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Unconventional opal-A and opal-CT diatomaceous reservoirs in the Monterey Formation of the San Joaquin Valley in California have produced for more than 30 yr. These opal-A reservoirs are characterized by high porosity (>50%) and low permeability ( 45% [Cymric and McKittrick fields]) or are very thick (>1000 ft thick [Lost Hills field]). These insights provide general guidance about characterization of unconventional diatomaceous reservoirs and the identification of factors that most dramatically influence performance.

A continuous oil accumulation is one that is pervasive throughout a large area and is not affected by natural hydrodynamic influences. Three source rocks in the San Joaquin Valley of California are actively producing hydrocarbons and represent potential continuous oil accumulations: the Monterey, Kreyenhagen, and Moreno Formations. The Energy Information Administration announced in 2014 that there are potentially 15 billion bbl of recoverable oil in the Monterey Formation in California, spiking huge interest. Such a resource would make the Monterey by far the largest continuous oil accumulation in North America. This number has since been reduced dramatically to 600 million bbl of oil for the state and 21 million bbl of oil within the San Joaquin Valley. In this study, the subsurface character of the Bakken Formation of North Dakota is compared with the three source rocks in the San Joaquin Valley at oil window depths. To characterize these reservoirs, hundreds of well logs, core descriptions, and mud logs were studied. A technique to rank character of oil show data was developed in which interesting, possibly interesting, or not interesting wells were located on thermal maturity maps. Interesting wells have significant oil shows, whereas not interesting wells have minor or no shows. In the Bakken Formation, the character of the oil show correlates with well productivity. Applying this same classification to the San Joaquin Valley source rocks leads to a more disappointing conclusion. Although there are oil shows in the source rocks of the San Joaquin Valley at oil window depths suggesting the presence of a continuous oil accumulation, the distribution of shows is both laterally and vertically heterogeneous and not predictable. Moreover, recent attempts to produce from source rocks at these depths have not been economically successful. We conclude that the three source rocks in the San Joaquin Valley represent heterogeneous and discontinuous oil accumulations at oil window depths in the subsurface. Likely there are billions of barrels of oil in these discontinuous oil accumulations. Source rocks in the Bakken are rated as world class; source rocks in the San Joaquin Valley are good to excellent quality. However, the quality of the oil shows in the San Joaquin Valley appears poorer than the Bakken Formation. It is possible that effective drainage between the source rocks and the up-dip reservoirs has left large volumes of the source rocks at oil window depths with only residual oil saturation. Complex structural and stratigraphic architecture, heterogeneity, and continuity create issues of predictability for optimal areas to target. Rapid rates of subsidence over the past few million years and accompanying thrusting and folding resulted in a complex subsurface pressure regime. The lack of clear hydraulic fracture targets, analogous to the middle Bakken, further complicates drilling decisions and likely deliverability. In addition, the oil windows in the San Joaquin Valley are significantly deeper than the Bakken Formation, which would result in substantially higher well cost. Recent drilling results support this study and suggest that heterogeneous and discontinuous oil accumulations in the San Joaquin are unlikely to become economic without dramatic changes in technology.

Water salinity in the San Joaquin Valley is a function of depth, location, and stratigraphy. This paper presents a reconnaissance study of water salinity within Kern County, California, using chemical analyses from oil field produced water and water wells as well as geophysical logs. Log analysis indicates that the base of underground sources of drinking water (USDWs) (

The Miocene Monterey Formation, long known as the critical source rock in California, also includes significant fractured chert and porous diatomite reservoirs. What is not widely recognized is that there are high matrix porosity reservoirs within the opal-CT and quartz-phase rocks. Using density, porosity, and mineralogy data, we have identified two distinct groups of Monterey Formation reservoirs. group 1 porosity changes gradually during silica diagenesis--porosities of 55-70% exist not only in opal-A-dominated samples but also in samples that have undergone the transition from opal-A to opal-CT. In group 2, porosity decreases abruptly at the transition, and rocks below the transition are tight. The main mineralogic difference between groups 1 and 2 is a higher clay content in group 1, resulting in different silica diagenesis pathways for the two groups. This led us to expect all San Joaquin basin samples to fall into group 1, and all coastal California samples to fall into group 2, because of the different depositional histories of the two areas. Although our prediction holds in most instances, we discovered that some San Joaquin basin samples exhibit group 2 characteristics. Therefore, it is important to use petrophysical, seismic, and/or geological information to determine if a Monterey Formation reservoir is likely to be a fracture-dominated type (group 2) or if there might be a matrix production component (group 1). Once one has made this diagnosis, the characteristic density/porosity relationships of each group allow one to easily calculate an accurate matrix porosity.

