ABSTRACT The Eocene Kreyenhagen Formation is a widespread siliceous, organic-rich mudstone within the San Joaquin Basin, but it is less studied than the Monterey Formation. This study characterizes the Kreyenhagen Formation in the Kettleman area to define its vertical and lateral variability on the basis redox conditions (Mo, U, Cr), paleoproductivity (biogenic SiO 2 , P, Ba), and detrital input (Al 2 O 3 , TiO 2 ) to determine the dominant environmental conditions during deposition. The Kreyenhagen Formation was correlated across 72 wells over a 4600 km 2 (1776 mi 2 ) area, which revealed an eastward thinning from 335 m (1100 ft) to less than 183 m (600 ft). We identified three informal members on the basis of log response and bulk/trace geochemistry: a lower calcareous silty mudstone, a middle organic-rich clayey mudstone, and an upper siliceous silty mudstone. Spatially, the greatest enrichment of total organic carbon, redox proxies, and biogenic silica occurs along Kettleman North Dome. These properties decrease eastward as clay volume, titanium, and aluminum increase. We interpret the Kreyenhagen Formation to record one transgressive-regressive cycle with contemporaneous climatic cooling: a transgression with initial suboxia and calcareous plankton productivity, a highstand with anoxic-euxinic benthic conditions and clastic starvation, and regression with elevated biogenic silica input. The upward transition from a calcareous to siliceous composition may reflect known cooling and upwelling intensification on the middle Eocene California margin. Mo/U and Th/U patterns suggest variable redox conditions across space and time. Lateral compositional trends indicate that eastern areas were proximal to a Sierran clastic sediment source, while western areas were distal and more anoxic.

ABSTRACT The structure and stratigraphy of the southeastern San Joaquin basin were reviewed for evidence that would document the impact on the basin of Miocene rotation of the adjacent Tehachapi and San Emigdio Mountains. Outcrops of basement rocks and volcanic intervals at the southeast margin of the basin contain paleomagnetic data indicating up to 59 degrees of clockwise rotation. The study used cross sections and maps of oil fields published by the California Division of Oil, Gas and Geothermal Resources. Information extracted included fault age and orientation, and stratigraphic data including gross unit thickness and net sand thickness. The geologic studies of the oil fields contain an abundance of evidence indicating Miocene extension. South of the Kern River, several fields contain numerous faults of early to middle Miocene age that generally fall on NW-SE or NE-SW trends. Fault offsets indicate a large amount of extension and correspond to the down-dropping of the floor of the Tejon embayment and break-up and collapse of the Edison high. Faults of similar age, present in fields north of the Kern River, have a slightly different NNW-SSE strike. Offsets on this latter set of faults are relatively minor and contributed in forming a wide shelf region. Sediments deposited during the middle and late Miocene reflect different styles of structural extension. South of the Kern River, the depositional gradient was very steep, and sand bodies representing deltaic, shallow-marine and deep marine environments are very localized in extent. North of the Kern River, sands deposited on the wide shelf are laterally extensive and represent deltaic and shallow-marine environments deposited at the terminus of a river system. The structural and depositional styles are similar between the Edison high and Tejon embayment area, indicating that the same structural events were responsible. The structural evidence is consistent with the rotation model of Goodman and Malin (1992) . However, if the Edison high block has rotated, then additional faults may be necessary to accommodate slippage against the adjacent Maricopa sub-basin block. The faulting style north of the Kern River is not consistent with rotation; thus rotation is likely limited to south of the Bakersfield arch.

ABSTRACT In this paper, we analyze source, seal, trap, reservoir, timing, and migration to explain petroleum occurrence and exploration prospectiveness in the southernmost San Joaquin basin. The factors that control oil occurrence and field size vary greatly among the three productive geologic domains in the area. We term these three domains the Foothills, the Basin, and the Upturn. The Upturn lies in between the structurally high Foothills and the structurally low Basin, and is a near-vertical panel of highly faulted strata with 5,000’-15,000’ of structural relief. Monterey Formation source rock in the southernmost San Joaquin basin has reached oil maturity mainly within the Basin. Oil generation there began about 4 Ma and continues today. However, the onset of anticlinal growth in the Basin at about 1 Ma dramatically changed oil migration pathways and delivery destinations. Prior to 1 Ma, oil migrating out of the Basin was delivered broadly across the entire east-west extent of the Upturn and Foothills. After 1 Ma, migration pathways became more complex and more east to west, resulting in very large oil pools in the southwestern San Joaquin basin. In the Basin, abundant oil has been generated, seal is ubiquitous, and early-formed traps are present. However, reservoir presence, reservoir quality, and the availability of migration pathways limit the amount of commercial production. Commercial oil is found mainly within gently dipping upper Miocene Reef Ridge and Stevens submarine-fan sand stratigraphic traps. The upper Miocene is sand poor, so sand mapping is critical. Fortunately, modern 3-D seismic combined with well data allow reliable mapping of sand fairways and fan facies. Little oil production exists above the upper Miocene because the high-angle faults that facilitate migration from the lower Monterey hydrocarbon kitchen do not penetrate high enough to allow charging of shallower strata. Little commercial production exists below the upper Miocene because depths are too great for sands to retain reservoir quality. The Upturn is well charged with oil, has abundant migration pathways, and contains numerous fault traps, many of which formed before oil migration began. However, it is sparsely drilled and difficult to image seismically, so exploration for the fault-footwall traps that are the typical targets is risky. Perhaps surprisingly given the active dense faulting, the presence of several oil fields in the Upturn indicates that seals can be effective. As in the Basin, sand is generally sparse, so sand presence is an important play control. The eastern Upturn toward the Tejon embayment is sandy, but the western and central parts generally are not. Where sand is present, reservoir quality is adequate even in lower Monterey and older strata that are tight in the Basin because burial of these strata was never deep. Most Foothills fields are located in Quaternary surface anticlines. However, central and eastern Foothills anticlines are undercharged because their present-day fetch areas are small and their trap timing is late: most of the anticlines are extremely young, so were not available to capture early-generated oil migrating from the Basin. To date more oil has been produced in the eastern than in the western Foothills, partly because sand content increases eastward.

