THE BATON ROUGE FAULT: CONDUIT OR IMPEDIMENT TO GROUNDWATER FLOW?

The Baton Rouge fault is a listric normal fault and part of a regional east-west trending fault zone that traverses the eastern to central portions of south Louisiana. The fault exhibits at least 2 periods of fault movement contemporaneous with sediment deposition. Fault generated rollover anticlinal structures and stratigraphic thickening of sediments across the fault indicate syndepositional fault movement in late Eocene to Oligocene time. Pronounced surface expression of the Baton Rouge fault indicates recent renewed structural fault movement.

The Baton Rouge fault is recognized as a major barrier to groundwater flow. Freshwater aquifers north of the fault serve as the municipal water supply and are in juxtaposition with saltwater aquifers immediately south of the fault. Recent saltwater encroachment north of the fault has resulted from increased pumpage and drawdown of the freshwater aquifers. Proposed mechanisms for saltwater encroachment include: 1. Saltwater flow across the fault between juxtaposed aquifers; 2) Saltwater flow along the fault plane from deeper aquifers.

The distribution and characteristics of hydrocarbon accumulations associated with the Baton Rouge fault suggests the fault may have failed to trap significant amounts of hydrocarbons in the past, allowing hydrocarbons to escape updip across the fault. A similar “leaky” fault mechanism, whereby saltwater flows across the fault between adjacent aquifers, is the likely explanation for the observed saltwater encroachment. However, recently inferred vertical saltwater flow from deeper, higher salinity aquifers utilizing the fault as a conduit cannot be dismissed.