The deep water Gulf of Mexico petroleum system: insights from piston coring, defining seepage, anomalies, and background

Cole, G. A., BHP Petroleum, Houston, TX; R. Requejo, Geochemical Solutions International, The Woodlands, TX; J. DeVay, BHP Petroleum, Houston, TX; A. Yu, BHP Petroleum, Houston, TX; F. Peel, BHP Petroleum, Houston, TX; J. Brooks, TDI-Brooks International, Houston, TX; B. Bernard, TDI-Brooks International, Houston, TX; J. Zumberge, GeoMark Research, Houston, TX; and S. Brown, GeoMark Research, Houston, TX

Abstract

In the early days of Gulf of Mexico piston coring, locations were chosen on a grid basis, or selected from loose 2D seismic surveys. Such locations resulted in some seepage "hits", but the majority had either a background signature or an "anomalous" value that was between a true visible seep and background (using fluorescence intensity and UCM content). A scale based on these early data identified anything less than 5000-10,000 fluorescence units as background, 10000-100000 as anomalies associated with seepage, and greater than 100000 as seepage.

Using new 3D surveys, it is easier to locate seepage-related seabed features. With better defined seepage sites and an extensive geochemical database, the old scale for background versus anomaly versus seepage has changed. By correlating true seepage to reservoired oil, most "anomalies" are not related to seepage or to the reservoired oils, therefore, not related to the subsurface petroleum system. The biomarker signatures can be used to define source origins, and when merged with regional understanding of source rocks in the greater Gulf of Mexico basin, a deepwater source model can be derived. 2D Temispack modeling confirms the seepage results based on a deepwater source rock model placing the primary source centered on the Tithonian and a possible secondary source rocks at the MCU and Oxfordian levels.

Based on oil-to-seep correlations, we can demonstrate:

1. that most piston cores <30000-50000 fluorescence (on the old scale) represent background, 30000-100000 are anomalies, and >100000 are true seepage.
2. biomarker signatures of most cores with <100000 fluorescence do not correlate to the reservoired oils.
3. geographical differences exist.
4. a pervasive background biomarker signature is present across the GOM, related to either river discharge sediments containing extractable oil and/or organic matter, or possible sediment de-watering carrying an oil-like signature, unrelated to the subsurface petroleum system.
5. there is a distinct pattern related to the Mississippi fan. The "background signatures" appear to contain real oil, but do not correlate to the active true seepage.

Using a rigorous approach when interpreting the detailed geochemical data from the piston cores, the "clean" seepage shows a regional trend that can be used to infer source rock type across the deepwater Gulf of Mexico. In areas where clastic sourcing is prominent, lower sulfur oils are predicted, whereas in areas dominated by carbonate signatures, higher sulfur oils will be present.