Abstract

The upper Miocene 24Z and 26R sandstones are lenticular multistory sand bodies which are significant oil reservoirs at several pools at the Elk Hills field, Kern County, California. Both units, which are in the uppermost portion of the Elk Hills Shale Member of the Monterey Formation, were deposited by sand-rich turbidite systems. Individual sand bodies are composed of medium- to coarsegrained graded sandstone beds and represent channel-fill deposits. At the margins of the lower sand bodies are interbedded fine-grained sandstone and mudstone which are levee and crevasse splay overbank deposits. Upper sand bodies grade laterally to very muddy overbank deposits or pinchout abruptly adjacent to siliceous shale of the Monterey Formation.

The deposition of the sandy and muddy elements of the turbidite system in an environment of syndepositional structural growth helped to create stratigraphic traps at the Elk Hills field. Initially, slight subsea relief of anticlines in the Elk Hills area directed turbidite deposition to synclinal lows, where well-developed levees formed at the channel margins. As anticlines grew, turbidite deposition was restricted to the central channel area, and only minor overbank deposits occurred at the channel margins. Following later uplift, oil was trapped in sand bodies with minor marginal overbank deposits because of effective permeability barriers between sandstone and adjacent shale. Sand bodies with significant levee or cravesse splay deposits would not trap oil because of ineffective permeability barriers at the updip edges of channel-fill deposits.