwave problem in shallow water by applying time-domain methodology, continuous profiling tools, and the use of CSEM for reservoir monitoring during production.
- North America > United States (1.00)
- Europe (1.00)
- Geology > Rock Type (0.93)
- Geology > Geological Subdiscipline (0.93)
- Oceania > New Zealand > South Pacific Ocean > Lau Basin (0.99)
- Oceania > Fiji > South Pacific Ocean > Lau Basin (0.99)
- Oceania > Australia > Western Australia > North West Shelf > Carnarvon Basin > Exmouth Plateau > WA-1-R > Scarborough Field (0.99)
- (3 more...)