In order to evaluate interaction between clay-rich faults and fluid flow in petroleum reservoirs, a fossil oil field was studied in outcrop (Lodeve basin, France). The late Hercynian Lodeve basin is a Saxonian half-graben with a cluster of ENE-WSW faults. Bituminous occurrences associated with Liassic uranium deposits are located in the footwall of the major faults. Today, differences of water composition and water-table levels exist on both sides of the major faults. These observations indicate that the faults are efficient lateral permeability barriers today and throughout their geological history.

Detailed observations show the presence of a thin continuous clay layer within faults. Mineralogical and geochemical studies were carried out to determine the nature and origin of the clay contents. Moreover, siltstones and shales of the host rock were also sampled and analyzed.

Increasing clay content towards the center of the fault zone is associated with grain size reduction, feldspar dissolution and Titanium concentration. This suggests that the clay layer resulted from gouge effect linked to initial fault movement. Results allowed us to precisely quantify the reduction in rock volume and to relate this reduction to the fault throw.

Although transverse sealing is developed early, later vertical migration of basinal fluids occurred. Deep-sourced hydrocarbons migrated along faults and present-day hydrothermal activity has resulted in smectite neoformation within the fault.

Detailed clay mineralogy allows us to decipher between smearing, gouge and authigenic origins. Two mechanisms co-exist in the studied faults and can be related to the specific periods of fault activity.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah