Deepwater GOM Imaging in OBM Reveals Paleotransport Direction of Turbidite Sands

Challenging imaging environment: OBM, high pressure, and low formation resistivity

Imaging deepwater depositional systems in the Gulf of Mexico with conventional imaging tools, even those that can operate in OBM, provides only limited information. Other than supporting structural analysis in tectonically disturbed sections and simple differentiation of sand and shale, the resolution of these images is insufficient to identify most internal features or sedimentary structures that are evidence of the depositional mechanism and orientation.

In addition to these inherent imaging challenges in a well accessing a deepwater slope system, a 9 7/8-in bit had been used, resulting a hole size larger than the 8-in hole coverage that imagers are designed for.

High-resolution imaging in oil-base mud

Quanta Geo photorealistic reservoir geology service is newly engineered electronically and mechanically to simplify the physics of the measurement to provide full-coverage, high-resolution microresistivity images and dip data in OBM that faithfully represent the formation geology. The innovative sonde design features 192 pad-mounted microelectrodes that are smaller than those used on conventional imagers to deliver excellent spatial resolution of 0.24 in × 0.13 in [6 mm × 3 mm] in the vertical and horizontal dimensions, respectively. Customized processing and interpretation apps and workflows in the Techlog wellbore software program enhance borehole image processing to render highly representative images and analysis.

Because Quanta Geo service independently applies each pad to the borehole wall instead of also using the pad arms to center the tool, inconsistent imager pad application is minimized in inclined wells and boreholes with wall imperfections. The dual-arm design makes it possible to image during the tool's descent, which minimizes the frequency and severity of stick-and-slip events and their effects on image quality.

Clearly imaged sands and their depositional trends

Quanta Geo service was run in the well in combination with the Sonic Scanner acoustic scanning platform. Images were acquired in both the down and up passes, with better quality achieved by the downlog images because less stickance occurred. Because circumferential coverage in the larger hole was approximately 80%, Discovery 360 multipoint statistics (MPS) processing was performed in the Techlog wellbore software program. By honoring the trends and textures visible in the original data, this method eliminates the between-pad gaps to produce a realistic full-coverage image.

The initial quicklook interpretation greatly reduced uncertainty in the identification of sand bodies and their structure. Quanta Geo service clearly imaged unconformities and faults along with numerous natural and drilling-induced fractures. The latter occur on the northeast and southwest sides of the wellbore as abundant en echelon subvertical features indicating the current maximum horizontal stress orientation.

In addition to channel sands and their related depositional facies, lobed sheet sands are readily identifiable in the high-resolution images. These coarse-grained, highly parallel beds with numerous scours are typically difficult to orient from dip data alone, especially where the sands were deposited flat on the basin floor. However, the clarity of Quanta Geo service's images makes it possible to easily distinguish the preferred orientation of the imbricated shale clasts, indicating the paleotransport direction.

The images also made it possible to differentiate slumped deposits at scales ranging from centimeters to tens of meters. The abrupt changes in dip and other internal features of the slumped sands would not be discernable on conventional images. The alignment of the slump axes with the paleoslope strike gives further context to the overall direction of the depositional system. However, the slumped nature of the sands means that their true stratigraphic thickness is less than half of their apparent thickness and also prompts reconsideration of their classification as a component of the net pay pending further investigation of their connectivity.