The north-central Gulf of Mexico area received rapid deposition of a basin-floor fan system consisting of interbedded muds, silts, and sandy turbidite deposits during the Pleistocene. Overpressure occurs at shallow depths when burial rates exceed the dewatering rates of sediment pore fluids. Two stratigraphic sequences in the region contain significant overpressure with elevated shallow-water flow risk within these units. We have used publicly available seismic and well data to identify the geomorphology and overpressure variation of these units. The previously described “Blue Unit” and its lateral extent, thickness, depth below sea level (BSL), and overpressure gradient have been revised. The Blue Unit extends from the northern portion of the Mississippi Canyon (MC) protraction area to as far south as the Atwater Valley (AT) protraction area. For the first time, the Green Unit’s lateral extent, thickness, depth BSL, and pore pressure are defined. The “Green Unit” was found to extend further south than the Blue Unit into the AT protraction area and further east in the Desoto Canyon protraction area. The tops of both units are highly incised by postdepositional erosional systems, whereas the base of each unit is well preserved. The top of the Blue Unit below the mud line (BML) varies from <70 m (<230 ft) in the north to as deep as 701 m (2300 ft) in the south, whereas the top of the Green Unit is as shallow as 300 m (985 ft) in the north to 901 m (2956 ft) in the south. Overpressure in the MC area has been reported just BML. The pore pressure gradient ranges from 0.47 to 0.52 psi/ft at the base of the Blue Unit and increases to 0.60 psi/ft within the Green Unit.

Deepwater basins around the world contain shallow sequences of overpressured, sand-prone sediments that can result in Shallow Water Flow (SWF) events. These events have frequently resulted in wellbore instability, increased man-hour exposure to potential HSSE risks as well as non-productive time (NPT) and have sometimes been the cause of the loss of the well while drilling the shallow (riserless) section for oil and gas exploration or development projects. Methods previously established to classify the magnitude of a SWF event have been used with partial success to identify the onset of a SWF event. The need existed to develop a system enabling early prediction, detection and mitigation of SWF events while drilling. Real-time monitoring of the riserless section of a marine well for SWF requires a system using a plurality of data feeds defined here as the SYSTEM. The data feeds include seismic data, remotely operated vehicle (ROV) video, and surface and downhole logging measurements. A SWF surveillance methodology, herein defined as a discharge category model (DCM), has been developed for early detection of a SWF event, prior to the onset of wellbore instability. The DCM focuses on baseline discharge categories (ranging from no flow to minor flow) prior to wellbore instability and taking into account the u-tube effects. Real-time monitoring of data feeds coupled with the DCM in the context of the SYSTEM has helped to mitigate SWF events. There have been no wells lost due to SWF events that have utilized the DCM in the context of the SYSTEM in various basins throughout the world. A total of 154 wells have been monitored globally using the DCM with 46 SWF events detected and mitigated before reaching a severity level that might compromise the well integrity from 2012 to 2019.