Abstract: Integration of Seismic and Surface Geochemistry Data for Evaluation of Hydrocarbon Systems in the Gulf of Mexico

Boettcher, S.; A. James and L. Wenger - Exxon Production Research Co.; O. Gross; S. Harrison and K. Hood - Exxon Exploration Co.

Exxon has developed an integrated approach and proprietary interpretive technology for utilizing hydrocarbon seeps to recognize active hydrocarbon systems away from well control. In the Gulf of Mexico, we have used surface geochemistry to discern hydrocarbon families and integrated this information with seismic data to delineate active migration conduits. Thermogenic hydrocarbons are documented in sea-bottom drop cores by GC, GC/MS, GC/MS/MS and maximum fluorescence intensity data (MFI). Biomarkers in seep oils correlate with reservoired oils and suggest regionally consistent oil-source facies and maturities are present throughout the study area. Direct rock/oil correlations and/or regional geologic interpretations indicate the likely sources of these oils are Lower Tertiary, Upper Cretaceous, and Upper Jurassic - Tithonian and Oxfordian. Significant levels of oil biodegradation and mixtures of oil with recent organic matter in the seeps result in complex gas chromatograph and biomarker patterns. Methodologies to ?see through? these effects have been developed and used to estimate the source facies and maturity encountered in the cores. These interpretations along with data from reservoired hydrocarbons and seismic surveys are used for making hydrocarbon systems maps and mapping potential migration pathways.

To establish which potential cross-stratal migration pathways are likely to be charged with hydrocarbons, we constructed a map of seafloor features from 3D seismic data and integrated the map with seep intensity data in the Green Canyon/Ewing Banks area (Fig. 1). Over an ~3500 km² area, 134 seafloor mounds were recognized, many of which are associated with strong seafloor amplitude anomalies, high MFI values from drop cores and sea-surface hydrocarbon slicks. The map shows that clusters of seeps consistently occur over areas where salt has ascended to shallow depths. Short, arcuate faults appear to be preferential conduits for hydrocarbon migration from the top or flanks of salt diapirs to the seafloor. Down-to-basin faults between salt highs do not show seismic evidence for fluid expulsion in this area. In one example, the spatial relationship between a seep, subsurface seismic wipeout zones and subsurface structure supports an interpretation of hydrocarbons actively moving along a fault that connects with a 160 MBOE oil field (Fig. 2). Specifically, circular mounds indicative of seafloor fluid expulsion are present along seafloor faults above a salt diapir located near the oil field. Seismic wipeout zones occur beneath the mounds and intersect faults and salt at depth. The wipeout zones can be broken into an upper zone of columnar, low amplitude data and a steeply-dipping lower zone of discontinuous, chaotic reflection. Whether fluids are migrating up discrete slip surfaces or are being carried in diffuse, disturbed zones that occupy the entire wipeout volume is unclear. However, the systematic occurrence of seeps along seafloor faults lowers the risk for charge into traps in an area where seismic data do not image down to source.

These observations lead us to conclude: 1) seafloor mounds observed on 3D seismic data are a product of fluid expulsion; 2) salt ascension zones and overlying faults are primary conduits for cross-stratal migration to the seafloor; 3) establishing a spatial relationship between seafloor fluid expulsion features and faults/diapirs lowers the risk for hydrocarbon charge from source along potential migration pathways.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil