Full Tensor Gravity Gradiometry (FTG) is aiding prospect evaluation in a formerly disputed zone of the Barents Sea, adjacent to the Norwegian-Russian border. Dr. Stephen Rippington, Dr. Neil Dyer, and Chris Anderson explain.
Fig. 1: Survey area.
The Norwegian Petroleum Directorate (NPD) is currently offering acreage for license in the formerly disputed zone of the Barents Sea, adjacent to the Norwegian-Russian border (Figure 1).
The NPD has provided comprehensive 2D seismic coverage of the area to assist in the evaluation of the geology and the nomination of the most interesting blocks. In addition, four 3D seismic surveys have recently been completed within the area.
ARKeX has also recently completed a multi-client Full Tensor Gravity Gradiometry (FTG) survey covering the same area.
Industry interest in the area has been very high, with over 30 companies reported to have subscribed to the 2D seismic database. The exploration potential of the Barents Sea in general has been recognized for decades and a variable record of drilling success and failure is testament to the complexity and consequent exploration risk. However, the former disputed zone has, until recently, not seen the same level of academic and industry interest given to the region as a whole, and is therefore still considered to be a new frontier.
FTG presents a relatively new way of measuring the Earth’s gravitational field and mapping subsurface structures based on varying density. Directly measuring the gravity gradient in addition to the usual measurement of acceleration due to gravity results in vastly improved data resolution.
Fig. 2: The cooperative Earth model. Broadband gravity, surface geology, seismic data and velocities and borehole data integrated into the construction of a common model that satisfies all the measurements. Images from ARKeX.
Gravity gradients have long been used as a derivative of a conventional measurement as a means of locating subsurface structure with greater precision. Measuring these quantities directly adds confidence, enabling familiar interpretational activities to be performed better while adding a range of novel techniques to be developed, which are compromised with conventional gravimetry measurement. Integration of gravimetry and gradiometry measurements develops the capability to deliver “broadband gravity measurement.” This is a valuable dataset, capturing the long wavelength components of the field that are driven by deep structure and the shorter wavelengths that result from mid-crustal and near surface structure.
Fundamental to the value of the broadband gravity measurement is the close relationship between density and seismic velocity. The two properties can be modeled in most circumstances with a common structure, allowing the two datasets to be interpreted cooperatively. This gives the gravity measurement direct relevance to the work of the seismic interpreter.
ARKeX deploys the Lockheed Martin Full Tensor Gravity Gradiometer, which not only delivers the vertical gravity gradient, but the horizontal components (Gxx and Gyy), as well as providing the full-tensor 3D measurement of the gravity field. The full tensor measurement adds resilience to the dataset, enabling the accurate construction not only of the gradient tensor but of a large range of special functions, which may be used to automate interpretive operations such as edge detection.
Interpreting FTG data from the SE Barents Sea
The Barents Shelf is a complex tectonic mosaic of cratons, platforms and basins, which amalgamated and deformed through a combination of compressional and extensional tectonic phases. The SE Barents survey area lies in a position where several important structural trends coalesce. Understanding the complex structural geology, at depth and in the shallow section, is crucial for understanding the regional tectonic framework, basin development and prospect-scale risks.
FTG data can be incorporated as part of a multi-physics approach to measure, map, integrate and interpret in context with seismic and other data. The combination of independent data provides additional constraints, which inform and improve the resulting Earth model. The ARKeX SE Barents FTG Survey was acquired in 2013 and 2014. The processed data became available in October 2014, after which a preliminary interpretation was produced to place the FTG data in a regional geological context and to demonstrate how the data can be integrated with other exploration datasets.
The area of interest contains elements of the Finnmark Platform, Tiddlybanken Basin, Fedynsky High, Nordkapp Basin and Bjarmeland Platform. The ARKeX FTG data enable a higher resolution of interpretation than has previously been possible. In particular, the morphology of salt, both in diapirs and grounded within the Permo-Carboniferous section, is defined in unprecedented detail.
Specific issues touched upon in the interpretation report are:
- The relative influence of Timanian, Caledonian and Uralian structural trends in different areas.
- Understanding the location and development of late Paleozoic basins.
- Investigating the location, extent, shape and volume of Permo-Carboniferous salt.
- The relationship between shallow deformation zones and the uplift, inversion and erosion of large thicknesses of Cretaceous-Paleogene overburden.
Fig. 3: (comprises 3 images, a, b, and c).
