Geological and Biological Relationships in the Puma Appraisal Area: from Salt Diapirism to Chemosynthetic Communities

Abstract

Salt diapirism to very shallow depths below seabed in the deepwater Gulf of Mexico Puma appraisal area has extensively tilted, thinned and faulted supra-salt strata, and permitted the migration of asphalt, oil and gas to the seafloor and into the water column. Hydrocarbon migration through the shallow strata has resulted in the formation of extensive areas of authigenic carbonate at, and immediately below, the seabed interface. Active hydrocarbon seepage continues to feed chemosynthetic fauna across much of the Puma area. However, population densities are sparser than might be expected based on the extent of authigenic carbonate interpreted from geophysical data.The integration of surface seismic and autonomous underwater vehicle geophysical data, synthetic aperture radar imagery acquired via satellite, and extensive video acquired in two remotely operated vehicle surveys, has been used to delineate the extent of the above geological and biological relationships to support exploration and appraisal activities in the Puma area. In areas of benign seafloor such extensive site investigation work would not be required. However, the evaluation of areas with complex shallow geology and rugged seafloor that may host environmentally sensitive fauna over extensive areas will benefit from the integrated approach described below.Puma Area. The Puma prospect was successfully tested by the Green Canyon (GC) Block 823 #1 well in 2003 ( Figure 1). The discovery well was followed by two appraisal wells (GC821 #1 and GC866 #1) which finished drilling in 2007. Reservoir level field appraisal, geotechnical studies to quantify the identified shallow geohazards, and investigation of potential development options are underway. The appraisal area is located in the southeast portion of the Green Canyon Protraction Area on the outer continental shelf immediately north of the Sigsbee Escarpment. Water depths in this area

Asphalt mounds and associated biota on the Angolan margin

“…This solidification and subsequent shrinkage leads to the formation of visible cracks in the asphalt surface, followed by fragmentation of the entire deposit (Brüning et al, 2010). Asphalt mounds have also been found in the Gulf of Mexico at two commercial hydrocarbon appraisal and development areas, Puma (Weiland et al, 2008) and Shenzi (Williamson et al, 2008), both near the Sigsbee Escarpment in the Green Canyon Blocks over 600 km to the NNE of the Campeche Knolls. Furthermore, a single photograph was obtained at another asphalt mound in the Gulf of Mexico (Pequegnat and Jeffrey, 1979).…”

“…In some cases there is also a chemosynthetic community, likely driven by microbial oxidisation of hydrocarbons. Microbial films and mats have been found at the Chapopote site in the Campeche Knolls (MacDonald et al, 2004), at Puma (Weiland et al, 2008), and at an asphalt mound site with active methane venting in the Santa Barbara Basin (Valentine et al, 2010). Vestimentiferan tubeworms (cf.…”

“…Lamellibrachia sp.) were common in close proximity to asphalt flows at Chapopote (MacDonald et al, 2004) and at Puma (Weiland et al, 2008), which they colonized by extending the posterior ends of their tubes into sediments beneath the flow edges or into fissures. Large bivalve shells, including the chemosynthetic family Vesicomyidae (cf.…”

“…Furthermore, a single photograph was obtained at another asphalt mound in the Gulf of Mexico (Pequegnat and Jeffrey, 1979). All the asphalt mounds identified in the Gulf of Mexico are associated with shallow buried salt structures (MacDonald et al, 2004;Weiland et al, 2008;Williamson et al, 2008). In the Pacific, asphalt mounds were first identified in shallow waters in Santa Barbara County, California, by SCUBA divers (Vernon and Slater, 1963).…”

“…and Solemya sp.) were found at Chapopote (MacDonald et al, 2004) and associated with a large mound composed of authigenic carbonate at Puma (Weiland et al, 2008). Heterotrophic fauna at Chapopote included galatheid crabs (Munidopsis sp.)…”