California Title 23 and the Solid Waste Assessment Test (SWAT) program, as administered by the Regional Water Quality Control Boards (RWQCB), require extensive characterization and monitoring of waste disposal sites. Typical SWAT/Title 23 investigations require multiple characterization boreholes in addition to groundwater and unsaturated zone monitoring. In an arid environment such as the southwestern San Joaquin Valley, many of these requirements become impracticable. Regions such as these are often characterized by extreme depths to groundwater, non-potable groundwater, and moisture deficient and hydrocompacted soils. Through the use of oil-field data, an accurate portrayal of subsurface conditions at a SWAT-ranked site was possible. Because of the extensive background material available and the relatively benign nature of the site, a single borehole was deemed by the REQCB to be an acceptable characterization of the site. As predicted from the background data review, this borehole was drilled to 1,000 ft in depth without encountering a water-saturated interval capable of yielding a sample. In addition, oil-saturated intervals were encountered beneath the site as predicted.

West Coast exploration accelerated in 1977, except in Washington where no wells were reported completed by year end. One significant oil discovery was made onshore in California on the west side of the San Joaquin Valley. Two additional discoveries were made offshore in the Santa Barbara Channel, 1 oil and 1 gas. These were reported by the media, but are not reflected in the CSD statistics. Additions to reserves in California outstripped production by 45 million bbl in 1977. Development activity increased in California and was concentrated in shallow drilling relating largely to steam-flood projects. Geothermal activity included the possible discovery of a new resource area in Inyo County, California. Development drilling continued at The Geysers and in Imperial Valley. The Geysers capacity will be raised to 908,000 kw over the next few years. Two 50-Mw plants are being planned for the Heber area in Imperial Valley.

Late Pliocene to early Pleistocene depositional patterns of upper Etchegoin and lower San Joaquin Formation sand and shale units in the Buena vista field area were controlled by changes in clastic input, eustatic sea level, structural growth, and circulation patterns in the south end of the San Joaquin Valley. Wireline and drill-strip logs, core depositions, paleontology, and petrographic data from these units suggest the interpretation of a series of shallow to marginal marine deposits with distinctive morphologic features and production characteristics. Late Pliocene marginal marine drainage systems transported clastics from southerly sources as structural and/or eustatic changes shoaled the southern area. An erosional hiatus and shallow marine transgression marked the extent of Plio-Pleistocene shoaling and rapid early Pleistocene foundering. Later Pleistocene changes in sediment supply and structural growth isolated the area from marine conditions as the basin filled with nonmarine sediments. Early field development was influenced by the areal distribution and reservoir characteristics of these sands as well as by the timing of such development activity. Depositional models derived from these data are useful in constructing paleogeographic models with regional hydrocarbon significance.

Thermal recovery of heavy oil is, to date, the most successful enhanced oil recovery process. Both steam stimulation and steam-flooding are widely used in California. They add almost 300,000 BOPD to the state's production. The Edison field, located on the eastern side of the San Joaquin valley, is just one of many heavy oil fields being produced by thermal recovery methods. The Edison heavy oil sands are offshore-bar and alluvial-fan deposits. There are two textural controls on the reservoir quality of these rocks: (1) grain sorting and (2) the amount of dispersed silt and clay. The reservoir properties affected by rock texture are permeability and capillary pressure. Capillary pressure is particularly important, as it traps oil, controls oil saturation, and limits oil mobility. The heavy oil at Edison is currently being produced by cyclic steam stimulation. The heat from the steam improves production by lowering the viscosity of the oil. Steam stimulation has doubled the recovery of heavy oil. The enhanced oil recovery project at Edison was recently expanded to include a pilot steam-flood. The pilot project was designed with the aid of a computer model. The model was used to simulate the movement of steam through the reservoir and predict oil and water production. Simulated production trends indicate that the success of the project will depend on the oil saturation in the reservoir at flood start. Steam injection into the pilot site began in February 1982. It is just one of many enhanced oil recovery projects being attempted by industry to try and offset a steady decline in new oil field discoveries. End_of_Article - Last_Page 1694