The data available for this project were interpreted in two ways:
- A map-based interpretation of faults, basins, structural highs and salt was constructed in ArcGIS, a geographic information system for working with maps and geographic information.
- Two 2D/2.5D density models were produced in the XField plugin for dGB’s OpendTect seismic interpretation platform (also available as a plugin for Petrel) using images of NPD seismic lines as a guide. XFIELD is a powerful geophysical modelling tool from ARKeX that enables explorationists to integrate and analyze potential field data alongside available seismic data.
The two interpretation methods were implemented in an iterative cycle, whereby one phase of map-based interpretation helps to inform the 2D/2.5 models, which then help to refine the next phase of map-based interpretation. Additional datasets (e.g. published maps and cross-sections) were loaded into ArcGIS to provide constraints on our interpretation.
Fig. 4: 2D/2.5D density model across the Nordkapp Basin, satisfying seismic, gravity and gradiometry data.
The following excerpts present some of the findings associated with the Nordkapp basin. The gravity data (Gz) show a single, major high-amplitude negative anomaly in the Nordkapp Basin, on which shorter wavelength anomalies are superimposed. However, the gradiometry data (Gzz) shows a cluster of high-amplitude negative anomalies. These high-amplitude lows correspond to individual salt diapirs, the tops of which have been imaged by the NPD seismic lines.
Figure 3 shows the gravity response over a small part of the Nordkapp basin and compares; a) satellite bouger gravity (Sandwell and Smith 2009), b) Survey Gravity (Gz), and c) Survey Gravity Gradient (Gzz). The gradiometry data contain significantly higher frequencies and provide a tool for delineating salt, but more importantly, windows through the salt into the Triassic and Jurassic succession. These windows may become targets for drilling ‘Pandora-type’ prospects, in which hydrocarbons are trapped in Triassic reservoirs against the salt flanks.
Figure 4 shows a modeled section based on an original seismic interpretation that was subsequently re-interpreted interactively so as to fit Gz and Gzz profiles plotted above. The profiles show the modeled response in red and the observed response in blue in each case. Note that the higher frequency signal of Gzz is not satisfied with this preliminary model.
The map-view gradiometry data can be used to identify smaller geological features and better define the edges of salt. Furthermore, by interactively fitting a seismic interpretation to an observed Gzz profile, the volume and shape of salt bodies can be better defined. Other features associated with more subtle density contrasts produce smaller amplitude, high frequency anomalies, as seen on the right side of the Gzz profile on Figure 3. These anomalies could be caused by seabed density variations related to the glacial history of the region, and/or shallow deformation zones resulting from the uplift, inversion and erosion of the Cretaceous-Paleogene overburden. Such deformation zones may generate small density variations in the Triassic-Jurassic succession.
This integrated seismic/gravity approach means that seismic interpreters can benefit from relevant high resolution information from gravity measurements in an interactive way. Interrogating the gravity dataset using the appropriate interactive tools (for example the XField plugin for Opendtect or Petrel) from the seismic interpreters’ working environment allows a rapid test of interpretational hypotheses, speeding the development of a comprehensive Earth model and increasing confidence This allows further investigation of features associated with sharp density contrast, which often limit seismic imaging.
The implicit coupling of geological insight, often significantly enhanced by the ability to visualize in map view the position and extent of structures interpreted on seismic sections, with geophysical rigor represents an improvement to the relevance of the interpreted Earth model as an active element of an exploration program.
Dr. Neil Dyer is chief technology officer at ARKeX. He has more than 15 years direct experience in the oil and gas sector in interpretation, QA, data processing and integrated ‘’Multi Physics’’ Earth modeling. Before joining ARKeX in 2006, Dyer held positions with HGS Limited and TGS-Nopec. Dyer has a first-class degree in geophysics and a PhD in environmental science (Volcanology) from Lancaster University.
Dr. Stephen Rippington is a senior geoscientist at ARKeX. Since joining ARKeX, Rippington has undertaken technical work on projects investigating the structural geology of the NE Greenland Margin, the SE Barents Sea and the Faroe-Shetland Basin. Before joining ARKeX, Rippington was as a field/structural geologist at CASP (formerly the Cambridge Arctic Shelf Programme). He has a PhD from the University of Leicester in the UK.
Chris Anderson is executive vice president for business development at ARKeX. He was previously director of sales at WesternGeco, and more recently VP sales and marketing at PGS.