Model Building, Depth Imaging and Drilling Campaign Case study: MP73 field, Gulf of Mexico Shelf

Ahmed Ammar and Ross Saunders, Energy XXI

Chuck Henry, Frostwood Energy

Tim Wilkinson, Talos Energy

Mike Frismanis, Mike Bradshaw, Jeff Codd and David Kessler, SeismicCity Inc.

MP73 Field was discovered in 1974 by Mobil Oil Company. Based on 2-D seismic data and well control, a salt model was developed. The model consisted of a salt dome with steeply dipping flanks and hydrocarbon bearing sands around the dome. In 1992 a 3-D dataset was acquired over the field. The 3-D dataset did not add a clear image of the salt dome, and therefore the historical model did not change. In 2007 Energy XXI acquired the field and found discrepancies between well data and seismic data. In order to resolve these discrepancies, Energy XXI decided to use depth imaging using the existing seismic data. SeismicCity took the challenge to construct an anisotropic model and apply depth imaging with the objective to clarify the image and interpretation around the salt dome.

Because of the lack of salt flank imaging, model building and depth imaging was done using a unique model building technique called the “salt expansion technique”. During the application of this process, we discovered that primary reflection seismic events were imaged inside the historical boundaries of the salt body. In order to preserve these seismic reflections an alternative salt model was created consisting not of a single dome type salt body, but several smaller detached salt bodies with sedimentary layers between the salt bodies. This seismic processing observation lead to a dramatic change of the historical salt model. In 2009, 35 years after the discovery of MP73 Field, a new salt model, consisting of several smaller and detached salt bodies was developed. This new model replaced the historical continuous diapir shape salt dome, leading to a new and optimistic interpretation of the producing sands.

Incorporating additional geological considerations and gathering salt markers from older drilled wells in the area of the producing field, the salt model was further modified during 2010. In the resulting model modification, the smaller salt bodies were connected through a series of thin salt welds, leaving a sedimentary cavity inside the salt body (as shown in the illustration above). The interpretation of the anisotropic depth migrated data using the new anisotropic model and salt body was completed in 2011 and resulted with two new drilling locations and well plans. The first well, named Ashton was drilled in 2011 targeting a series of seismic amplitudes located inside the historical salt body and outside the new model. The well encountered multiple hydrocarbon sands and was completed in 2011. The second well, named Onyx was drilled in 2011 into a series of sand layers that were not imaged on any previous prestack time migrated data. Onyx initial production rate reached 5,500 BO/D, which made it the highest producing oil well on the GOM shelf at the time. The drilling success of Ashton and Onyx wells resulted in the identification of several other new prospects located ‘inside’ the boundaries of the salt dome model.

With the application of advanced model building techniques, the construction of more accurate anisotropic models, and by incorporating all available subsurface information, legacy salt models can be updated and improved. Successful drilling that was executed using the new PSDM data resulted in significant new finds of additional oil and gas reserves in MP 73 field, validating the viability of the model building and depth imaging techniques used. We expect that similar success can be applied to other salt dome related prospects both offshore and onshore.

The authors wish to thank Energy XXI for the permission to present this work and drilling results.