|The use and integration of gravity gradiometry into the exploration workflow – Examples from Africa, Middle East, and the Gulf of Mexico|
Gravity gradiometry can provide valuable additional data to incorporate into the exploration workflow. Once considered a niche technology, gravity gradiometry imaging (GGI) is today becoming a significant line item in exploration budgets. Although 2D and 3D seismic continue to be the technology of choice in frontier exploration – both onshore and offshore, the deeper and more complex geological settings, the challenging and environmentally sensitive conditions, and the continued need to explore large areas while controlling costs have focused attention toward additional technologies such as GGI. Airborne and marine GGI surveys offer non-invasive passive technology, and allow the exploration of vast regions quickly, accurately, and efficiently. By flying above the terrain, large areas can be cost-effectively surveyed, and areas can be pre-screened for additional spend with no adverse environmental impact. The data obtained can be an important complement to existing traditional seismic, and allow efficient screening of areas to focus future spend on high-graded areas. A series of examples from the Middle East, Africa and the Gulf of Mexico are discussed to show how GGI has been incorporated into the exploration workflow.
|MIDDLE EAST EXAMPLE|
The study area is located on the western deformation front of the northern Oman Mountains in Dubai. Thrusting along the front of the belt trends in a north-south orientation. Complex geology makes conventional exploration challenging, because the seismic response is poor over a significant area. Dubai Supply Authority commissioned an airborne gravity gradiometry survey to improve the confidence in top reservoir location and to aid in ongoing exploration activity. Following data acquisition and processing a series of 2D seismic lines were iteratively re-interpreted by integrating acquired gravity gradiometry data with existing seismic interpretations, and well data. Essentially interpretation was driven by the need to match both the density profiles (guided by gravity response) and structurally balanced cross-sections. Figure 1 shows a 3D model from the Middle East Study incorporating the integration of all the component datsets. (after Protacio et al, 2010)
|WEST AFRICA EXAMPLES|
The first example is from onshore West Africa area where access was difficult, as exemplified by Figure 2. The hydrocarbon play is successful where structural closure occurs at the base salt interface. However, interpretation of the 2D seismic dataset was subject to uncertainties arising from a thick and variable weathering layer, velocity uncertainties, and the complex 3D architecture of the salt bodies.
In order to image the deepest structures it is necessary to build a shallow earth model and subtract its contribution from the data. This is termed “backstripping” and requires some independent data that can be used as constraints to guide the process. (Barraud et al, 2010)
Figure 3 : Combining interpretation of a single regional seismic line with regional gravity gradiometry data.
Figure 4 : Modelling of Interpreted Seismic Line to match observed and calculated gravity and gravity gradiometry response.
|GULF OF MEXICO EXAMPLE|
This is another example where gravity gradiometry provides a useful supplementary dataset to a grid of sparce regional seismic lines, and allows salt architecture and fault linkage to be better interpreted. Having established the causal features of the “highs and lows” in the gradiometry signal by iterative gravity modelling of interpreted seismic lines, fault linkages and salt geometries can be better constrained. Figure 5 demonstrates how 2 widely spaced seismic lines, without the infil capcity offered by the gravity gradiometry image, would be interpreted showing 2 faults which occur on both sections. The gravity gradiometry image supports the direct linkage of only one of the faults, with the gravity gradiometry suggesting that the second fault is offset by a transfer fault, or are 2 different faults ending at offset fault tip lines.
John Alfred Protacio, Jonathan Watson, Frank Van Kleef, David Jackson, 2010 : The Value of Integration of Gravity Gradiometry with Seismic and Well Data – an example from a frontal thrust zone of the UAE-Oman Fold Belt. 72nd EAGE Conference & Exhibition incorporating SPE EUROPEC 2010
Joseph Barraud, Frederic Assouline, Neil Dyer, Jon Watson, 2010 : Interpretation of Gravity Gradiometry data and Integration with PSDM Workflow – Imaging Sub-salt structures in Gabon. 72nd EAGE Conference & Exhibition incorporating SPE EUROPEC 2010