Calculations of the tectonic component of the subsidence history of California's southern San Joaquin basin indicate 200-400 m of abrupt tectonic uplift during the Relizian, about 15 Ma. This uplift can be explained as an isostatic response to the movement of thin, young lithosphere south of the subducted Mendocino fracture zone to a position under the basin. The calculated uplift cannot be a result of extension and subsidence caused by instability of the Mendocino triple junction, but earlier (early Miocene) subsidence of the basin may have resulted from triple-junction-related extension. Marine strata deposited in the San Joaquin basin during the middle Miocene are rich sources of petroleum, so the inferred uplift and facies shifts may have important implications for ex loration.

The Eocene to lower Miocene fill of the southern San Joaquin basin contains three complete depositional sequences--the Tejon, San Emigdio, and Pleito--each of which corresponds to a formal formation. The Tejon sequence (lower to middle Eocene) is marine and incorporates nearshore, shelf, slope, and basinal deposits. The San Emigdio (upper Eocene to lower Oligocene) and Pleito (upper Oligocene) sequences intertongue eastward with alluvial-fan deposits of the Tecuya Formation. The lower part of the San Emigdio sequence was deposited by a westward-prograding Gilbert-type delta. The upper part of the San Emigdio sequence and lower part of the Pleito sequence were deposited by a system of shelf fan-deltas that prograded at least 10 km to the west. The middle and upper parts of the Pleito sequence were deposited by a slope fan-delta in relatively deep (hundreds of meters) water. Regional transgression during the early Eocene initiated deposition in the southern San Joaquin basin. The lower San Emigdio Gilbert-type delta prograded from the shelf edge during a lowstand in eustatic sea level at approximately 40 Ma. Relative highstand deposits in the San Emigdio and Pleito Formations consist of widespread progradational shallow-marine and nonmarine facies. The Eocene to early Miocene tectonic history of the southern San Joaquin basin included three distinct periods of increasingly intense activity. Rapid early to middle Eocene subsidence of the basin was associated with the emplacement of the Salinian block. Late Eocene to early Oligocene uplift and subsidence occurred in two discrete pulses, and a seismically(?) generated submarine-slumping episode triggered the change from shelf fan-deltas to slope fan-deltas. During the late Oligocene to early Miocene, the southern San Joaquin basin was disrupted by major uplift, proximal volcanism, and syndepositional faulting, as the Mendocino triple junction migrated past the study area, and San Andreas fault-related tectonism was initiated. In general, local tectonic events, rather than eustatic ea level events, seem to have exerted the predominant control on middle Cenozoic sedimentation in the southern San Joaquin basin.

Ninety-two core samples of Cretaceous through Miocene rocks, from 13 wells in the western San Joaquin Valley, California, were examined in fluorescent light to assess kerogen types and source potential. The results are compared to those obtained from transmitted light analyses. Samples of different ages show more similarities than differences in kerogen types and source potential. The samples are mostly of fine-grained lithology from the Monterey, Kreyenhagen, and Moreno Formations, which are known or suspected source rocks in the San Joaquin Valley. All samples examined in this study are immature or marginally mature with respect to oil generation. Transmitted light analyses show that the organic matter of most of the samples consists of aggregated amorphous kerogen. Liptinitic (pollen, spores, cuticle, resin) and woody or coaly debris are generally minor components. Very finely divided amorphous kerogen is also present, usually in minor amounts. Based on the predominance of aggregated amorphous organic matter, most samples are interpreted to be oil prone. Fluorescent light analyses show that most of the organic matter consists of orange to brown-fluorescing Amorphinite B. Many samples also contain common to abundant amounts of yellow-fluorescing Amorphinite A. Liptinitic and nonfluorescing debris are generally less common. Grayish-fluorescing Amorphinite C ismore » abundant to predominant in several samples in one well. Based on the predominance of Amorphinite B, most samples are interpreted to be oil prone. Additionally, most of the amorphous organic matter is interpreted to be of marine origin, probably derived from the decomposition of phytoplankton, zooplankton, and other marine organisms.« less

