The Pliocene Calitroleum and Gusher producing zones of the Etchegoin Formation at Elk Hills and Coles Levee oilfields were deposited in a shallow marine environment with fluctuating sea levels. Net sand isochore maps indicate that sediment was supplied to the study area from both the east and the west. A sand-poor, low energy depocenter occupied the eastern Elk Hills/western Coles Levee area in the center of the study area throughout much of the time represented by Calitroleum and Gusher deposition.
Gross interval isochore maps show the Calitroleum zone thickening to the west and the Gusher zone thickening to the east. Both zones thin over the eastern and western anticlines of Elk Hills. This suggests that these anticlines were actively growing during deposition. Neither zone thins over the Coles Levee anticlines.
At least one minor transgression is documented within the Calitroleum zone. This is indicated by migration of sandy nearshore facies toward the margins of the study area and the subsequent expansion of the area occupied by fine-grained deposits in the central part of the study area. On the eastern margin of the study area, blocky and fining-up electric log facies characteristic of channel sands are replaced upsection by coarsening-up nearshore deposits and then spiky, sand-poor offshore facies. This transgression is followed by a regression that occurred within the uppermost part of the Calitroleum zone.
The eastern source area achieved dominance during deposition of the Gusher zone and the finer-grained, low-energy facies of the basin depocenter migrated westward into western Elk Hills. This is best illustrated by the thick sequence of blocky and fining-upward sands that appear within the upper part of the Gusher zone. This marks the first pronounced progradation of the Kern River fan delta into the eastern part of the study area.
The Calitroleum and Gusher zones form the lower part of the Lower Shallow Oil Zone (LSOZ) at Elk Hills. Eight gas samples from Elk Hills reservoirs ranging in age from mid-Miocene to late Pliocene were collected and analyzed for molecular and stable isotope composition. The gases fall into two groups. Group I is a bacterial gas rich in methane with a lighter isotopic composition. This gas is found in younger reservoirs of the lower San Joaquin and uppermost Etchegoin formations as well as in the deeper Gusher zone (LSOZ) of eastern Elk Hills. Group II gases are heavier both in molecular and isotopic composition and represent gases derived from thermogenic processes. These gases are found in the deeper Miocene reservoirs and in the Gusher and Calitroleum reservoirs (LSOZ) in western Elk Hills.
The bacterial gas of the eastern LSOZ occurs in a structurally lower position than the thermogenic gases of the western LSOZ. The gases may be prevented from mixing by listric normal faulting in eastern Elk Hills and western Coles Levee or, more likely, by the presence of the low-permeability, sand-poor depocenter which occupies the same area.
