The K-2 Field is located in the deepwater Gulf of Mexico, in Green Canyon Block 562 at a water depth of 3,900 ft, approximately 180 miles south of New Orleans. This subsalt field was discovered in September 1999 by the Conoco GC-562 No. 1 well. Anadarko gained entry into the K-2 unit in October 2001 (Figure 1). Subsequent drilling encountered over 339 ft of pay in August 2002, below salt. Following up on this success, Anadarko discovered the K-2 North Field in the adjacent block, Green Canyon 518, in 2003. In May 2005, production from K-2 began from the first well, producing at 12,000 boepd through an 8-mile flow-line tie-back to the Anadarko-operated Marco Polo TLP in Green Canyon Block 608.

Prestack depth migration has played a vital role in exploring K-2, providing excellent definition of the salt body and of the underlying sedimentary section in areas where the salt has a tabular morphology (Figure 2). In these areas, we can image the base of salt (~20,000 ft) and also the underlying sedimentary section

Figure 1. Location map of the K-2 Field, Green Canyon 562, offshore Gulf of Mexico.

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down to a depth greater than 40,000 ft. This represents an outstanding subsalt imaging achievement.

While prestack depth migration has yielded some excellent imaging results, the technique is not without problems. The eastern flank of the K-2 salt has a steep face (~60°) whereas the salt top displays a prominent structural peak immediately west of this flank. Prestack depth migration using a Kirchhoff algorithm fails to image the base of salt and the subsalt section in this area, resulting in uncertainty in determining the updip extent of the K-2 Field and in estimating field size. This also impacts the appraisal drilling program: If the field is believed to extend farther updip then this provides additional drilling targets. Conversely, if we can determine that the field is more limited in the updip direction we may avoid drilling some costly deepwater wells.

To address this problem we employed two independent, complementary geophysical techniques: full tensor gravity gradiometry (FTG) and wave equation prestack depth migration. FTG measures the gradient of the earth’s gravitational field and provides a superior measurement compared with that obtained with conventional gravity surveys.

• Effects induced by platform motion are suppressed because FTG measures differences in the gravitational field at the various sensors, all mounted on a common platform, and
• FTG is inherently more sensitive to the distance of a mass anomaly.

Figure 2. Kirchhoff prestack depth migration profile through the K-2 salt body provides excellent image definition beneath the tabular section of the salt body. However, the seismic image underneath the peak of the salt and the eastern salt flank is very poor and interpretation of the base of salt and subsalt section is highly uncertain in this area. Seismic data are shown courtesy of WesternGeco.

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To analyze the FTG data a 3-D earth model was first constructed using all available geological and geophysical information, including the base of salt horizon where this could be interpreted with confidence. We then performed an inversion of the FTG data to determine the configuration of the base of salt in the “noimage” zone which would provide the best fit to the data. In this way we formulate the problem as a very specific question, incorporating all the known subsurface information, while focusing the inversion on that part of the earth model which we really need to determine.

In seismic imaging, wave equation prestack depth migration addresses two of the critical issues that can arise with Kirchhoff migration:

• Multipath effects, and
• Migration artifacts that mimic the Kirchhoff operator.

Using a wave equation approach, we reimaged the 3D seismic data which had previously been depth migrated with a Kirchhoff algorithm. Both migrations used a similar, but not identical, velocity field. By avoiding the Kirchhoff-induced migration artifacts, the wave equation depth migration succeeded in imaging the base of salt in what had previously been a no-image zone. Both FTG and wave equation prestack depth migration proved effective in locating the base of salt in the area updip of the field, where Kirchhoff migration had previously failed. Both indicate that the salt has a keel structure, effectively delineating the updip extent of the field. Because the two methods measure very different properties of the subsurface, the agreement between the results provides a high level of confidence in the final interpretation.

As a result of this two-pronged approach we can confidently identify the updip extent of the pay sands and also resolve the need for further delineation in this part of the field.

For more information read: “Resolving the K-2 Salt Structure in the Gulf of Mexico; an Integrated Approach using Prestack Depth Imaging and Full Tensor Gravity Gradiometry,” John O’Brien, Mark A. Davies, Arnold Rodriguez, David Sixta and Phillip Houghton, The Leading Edge, Vol. 24, p. 404, April 2005.

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