Drilling activity in the West Coast region in 1985 maintained the brisk 1984 level, the number of wells drilled decreasing by only 2%. Exploration efforts continued at the pace of the previous year with 265 exploratory wells drilled in 1985, 4 less than in 1984. Exploration offshore California continued to decline as development plans progressed for the nearly 2 billion bbl of oil discovered in recent years. Three new platforms were launched in 1985, and 13 others are planned for the Santa Maria and Santa Barbara Channel offshore areas. The Monterey Formation continued to be a hot exploration target, with 4 of the 12 exploratory wells drilled in 1985 each testing combined rates of more than 3,700 b/d of 8-16° oil. Enhanced recovery techniques for low-gravity crude are partly responsible for the success of the recent testing. Onshore California exploratory activity increased slightly, with 229 wells drilled in 1985 compared to 223 in 1984. The San Joaquin basin had the most significant find in over a decade. The Channel Exploration 63X-30 Tenneco-San Emidio was completed in Stevens sand for 2,064 BOPD and 405 MCFGD. Drilling in Oregon and Washington continued its active trend with 25 wells drilled in the 2 states in 1985, 5 more than 1984. Activity centered in Oregon's Mist field, with 16 wells resulting in 6 new pools and 1 field extension. Geothermal activity in California decreased slightly, with 62 wells drilled in 1985 compared to 76 in 1984. Activity was concentrated at The Geysers field as 3 more plants were brought on line. The Imperial Valley also saw significant activity when 3 new power plants went into operation.

Exploration activity declined in offshore California as operators focused on development plans for the nearly 2 billion bbl of oil discovered in recent years. Fourteen planned platforms could more than double current offshore production to 400,000 BOPD. Highlighting the offshore Santa Maria basin was the Reading & Bates Tricia Prospect discovery. The Reading & Bates 1-OCS-P-0416 tested a 1,570-ft Monterey section at a combined initial potential of 2,400 b/d of low-gravity oil. Elsewhere in the basin, successful extension work was recorded in San Miguel and Point Pedernales fields. In the Santa Barbara Channel, exploration activity was highlighted by Union's new-pool discovery in the Santa Clara unit. The Union S-26-OCS-P-0216 flowed 356 b/d of 20.7° oil and 245 MCFGD from lower Repetto sands. Renewed optimism in 1984 increased exploration activity in onshore California basins. 183 wildcats resulted in 51 discoveries: 10 new fields, 7 new pools, and 34 extensions. Most of the success centered in the Sacramento and San Joaquin basins. Drilling in Oregon and Washington reached a near record level in 1984. Activity was quite high in the Mist gas field in Oregon. The 12 wells drilled resulted in 4 new-pool discoveries. The deepest well drilled to date in Washington was abandoned late in the year. The well, drilled in the Columbia basin as a joint venture of Shell and Arco, reportedly reached a total depth of 17,518 ft and tested gas at noncommercial rates. Geothermal activity in California was also at a higher level in 1984, up 32% over 1983. The increase mainly was due to renewed activity in the Imperial Valley gearing up for the installation of 2 new power plants.

