Don L. Hall¹, S. Michael Sterner², Wells Shentwu²
(1) Fluid Inclusion Technologies, Inc, Broken Arrow, OK
(2) Fluid Inclusion Technologies, Inc

Abstract: Fluid inclusion evidence for hydrocarbon microseepage and applications to prospect evaluation

Surface geochemical methods have documented the presence of organic and inorganic geochemical anomalies over productive reservoirs that are attributed to near-vertical microseepage of light hydrocarbons accompanied by fluid-rock redox reactions. These same fluids and processes are recorded within the fluid inclusion record and can be detected with Fluid Inclusion Stratigraphy (FIS), an analytical system for mapping volatiles released from crushing rock material. Application of this technique suggests:

1. Chemical indications of microseepage include the presence of light hydrocarbons (generally C1-C5), CO₂, acetic acid, simple aromatics and sulfur compounds (e.g., H₂S, COS, CS₂). Alkane/cycloalkane ratios reflect the extent of biodegradation.
2. Signals appear to be most prominently developed offshore.
3. Compounds are attributed to sulfate-reducing bacteria that utilize seeping hydrocarbons and dissolved sulfate to fuel life processes.
4. Anomalies tend to be chimney-like, and have been identified above petroleum accumulations as deep as 6,000 m (particularly gas-condensate and volatile oil reservoirs).
5. Indications are strongest at shallow depths and often disappear abruptly below 1000-2000 m, probably due to nutrient and/or temperature controls on bacterial activity.

Recognition of FIS subsurface seeps in areas limited by shallow well control can be used to infer deeper prospectivity and focus exploration efforts. Analysis of approximately 200 wells from the GOM suggests that drilling success over FIS subsurface seep anomalies will be 75%, while drilling success in areas without FIS subsurface seep signals will be 10%.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana