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Realistic Sequence Stratigraphy in the Deepwater Gulf of Mexico Using High Resolution Borehole Images


Over the years, geoscientists have interpreted the sequence stratigraphy of fields, or basins, or regions using seismic data. In the deepwater Gulf of Mexico (GOM), this science may be possible in some places, but in many regions, where the reservoirs are subsalt, the geoscientist is not afforded this luxury. Workers often use the Gamma Ray log from wells for sequence stratigraphic interpretation. Even though, the Gamma Ray log is quite useful in many places, it has its limits. One geological phenomenon that defies the gamma ray log is mass transport. A gamma ray log cannot differentiate between a quiet environment hemi-pelagic shale and an ultra-high energy mass transport deposit. For this, a much more advanced technology is needed.

Most wells in the deepwater GOM are drilled using synthetic oil-base muds (SOBMs). A side effect is that this prevents the use of the decades old technology of micro-electric borehole images that have been widely used in wells drilled with water-base muds. Recent advances in technology have enabled acquisition of high quality and high-resolution borehole images even in the SOBMs. Among many uses of these images, one important application from the deepwater GOM is being able to distinguish between hemi-pelagic shales and mass transport deposited shales. In addition to the obvious relevance, this distinction has an impact on sequence stratigraphic interpretations, too. An interval that might look shaly on the Gamma Ray log would likely be interpreted as a highstand deposit or possibly a deposit in a transgressive systems tract. Presence of a sand right above such a deposit would likely create some confusion in a sequence stratigraphic context. Once the borehole image is brought into the discussion, it might become evident that some of those high Gamma Ray readings are, in actuality, of a mass transport deposit. Now the presence of the sand above this shale fits as part of the lowstand after a falling stage systems tract in a regressing sea.

This paper discusses using the new high-resolution borehole images for sequence stratigraphy in the deepwater GOM as especially relevant to the subsalt plays where seismic data clarity is limited. With this technology, geoscientists and sequence stratigraphers are now empowered with a strong tool to make more realistic field and regional sequence stratigraphic interpretations, and ultimately, develop a more accurate field and reservoir model for the asset manager.