The Oligocene Vedder formation consists of well-sorted medium to fine-grained marine sand and shale in the subsurface of the eastern San Joaquin Valley. Updip, this formation interfingers with nonmarine/lagoonal facies known as the Walker Formation. This relationship appears to be transgressive because the marine Vedder generally overlies the Walker Formation. Downdip, the Vedder sands interfinger with middle to lower bathyal shale in a progradational manner, forming upward-coarsening patterns in well logs. Depositional water depths for the shale were determined from benthic foraminifera assemblages. The Vedder formation is approximately 750 ft thick along its updip part, and gradually thickens to 1500 ft downdip. Overall deposition geometry, determined from well-log correlations and seismic data, is generally parallel and downlapping. A prominent shelf-slope break is not evident. Rather, depositional surfaces are tabular or broadly lobate, with a depositional slope of 5/sup 0/-10/sup 0/. This geometry of constant slope between nonmarine and deep marine water depth is termed a ramp. The depositional style and geometry are similar to that of the Oligocene upper Pleito Formation, which crops out in the San Emigdio Mountains on the southern margin of the San Joaquin Valley. The Vedder formation was deposited subsequent to a period of rapid subsidencemore » (about 50 cm/1000 years), as determined from geohistory analysis of well data on the Bakersfield arch. This rapid subsidence may have induced deposition in a ramp geometry, rather than a shelf-slope configuration.« less

In 1937 the writer defined the Lodo formation exposed along the west side of the San Joaquin Valley, California, as a shale unit with an included sandstone member (Cantua sandstone). The formation is stratigraphically above the Moreno shale (Upper Cretaceous) and below the Domengine sandstone (middle Eocene). The Domengine in that paper erroneously was considered to be the "Domijean sands" of F. M. Anderson (1905). It included the "Tejon" of Anderson and Pack (1915), or the restricted Domengine of Clark and the underlying sandstone referred by him to the Meganos. However, a critical reading of the original definition of Anderson's "Domijean sands" shows that it included all strata from the base of the Cantua sandstone member to the base of the Kreyenhagen shale of later workers. It is, therefore, an unsuitable cartographic term. The Domengine as redefined by Clark is a well defined mappable unit. The name Yokut is here proposed for the sandstone unit that lies between the top of the Lodo shale and the pebble bed at the base of the Domengine as redefined by Clark. The Yokut is overlapped by the Domengine at the axis of Coalinga anticline on the south, and approximately one mile northwest of Tumey Gulch on the north. The lower contact of the Yokut is gradational with the underlying carbonaceous sandy siltstone in the upper part of the Lodo. The type locality of the Yokut sandstone is near the west boundary of the SE. ¼, Sec. 29, T. 18 S., R. 15 E., M.D.B., in Domengine Creek where the formation is 45 feet thick. A maximum thickness of 305 feet was observed between Salt Creek and Martinez Creek. Yokuts is the name of an Indian tribe who inhabited the San Joaquin Valley.

Several areas with altered, highly acidic sediments are present on the western margin of the San Joaquin Valley near O'Neill and San Luis Reservoirs. Alteration occurred mostly in the upper part of fluvial deposits of the Plio-Pleistocene Tulare Formation. Surrounding deposits, in general, have normal basic pH of 7.5 to 8.5. Altered sediments with pH values ranging from 1.5 to 4.5 contained some native sulfur, gypsum, iron sulfide, and alunite-jarosite. Three samples had pH values of 0.7 and two had values of 0.5. Hydrogen sulfide odor had been noted in many auger holes during exploration for construction materials in the vicinity, and methane-bearing gas with a hydrogen sulfide odor was found in two test holes, 25 and 80 ft deep. The alteration is similar to that resulting from low temperature hydrothermal processes, but the association with methane-bearing natural gases and hydrogen sulfide suggests a different origin. The gases rising from deeper beds were trapped locally under a clayey cap, where reactions between hydrogen sulfide, calcium-iron compounds in the sediments, and oxygen in groundwater and soil atmosphere produced an acidic environment, with iron sulfides, native sulfur, gypsum, alunite, and related minerals.

