EXPLORATION Advantages seen in integrated offshore 3D seismic, geochem data

John Q. Belt Jr., Gary K. Rice
GeoFrontiers Corp.
Dallas

This is the second and concluding part of an article on the advantages of integrated 3D seismic and geochemical data in the Main Pass area of the Gulf of Mexico.

The data discussed were acquired in mid-1992 in a bottom cable, 3D seismic and shallow piston-core geochemical survey on Main Pass Blocks 41 and 58 off Southeast Louisiana. The 15.5 sq mile study area is in 11-24 m (36-79 ft) of water (OGJ, Apr. 1, p. 76).

Data integration

Numerous horizontal time slices and vertical profile lines were available from the 3D seismic data. These seismic sections were compared to various geochemical concentration and composition maps. The purposes of integrating the seismic and geochemical data were to determine and validate subsurface structures and vertical migration pathways and locate potential hydrocarbon accumulations associated with geophysically defined traps.^{10 11}

Fig. 14 [92116 bytes] compares the horizontal time slice at 500 msec to the three-ring aromatic concentration data from fluorescence analysis. The correlation between the faults in the seismic data and aromatic anomalies is very good. The seismic data show some of the conduits (macro-fractures) for liquid petroleum seepage into the near-surface sediments as faults. The shallow, seismic time slice also validates several aromatic data trends in Fig. 14 [92116 bytes]as fault related. Fluorescence fingerprinting indicates liquid, petroleum seepage along the faults is very similar to mid-depth oil (Fig. 8).

Fig. 15 [75669 bytes] compares the horizontal time slice at 1,500 msec to the ethane concentration data from acid extraction analysis. The 1,500 msec horizon was selected because it is closest in depth to the producing zone (mid-depth oil) matched using fluorescence fingerprinting. Numerous ethane concentration anomalies appear associated with features other than faults. Five significant northeast-southwest trending seismic features parallel geochemical anomalies located between faults in Block 58 (labeled 1 through 5). Four areas (labeled 6 through 9) appear significant in highly productive Block 41 to the north. Therefore, ethane and other low-molecular-weight hydrocarbons appear to have migrated near-vertically where faults are not apparent in the seismic data.

Fig. 16 [101,656 bytes] compares a vertical profile line to both acid extraction analysis (ethane concentration) and fluorescence analysis (three-ring aromatic concentration). Line A-A' (located on Figs. 14 [92116 bytes] and 15 [75669 bytes]) runs north-south, through the central portion of the study area. The vertical profile line shows that some ethane and three-ring aromatic concentrations are associated with visible faulting. Other high ethane concentrations are not located over visible surface fault traces. They appear associated with possible traps identified in the 3D seismic survey data near the productive horizon. The areas labeled 2, 3, and 7 in Fig. 16 [101,656 bytes] are those identified in the 1,500 msec horizontal time slice in Fig. 15 [75669 bytes].

What was learned

The following conclusions were made from the integration of 3D seismic survey and geochemical data:

Bottom cable, 3D seismic data, and shallow piston-core geochemical data are complementary. The integrated, synergistic results far exceed the information obtained with either data set by itself.
Shallow, 2 m piston-core samples can be obtained free of terrestrial contamination at lower costs than deeper coring operations.
High-density, grid sample design programs give useful geochemical data with meaningful spatial detail. Targeting cores over major faults is only necessary for collecting high concentrations of liquid petroleum.
The 3D seismic data, in both vertical and horizontal displays, indicate a highly faulted area. This is further validated when integrated with fluorescence data. Fluorescence fingerprinting also matched a known producing horizon (mid-depth oil) to the sediment samples.
Both the light hydrocarbons (acid extraction analysis) and aromatics (fluorescence analysis) indicate the data are highly correlated and free of contamination. A single thermogenic source at depth is indicated in the Pearson Product-Moment Correlation Coefficients, linear cross plots, and ethane composition index (ECI) histogram.
The C₁₅₊ capillary gas chromatographic (GC) data also indicate a thermogenic source at depth similar to mid-depth oil, as opposed to a contaminated fluvial source.
The light hydrocarbon data in Figs. 15 and 16 show ethane concentration anomalies are associated with subsurface, geophysical features other than visible faults. Some anomalies appear to trend northeast-southwest adjacent to faults, which could indicate hydrocarbon traps at depth.

Figs. 14 [92116 bytes], 15 [75669 bytes], and 16 [101,656 bytes] show a noticeable decrease in hydrocarbon concentrations (eth- ane and aromatic data) in the highly produced northern portion of the study area (Block 41). Low production in the southern portion (Block 58) has resulted in relatively higher hydrocarbon concentrations. Decreasing reservoir pressure resulted in decreased liquid petroleum seepage up faults (aromatic concentrations) and de- creased vertical migration in light gas concentrations (ethane concentration). Production-dependent concentrations are further evidence the groups of hydrocarbons detected at the surface were from a source at depth and not fluvial contamination.⁸

The 3D seismic survey data delineated subsurface structures, possible traps, and major vertical migration pathways such as faults. Geochemical analyses of shallow, piston-core samples detected and measured hydrocarbons associated with these subsurface structures and migration pathways. The information derived from integrating 3D seismic survey data with geochemistry is synergistic. The integrated product is invaluable in developing an offshore, petroleum geological model that is both pragmatic and cost effective.

Acknowledgments

The authors thank Bill Haworth, staff geologist, and Carol E. Avery, petroleum geologist, of Chevron USA Production Co., New Orleans, for providing geological information, constructive comments, and suggestions. The quality of this article was assured through their numerous efforts and contributions. The authors also thank for their geological review and suggestions: J. Michael Usseglio, president, Surface Oil & Gas Co.; Robert A. Cannon, consulting petroleum geologist; and Robert H. Springer, petroleum geologist. For review and comments on statistical quality control measures the authors thank: Janice M. Hiroms, manager; David E. Hunt, laboratory services superintendent; Patrick K. Eaves, laboratory services supervisor; Joseph B. Boswell, quality control superintendent; Arthur G. Bettes and Phillip J. Plauch, chemical analysts, all of Lyondell Petrochemical Co., Houston. Finally, thanks to all those offering comments and suggestions after presenting this article, in part, at the 1994 AAPG Hedberg Research Conference in Vancouver, B.C.

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