--> --> Thickness and extent of clay layers in the Pleistocene Tulare Formation near Elk Hills, California: Implications for water disposal operations and timing of structural growth

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Thickness and extent of clay layers in the Pleistocene Tulare Formation near Elk Hills, California: Implications for water disposal operations and timing of structural growth

Abstract

The lacustrine Amnicola Clay (2.2 Ma) separates the upper and lower members of the Pleistocene Tulare Formation at Elk Hills oilfield, California. The Tulare Clay is found within the upper Tulare Formation below the Corcoran Clay (0.64-0.8 Ma). Logs from 650 wells were used to correlate the Amnicola and Tulare clays across Elk Hills and into adjacent fields to map the structure, thickness and lateral extent of the clays. These maps help bracket the timing of late stage structural and topographic development at Elk Hills. The thickness and continuity of these clays also have important implications for water disposal operations in the Tulare Formation in the map area, as continuous clay layers are more likely to confine the disposed produced waters and keep them from reaching overlying protected aquifers. The structure maps reveal a single northwest-trending anticline in the Elk Hills area. The isochore map shows that the Amnicola clay is continuous across the Elk Hills area and forms a coherent confining layer across the study area although it thins over the crest of the anticline. This demonstrates that Elk Hills was covered by the paleo lake approximately 2.2 Ma and most of the recent structural and topographic growth occurred after deposition of the Amnicola Clay. The younger Tulare Clay does not extend over the structural high and forms only a local confining layer. It thickens into the adjacent synclines and is absent over the crests of the anticlines, demonstrating that significant structural and topographic growth occurred at Elk Hills before, or during, deposition of the Tulare Clay (prior to 0.8-0.64 Ma–the age of the overlying Corcoran Clay). Therefore it appears that late stage structural growth and topographic development of the Elk Hills anticline occurred after 2.2 Ma but began before 0.8-0.64 Ma.