Membership in the Pacific Section of the American Association of Petroleum Geologists has declined 18.8% from its all-time high of 1,362 members in 1956. The rate of decline appears to have leveled off during the year 1959-60. During the 15 years prior to 1956, the Pacific Section grew rapidly from 247 members in 1941. This rate of growth was at least equal to the phenomenal growth rate experienced by national A.A.P.G. The present analysis shows the trends of Pacific Section membership throughout California during the 9-year period 1951-1960. Trends by areas corresponding with those of local geological societies and oil-compnay operations are shown. Statistics used are obtained from membership lists, newsletter mailing lists, and five Pacific Section directories published to date. By the use of these statistics, it is possible to show that while the number of members has changed, the percentage distribution of Pacific Section members in each of the Los Angeles, San Joaquin Valley, Coastal, and Northern California operational areas over the past 9 years has remained fairly constant, but that members outside of California have increased from 4% to about 15% of the total organization. A study of the percentage distribution of members by organizational affiliation shows that between 40 and 50% of the members worked End_Page 130------------------------------ for 15 major oil companies over most of this period, while 15 independent oil companies employed an average of 7%. All oil companies together accounted for 75% of the total membership in all areas. Membership trends in California alone for each year bear a resemblance to exploration activity over the period analyzed. The peak year in California's drilling record for exploratory holes completed and footage drilled corresponds closely with the high point in Pacific Section membership. Membership figures used in this analysis consist of the total number of active and subscriber members. Only paid members could be counted inasmuch as all available lists include only this group. A check was made against the geographical roster of the national A.A.P.G. March Bulletin for each year to find the relation between national and Pacific Section members. End_of_Article - Last_Page 131

A study of the depth-distribution of recent ostracods in the San Pedro Channel from Long Beach to Avalon Bay, Catalina Island, has revealed a new tool to aid in the solution of paleogeographic and ecologic problems in California stratigraphy. These microscopic crustaceans are very abundant in shallow salt-water lagoons, in mainland continental shelf sediments down to a water depth of 500 feet, and along protected island shores. The samples collected by Alex Clark and Manley L. Natland show by the presence of soft parts that the more abundant species of ostracods were alive on the bottom when dredged. Comparison with the 26 fossil forms recorded by L. W. LeRoy in 1944 reveals eighteen of his species and varieties surviving to Recent time. Total recent forms recognized to date number approximately 22 genera, 52 species, and 7 subspecies. The following geologic formations and Recent ostracod habitats are grouped together to suggest similar environments, as inferred from the occurrence in each of certain characteristic genera and species. End_Page 2087------------------------------ I. Lower San Joaquin Formation (Upper Pliocene) and Salton Sea deposits (Recent?) II. Upper Etchegoin Formation (Pliocene) and Sunset Lagoon (Recent), Salt marshes of Mission Bay (Recent) III. Upper Imperial Formation (Pliocene) and shore waters of the Gulf of California (Recent), mouth of Mission Bay (Recent) IV. Upper Pico Formation (upper Pliocene or Pleistocene), Lower Etchegoin Formation (Pliocene), lower Imperial Formation (Pliocene or Miocene), Upper Wildcat Series (Upper Pliocene) and Pacific Coast, exposed, continental shelf bottom deposits V. Lomita marl (Lower Pleistocene or Upper Pliocene) and Catalina Island lee shore marl (Recent) Slides are presented to show locality map, anatomy, representative species. and occurrences in the Pliocene of the Midway-Sunset oil field, Kern County, California. End_of_Article - Last_Page 2088

The structure of the Santa Ana oil field in central Anzoategui comprises a subsurface thrust fault (Anaco thrust), with approximate strike N. 65° E. and northwest dip of about 45°, and three elongated oil-bearing domes oriented along the fault in its northwestern segment. The field is part of the Anaco structural trend which, with the same characteristics, continues northeastward for more than 40 kilometers to determine the San Joaquin, Guario, and Santa Rosa oil fields. Repeated sections measured in Santa Ana wells, ascribed to the Anaco thrust, vary from 80 to almost 1,500 feet, but stratigraphic throws are still larger in San Joaquin. In Santa Rosa, where no wells have yet penetrated the thrust, displacements may be of the order of 3,000 feet or more. The thr st fault is best known in the Santa Ana area, where ample subsurface control is available; it appears to die out upward at the Oficina-Freites contact. A detailed thickness analysis of the sedimentary column (upper Periquito and Oficina formations) in the Santa Ana area, supplemented by similar but less detailed information from adjoining areas, has revealed a continuous record of deformation, in concomitance with sedimentation, and has resulted in a structural history which confirms an earlier outline postulated on the basis of seismic evidence. According to this history, normal faulting (the present-day thrust) and doming were already present in Periquito time, that is, from the earliest available record. While folding continued uninterruptedly possibly to the Recent, normal fault growth stopped toward the close of Naranja time. A reversal of fault movement started before the end of Oficina time to produce the Anaco thrust, which h s the same attitude as the original normal fault. Several lines of evidence, such as the thickness trend of the Miocene Freites formation, indicate the uplift continued during post-Oficina sedimentation. The writers are not aware that a similar history has been proposed for other thrust faults, but it is quite possible that the Santa Ana-type fault is not unique. The data and conclusions of this work may be of interest to those concerned with the mechanics of deformation, which reason has prompted the writers to present their findings.

