Abstract

Geoscientists typically interpret the sequence stratigraphy of fields, or basins, or regions using seismic data. In the deepwater Gulf of Mexico (GOM), in many subsalt plays, the geoscientist is not afforded this luxury. Workers often use the gamma ray log for sequence stratigraphy. Even though, the gamma ray log is quite useful in many places, it cannot differentiate between a quiet environment hemi-pelagic shale and an ultra-high energy mass transport deposit (MTD). For this, much more advanced technology is needed.

Deepwater GOM wells are drilled using synthetic oil-base muds (SOBMs). Recent advances in technology have enabled acquisition of highresolution borehole images even in the SOBMs. An important observation from the images, in the deepwater GOM, is the distinction between hemipelagic shales and MTD shales. From a sequence stratigraphy perspective, a shaly looking interval 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. 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 an MTD. 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.