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October 2005

Natural Seepage Investigation in Green Canyon

TDI-Brooks recently conducted an analytical and historical seepage investigation for a deep water Gulf of Mexico operator around a field that came on stream in late 2004. On December 16th, pronounced oil slicks were observed approximately 3-miles to the northwest of the operator's field in the Green Canyon lease area. Figure 1 shows a photograph of these sea surface slicks taken on a helicopter over-flight of the area to the northwest of the facility. In order to check that these surface slicks were not originating from its production facility and associated pipelines, a remotely operated vehicle (ROV) inspection was conducted. This survey showed no evidence of leakage from the production and transportation complex. The operator also collected samples of the oil slicks that were analyzed by TDI-Brooks.

Figure 1. Surface slicks found in GC-600 to the northwest of a production facility evident in a photograph taken on 16 December 2004.

A Synthetic Aperture Radar (SAR) audit of the natural seeps in GC-600 area conducted by NPA Group and Dr. Ian MacDonald at TAMU-Corpus Christi indicates that these surface slicks have been a long-standing feature of the area and originate from previously known natural seepage. This was confirmed by historical and recent satellite imaging of the block as well as previous geochemical coring showing macro oil seepage near the center of the block. All available evidence indicates that natural seepage in GC-600 has been a prevalent, long-term feature of this block.

Three separate synthetic aperture radar (SAR) studies confirm that natural oil seepage in GC-600 is prolific; creating massive slicks trailing over many kilometers, in different directions at different times. These directions depend primarily on surface and subsurface currents. They do not necessarily trail downwind because the seepage-slick films are ultra-thin, responding to current rather than wind. Three (3) sources of SAR imagery were examined and acquired as follows: 1. NPA Group SAR Images from 1998; 2. Ian MacDonald (TAMU-CC) SAR Images from 2001 & 2002; and 3. New SAR images acquired by NPA Group from 3 & 6 December 2004. All three of these sets of SAR images from 1998 to 2004 show major slicks in GC-600 confirming the significant and pervasive natural oil seepage in this block. Figure 2 illustrates the prevalent and persistent slicks shown by SAR images acquired at this site in 2001/2002.

Figure 2. SAR imaged natural seepage slicks from 2001 & 2002 in the GC-600 region. Depth contours are in meters. Legend shows the separate image collections that detected natural slicks in the region. Also plotted are results of SLIKTRAK simulation of oil drops rising from the location of a known mud volcano and probable seep source.

Green Canyon is a region of intensive natural hydrocarbon seepage. The original discoveries of macro oil seepage in cores, oil seep related chemosynthetic organisms (clams, mussels and tube worms), gas hydrates and extensive authigenic deep-sea carbonates were made in the Green Canyon lease area (Brooks et al. 1984, 1985, 1986, 1987a, b, and 1989). Two (2) macroseepage sites with oil-stained and gas charged cores have been previously located in GC-600 as part of TDI-Brooks' consortium surface geochemical exploration (SGE) coring programs. Figure 3 shows the locations of these oil-stained cores overlaid on the SAR slick images for the NPA Group's 1998 slick image. These sites are the source of the natural seepage in GC-600 producing the surface slicks.

Figure 3. Prolific oil seepage slick observed in a SAR image from July 1998. Overlain on this image are locations of two (2) oil stained cores as well as the geochemical analysis of the cores.

Liquid hydrocarbon seepage in the deep water Gulf of Mexico is associated with deep cutting faults that provide conduits for the upward migration of oil and gas. These sites are identified based on 2-D and 3-D seismic data. The 3-D seismic data that now covers most of the deep-water Gulf has become a very effective tool for delineating the most likely sites of natural hydrocarbon seepage into seafloor sediments. The most prolific seep sites in the deep Gulf are most often associated with mud and/or hydrate mounds that result from the migration of oil and gas up from the underlying fault system. The fault systems in Green Canyon are mostly controlled by the prolific salt tectonics, which provide often numerous potential conduits for hydrocarbons leaking from subbottom reservoirs.

In the 3-D seismic records, these seep features can often easily be detected by surface amplitude anomaly and edge maps correlated with the underlying deep cutting fault system. The surface amplitude anomaly maps show hard features often associated with authigenic carbonate and chemosynthetic ecosystems associated with natural seepage. Figure 4 displays a Chirp subbottom profiler section with two mounds that have active macroseepage in GC-600. Figure 5 shows the surface bathymetry and surface amplitude anomalies associated with these two seepage features. Figures 4 and 5 illustrate classical examples of macro oil seepage sites in Green Canyon identified in seismic sections and surface amplitude anomaly maps generated from 3-D seismic data sets.

Figure 4. Chrip subbottom profler line across macroseep sites in GC-600. The macroseepage sites are the two mounds in the center of the section.

Figure 5. Overlay of oil slicks inferred from 2001 SAR images on seafloor bathymetry (A) and amplitude anomaly response derived from 3-D seismic data (B).

The seep oil from the cores in GC-600 was further analyzed to determine its history using standard geochemical oil/seep analysis techniques (i.e., biological markers, isotopes, fractionation data, and whole oil GC). The oil extracted from the piston cores were highly biodegraded consisting of a large UCM with all the n-alkanes biodegraded from the seeping oil. The biomarkers indicate an Upper Jurassic (Tithonian) marl/carbonate sourced oil, which is common in oils and natural seeps from this part of the deep water Gulf (GeoMark Research).

Figure 6 shows a comparison of the produced oil from the facility with the slick/surface water sample and oil stained cores. The figure show whole oil gas chromatograms with Flame Ionization Detection (FID) of these four samples. While the oil produced at the production facility is a light oil dominated by liquid hydrocarbons with carbon numbers mostly less than n-C17 and with the high concentration of peaks in the gasoline range fraction. The hydrocarbons in the oil-stained cores from GC-600 (lower panels in Figure 6) are mostly composed of an unresolved complex mixture (UCM) of hydrocarbons that cannot be resolved gas chromatographically. The few peaks on top of the UCM in the seep oil samples are mostly aromatic hydrocarbons that are more resistant to biodegradation than the n-alkanes. The n-alkanes are virtually absent from the oil stained cores because of the extensive biodegradation of the oil in the shallow cores. The whole oil hydrocarbon composition of the sea surface seep oil (upper right panel) is composed of a large UCM and n-alkanes that have been mostly weathered away below the n-C12 to n-C15 hydrocarbon range. In temperate climates, hydrocarbons below n-C15 on the sea surface are rapidly weathered within a few hours through evaporation and dissolution. Our interpretation of the gas chromatograms is that the slick oil cannot originate from the produced oil because of the large UCM found in the slicks. If the source of the slick hydrocarbons was the production oil, it would not contain the large UCM found both in the slick and oil-stained core samples. The hydrocarbon chemistry of the oil stained cores and slicks suggest a common source --- i.e., natural hydrocarbon seepage.

Figure 6. Whole oil gas chromatograms of the surface slick sample in GC-600 (upper right) compared to the produced oil (upper left) and sediment oil stained cores (bottom two panels).

Geochemical sampling with piston cores, identification of direct hydrocarbon features on 3-D seismic data, analysis of the sea surface slicks and seafloor sediment, and detailed seafloor topographic mapping confirm the presence of significant natural oil seepage in GC-600.