The paper discusses the importance of the overlaps in the southern part of the San Joaquin valley. Two of these overlaps are of remarkable magnitude: one existing at the base of the Vaqueros (lower Miocene) and the other at the base of the Santa Margarita (uppermost Miocene). Each overlap was preceded by an intense folding, and the structural features in these different periods of folding are evidently independent of each other. This is well illustrated in the lower Santiago Canyon in the San Emigdio district where Vaqueros beds are folded into an anticline that is not reflected in an underlying, apparently monoclinal, series of Oligocene beds. Not far from this locality a broad syncline of Etchegoin (Pliocene) overlies vertical beds of Vaqueros. The great overlap of the Santa Margarita and Etchegoin brings these formations into contact with all the older ones. They are therefore able to accumulate the oil produced in different strata of the older rocks. This may explain the great quantities of oil now present in the Etchegoin, without making it necessary to admit that the oil accumulated by migration over long distance.

Gas and oil occur in the Pliocene in a number of fields located along three major northwesterly-trending anticlinal structures of low relief in the east-central portion of the San Joaquin Valley. The upper member of the Pliocene, the San Joaquin Formation, is 1,200-1,800 feet of alternating brackish and marine clays and thin sands; the lower member, the Etchegoin, is 3,000-5,000 feet of largely marine shales and tight sandstones. The majority of the production comes from the First Mya-B Zone of the upper portion of the San Joaquin, at depths ranging from 2,200 feet at Semitropic to 2,800 feet at Harvester. Thickness ranges from 5 to over 50 feet, averaging about 10 to 15 feet. The more important fields (Trico, Buttonwillow, Semitropic) are primarily structurally closed el ngate domes, but lensing and stratigraphic trapping are important contributory factors to the accumulation in each field, and are the primary causes at Harvester. The Atwell Island sandstones, one or more of which are productive at Trico, Harvester, and Garrison City (?), are next in productive importance. They occur in the lower portion of the San Joaquin and are of cyclical or repetitive depositional character in the Trico-Harvester area. Additional productive zones in the San Joaquin are present at Northwest Trico, Semitropic, Buttonwillow, and Bowerbank. The Etchegoin is of considerably lesser productive importance than the San Joaquin, primarily because of lack of permeability. Productive gas zones include the Mulinia (Semitropic, Garrison City, and Bowerbank), Mitchel (Garrison City), and the "E-7" (Shafter). Indications of oil in the Pliocene so far have been observed only in the Etchegoin in this area, and only one zone, the Randolph (at Semitropic), is productive. The Randolph is a series of fine-grained, silty sandstones in the lower portion of the Etchegoin. Five wells are currently producing a total of about 110 B/D of 30° oil from depths of 7,000-7,650 feet. At least 700 acres so far have been proved productive. A combination of faulting and permeability changes probably controls the accumulation here. Non-commercial oil showings in the Etchegoin have been encountered at Buttonwillow, Wasco, and in additional zones at Semitropic, but none have been reported in the several deep tests at Trico. Recent drilling at Semitropic suggests that the Pliocene structure may be the result o deep-seated faulting and, consequently, prospects for deeper production from Miocene and Eocene sandstones may be considerably greater than previously suspected. End_of_Article - Last_Page 1082