ABSTRACT Study of Aqueduct field has revealed insights about the seismic signature, reservoir distribution, and possibly oil charging of Stevens fans. These relationships are especially well revealed at Aqueduct because of the field’s structural simplicity and the availability of a diverse data set including high-quality 3-D seismic. Aqueduct field is a stratigraphic trap formed by pinchout of the upper Monterey Stevens submarine-fan reservoir sand combined with a gentle structural bowing. The sand is time equivalent to updip interfan pelagic siliceous shale that provides the seal. Aqueduct field lies within the Aqueduct fan system, one of several southernmost San Joaquin basin upper Stevens fan systems deposited within the Monterey. The fan system is expressed on seismic profiles, cross sections, and interval isopach maps as a thick, which is typical for Stevens fans. The distal part of the system is fan shaped, but the proximal part is narrow and linear. Aqueduct field lies in the proximal part of the fan, where the lateral pinchouts of sand from the fan axis to flanking shale are abrupt. In spite of the linear proximal fan shape, we interpret that erosion at the base of the fan was minor. In the distal fan, sand passes gradationally from the fan axis to siliceous shale in the interfan area. Seismic, geochemical, and oil-water contact data suggest that Aqueduct field probably is compartmentalized into at least three separate oil pools. Sands at Aqueduct field may have been deposited as a series of prograding shingled fan lobes. If so, we interpret that the shales between these shingled lobes isolated the pools from one another. Aqueduct field has smaller reserves than nearby fields that are otherwise similar and is remarkably unfaulted. The small size may be due to charge limitation. At Aqueduct field, the absence of faults to facilitate vertical oil migration from the lower Monterey hydrocarbon kitchen upward into the reservoir may limit field size.

ABSTRACT 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.

ABSTRACT This study provides a seismic-scale sequence stratigraphic framework for the 1300 meter upper Neogene succession of the Bakersfield Arch, southern San Joaquin basin, California. An evaluation is made, within the context of time-significant stratigraphic surfaces, of the shallow-marine siliciclastic strata of the late Neogene Etchegoin (upper Miocene to Pliocene) and the San Joaquin (Pliocene) Formations which have produced 10.4 billion m 3 (367 billion ft 3 ) of dry gas within and adjacent to the study the area. Three-dimensional seismic data allow seismic-scale (50-150 m) interpretation of eight unconformity-bounded stratigraphic sequences. Well log data enable interpretation of reservoir-scale (meter) parasequences and parasequence sets within each sequence. These sequences, labeled ‘A’ through ‘H’ in ascending stratigraphic order, prograded and thickened west-southwest to fill the basin. Each sequence contains northeastward-onlapping transgressive systems tracts, separated by a flooding surface from the overlying southwestward-downlapping highstand systems tracts. A third systems tract, the lowstand wedge, is represented by incised valleys (widths of 180-600 m) on the uppermost highstand deposits of sequences ‘B’, ‘C’, ‘E’, and ‘F’ Each transgressive and highstand systems tract contains 30- to 80-m thick parasequence sets with characteristic stacking patterns composed of 6- to 20-m thick parasequences. Rapid subsidence plus high sediment flux in the Bakersfield Arch area resulted in the preservation of systems tracts and the compartmentalization of basinward lowstand wedges in the Arch. Detailed mapping at the North Coles Levee field of 6- to 18-m thick lower Gusher gas sands in the lowstand wedge of sequence ‘E’ reveals a wave-modified, tidal environment with a retrogradational stacking pattern, indicative of deposition at or near the onset of a relative rise of sea level. Pliocene deformation of the basin combined with a eustatic sea level fall resulted in a distinct change in depositional style from coarsening-up and fining-up parasequences capped by shales to a continuous series of coarse-based, thinning and fining-up sandstones, seen at both the seismic- and reservoir-scales of observation during deposition of sequence ‘H’. Following this change, shallow-marine facies were displaced upsection by non-marine facies of the Pleistocene Tulare Formation. This study of the integrated effect of tectonics, relative sea level, and sediment flux, in the context of a sequence stratigraphic framework, will enable better prediction of the geometries and facies distributions of gas-prone Neogene sands in the San Joaquin basin.