The continental slope of the Deepwater Gulf of Mexico is characterized by complex seafloor topography caused in part by shallow salt deformation. Locally, faults provide pathways for hydrocarbon migration through the suprasalt sediments to the seabed. Seafloor hydrocarbon seep sites can provide habitat for unique biologic communities. Seafloor features and associated fauna present significant challenges to site selection for wells, moorings, subsea equipment, and flowlines associated with petroleum field developments. This paper documents a case study of how these challenges were addressed during development of the Shenzi Field through a series of site investigations beginning during the early planning phases of the project. The Shenzi case study presents a proactive approach for addressing deepwater marine biology during field development. Integrated site investigation yielded a successful development plan from both engineering design and regulatory permitting perspectives consistent with the Operator's charter commitment to sustainable development of petroleum resources in an environmentally responsible manner. Introduction Project Background. The Shenzi Field was discovered in 2002 by the GC 654 #1 well. This discovery well was followed by five appraisal wells that confirmed commerciality of the field. The field development concept for the initial phase of production consists of multiple subsea wells around three drill centers tied back to a dedicated Tension Leg Platform (TLP) processing facility via production flowlines, gas lift/injection flowlines, and umbilicals. Third party oil and gas export pipelines will provide transportation back to market. Geologic Setting. The Shenzi Field lies approximately 104 nautical miles south of Fourchon, Louisiana (Figure 1a), on the continental slope of the northern Gulf of Mexico (GOM) approximately 5 miles north of the Sigsbee Escarpment in Green Canyon Blocks 609, 610, 653, and 654. Water depths in Shenzi Field range from approximately 4,150 feet in the northeast quadrant of Block 653 to 4,480 feet in the southeast corner of Block 654. Site Investigations Desktop Study. A shallow hazards desktop study was conducted during the Concept Phase of the Shenzi Project to identify seafloor and shallow geologic features pertinent to field development. This study was based principally on a spectrally enhanced 3-D seismic volume, existing shallow hazard reports submitted in support of Exploration Plans (EP) for well locations, and other readily available data. Numerous fault scarps were evident from seafloor renderings derived from the Shenzi 3-D seismic volume. A zone of seafloor faults was apparent above a shallow salt ridge that extends from the northeast quadrant of Block 653 across the southern boundary of Block 610. An area of hummocky topography was apparent adjacent to fault scarps near the boundary between Blocks 653 and 654. A few isolated areas indicative of possible hydrocarbon seeps were identified from a seafloor amplitude rendering generated from the 3-D data.
This paper describes a methodology for integrating Autonomous Underwater Vehicle (AUV) geophysical data with Remotely Operated Vehicle (ROV) video in order to predict environmentally sensitive deep-water chemosynthetic habitat. The methodology was developed using AUV geophysical data and ROV video acquired in BP's Puma appraisal area, in the U.S. Gulf of Mexico. The results from the AUV survey indicated dozens of hydrocarbon seepage sites with the potential to support high-density chemosynthetic communities. These communities are protected by U.S. environmental regulations and, thus, could significantly impact field development plans. The ROV video revealed a few high-density chemosynthetic communities and cleared certain critical areas for potential development, but it still left numerous possible hydrocarbon seepage sites uncleared. It would have been a costly undertaking to inspect each site by ROV when they are so numerous and widely separated. Therefore, an alternative means of clearing as many of these sites as possible was needed.Upon integration of the ROV video with the AUV data, the following important observations were made: (1) Densely populated chemosynthetic communities in the Puma area only occur at hydrocarbon seepage sites with relatively large (several 10's of cm or larger) carbonate outcrops; (2) there is a strong correlation between this carbonate substrate and highintensity backscatter in AUV multibeam sonar data; and (3), carbonate outcrops are distinctly visible in the AUV sidescan sonar data as anomalous seafloor mounds. These observations suggested that ROV observations could be used to calibrate the AUV geophysical data. The calibrated AUV data could then be used to predict what other sites away from the ROV survey track (without direct visual inspection) have the most potential for supporting high-density chemosynthetic communities.Based on the calibration results, inferred hydrocarbon seepage sites that exceed a specific seafloor backscatter intensity level, and contain anomalous seafloor mounds interpreted as authigenic carbonate, are the areas with the most potential for supporting high-density chemosynthetic communities at Puma. Other inferred hydrocarbon seepage sites that do not meet these criteria have a low potential for supporting high-density chemosynthetic communities, and are less likely to be subject to environmental regulations. The methodology utilized here helped prioritize areas of environmental concern at Puma, incase additonal ROV investigation is required. Additionaly, this method may be applicable in other settings where suitable AUV data and sufficient "ground-truth" from ROV video exist.
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