The West Edison oil field is on the east side of the San Joaquin Valley about 9 miles southeast of Bakersfield. Oil occurs principally in the non-marine Chanac formation (Pliocene-Miocene) and and marine Santa Margarita sand (upper Miocene), and minor production is developed from marine Nozu sand (middle Miocene) and Olcese(?) sand (lower Miocene). The total thickness of strata overlying basement varies from 4,000 to 6,500 feet, and average depth of wells is about 4,000 feet. A diagram based on unusually good well control shows the transitional relationships between the Chanac and Santa Margarita formations. Pre-Santa Margarita stratigraphy and structure are not clearly understood because of the paucity of data, but stratigraphic cross sections are presented in an attempt to portray possible conditions during that time. The field is on a general regional homocline dipping southwest, and oil accumulation is largely due to normal faulting with partial help from lensing sands. Some of the larger pools in the field are trapped on the down-thrown side of normal faults. The field was discovered in 1935 and has had five periods of activity as a result of discovery of new zones and new fault blocks. There are now 1,300 acres productive from 180 wells. Total production is more than 8 million barrels or 6,200 barrels per acre, and 1951 showed the highest annual production in the field's history, 1,400,000 barrels. Current production is 4,200 barrels per day, or an average of 23 barrels per day per well. End_of_Article - Last_Page 186

Branchlike, 4-5 mm diameter concretions and a few associated irregular nodules of iron sulfide and its oxidation products found near the western margin of San Joaquin Valley, west of Los Banos, California, are the result of epigenetic cementation of fluvial sandy-clayey sediments of the Tulare Formation. The concretions commonly are oriented nearly vertically and form isolated clusters in an area of seepages of natural gas which contain both methane and hydrogen sulfide. Cementation was caused by chemical reactions of sediments and ground water with gases migrating along miniature, near-vertical passages.

The Pliocene of the San Joaquin Valley is defined and a description given of the sediments and the invertebrate fauna. Attention is called to the diastrophic history, the geologic occurrence of land vertebrates, and to the physical conditions under which the sediments were deposited. To facilitate the presentation of the subject several new names for stratigraphic units, faunal zones and diastrophic disturbances must, unfortunately, be used. To chronologize the Pliocene it will be necessary to refer to some heretofore unused time names. All new names, however, are used informally. End_of_Article - Last_Page 1877

The Corcoran clay represents a unique departure from the continental alluvial fan deposition of the Quaternary of the San Joaquin Valley. It is massive, silty clay, highly diatomaceous, 50-120 feet thick, generally 200-800 feet beneath the surface, extending over an area greater than 4,000 square miles. The Corcoran clay is involved in a southward plunging, open syncline of which the west flank is the steeper. Present evidence limits the age between extreme upper Pliocene and mid-Pleistocene. The lake in which it was deposited was probably formed by diastrophism in the Coast Range, by drowning of the San Joaquin Valley during a sea-level rise, or by construction of a fan by the Sacramento River and others east of Carqinez Strait, thereby damming the outlet of more southerly streams.

The Kettleman Hills oil field is one of California's largest oil fields and will be numbered among the major oil-producing areas of the world. It is in Fresno and Kings counties, California, along the western foothills of the San Joaquin Valley, approximately 180 miles southeast of San Francisco. The Kettleman Hills constitute a topographic feature more than 30 miles in length. Structurally they are composed of two well defined elongate domes, the North and Middle domes, and the northwestern end of what may be a third dome, frequently spoken of as the South dome. The North dome, the largest of the three, may be expected to be productive in an area of 15,000 or 20,000 acres. Oil and gas are produced prolifically from the Temblor formation of Lower Miocene age, which in the North dome is more than 1,500 feet in thickness. The writers discuss briefly the stratigraphy, structure, and mode of occurrence of oil and gas in this field.

The mineral composition of 192 outcrop and core samples from sandstones of late Upper Cretaceous, Paleocene, and Eocene age in the Los Banos district are tabulated. The mineral composition appears to afford few data for differentiating and correlating late Upper Cretaceous and Paleocene sandstones of the Los Banos district. The Eocene Domengine formation in the southern part of the area shows a marked increase in glaucophane as compared with the Paleocene and late Upper Cretaceous sandstones. The mineral suites are interpreted as indicating derivation from eastern and western sources, glaucophane being the most reliable indicator of a western source. Kaolinite, possibly of the anauxite type, occurs in white sands of Ione type in the Martinez (?) formation of possible Pale cene age, in outcrop sections and in a subsurface sand.