The upper Miocene Monterey shale units of the Midway-Sunset field and the San Joaquin basin are the main source rocks for many of the oil producing formations. The Monterey Formation is derived from diatom frustules making the rocks rich in biogenic silica. The four siliceous members of the Monterey Formation in Midway-Sunset are the McDonald, Antelope, Belridge, and Reef Ridge shales with the entire section being over 5000 feet thick and interbedded with the Williams, Republic, Spellacy, and Potter sandstones. These siliceous rocks are found as unaltered amo rphous opal-A, and its diagenetic equivalents of opal-CT and quartz. The rock is characterized by porosities ranging from 70-10% and permeabilites less than 0.1 millidarcy. Since 1978, the members of the Monterey Formation have become major producing reservoirs on the west side of the San Joaquin basin as a result of hydraulic fracturing and/or cyclic steaming. Commercial production was established at Midway-Sunset by Santa Fe Energy Resources in 1985 in the Reef Ridge Shale and to date over 160 wells have been drilled to develop this reservoir. At least seven other areas in Midway-Sunset have had drilling and completion attempts aimed towards establishing commercial potential with mixed results to date from Miocene Shales.

The Midway-Sunset Oil Field produces primarily from Neogene sands and gravels which are found within the upper Monterey Formation, the Reef Ridge Shale, the Etchegoin Formation, the San Joaquin Formation and the lower and middle Tulare Formation. Age dates based on radiometric and paleontological analyses suggest that these producing horizons range in age from about 8.9 Ma (late early Mohnian) to about 1.0 Ma (Pleistocene).

ABSTRACT The South Belridge field is located in western Kern County, California. One reservoir in the field, accounting for about 20% of the daily field oil production, is the Belridge diatomite member of the Reef Ridge Shale. The field is developed on a elongated anticline. Within the field, normal faulting appears in three episodes: early Pliocene large offset lystric faults, the middle Pliocene large offset Middle Belridge fault, which geologically separates the northern portion of South Belridge field from the southern portion, and late Pliocene small offset vertical faults. The small offset faults, along with the natural fracture system, are the primary permeability conduits in the Belridge diatomite reservoir. In northern South Belridge field the natural fracture system trends Ν45°Έ and is near vertical. Around section 12, T28S, R20E, MDB&M, four intervals within the Belridge diatomite are productive: the 12 Zone, upper diatomite, middle diatomite, and the lower diatomite; all of the lower Belridge diatomite north of the Middle Belridge fault lies in the opal-CT facies and is a distinct hydrocarbon reservoir. Lithologies of the three opal-A intervals vary from diatomaceous mudstone to silty diatomite. Productive zone thicknesses range from an average of 100 ft to 775 ft. Average effective porosity is 36.7% to 55.4%, average helium permeability is 1.86 md to 103 md (0.11 md to 0.56 md to brine), and average free water saturation is 55.4% to 69.0%. Formation waters vary from 25,085 ppm TDS to 40,000 ppm TDS with resistivities of 0.35 ohms to 0.17 ohms at 25° C. Oil gravities range from 22.2° API gravity to 32° API gravity. Diatom floras from wells in South Belridge field show the middle and lower diatomite to be late Mohnian to middle Delmontian and the upper diatomite and 12 Zone to be late Delmontian. The section is probably correlative to the Bitterwater Creek Shale of the southern-most Temblor Range and is time-correlative to the Sisquoc Formation diatomites of coastal California. The diagenetic transition of Belridge diatomite from opal-A to opal-CT occurs between 2000 ft and 2300 ft and is coincident with the top of the lower diatomite interval. The transition from opal-CT to quartz occurs at about 3700 ft. Formation temperatures within the Belridge Diatomite suggest that there has been 925 ft of post-opal-CT/quartz transition uplift and 300 ft of post opal-A/opal-CT transition uplift in South Belridge field. The coincidence of the opal-A/opal-CT transition with the top of the lower diatomite suggests that diagenesis preceded structural deformation. Using average compacted sediment deposition rates, the estimated time to deposit the Belridge diatomite was 1,643,400 years. Comparison of the lower Belridge diatomite sections at the north and south ends of South Belridge field suggests that the relative basinal subsidence during deposition was -0.48 ft/1000 years.

ABSTRACT A light oil steamflood pilot (LOSF) in the Shallow Oil Zone (SOZ) is under evaluation at the Naval Petroleum Reserve No. 1 (NPR-1), the Elk Hills field in Kern County, California. The Shallow Oil Zone is defined by the Elk Hills Engineering Committee as all oil and gas bearing rocks of Pliocene age above the Reef Ridge Shale, except the dry gas-bearing rocks above the top of the Scalez marker bed. The Sub-Scalez I (SS-1) of the SOZ is the formation steam being injected. The SS-1 is a unit of the Scalez sand zone in the San Joaquin Formation of Pliocene age. From a reservoir perspective, the steamdrive process is behaving in a sequence of predicted events. The first expected event was the appearance of fresh water accompanied by carbon dioxide at the producing wells; this happened 3 months after the start of steam injection. The second event, an increase in API gravity of produced crude, appeared 3 months later or 6 months into the project. Finally, the arrival of the heat front at producing wells was detected 13 months after startup.

ABSTRACT Surface subsidence due to reservoir compaction during production has been observed in many large oil fields. Subsidence is most obvious in coastal and offshore fields where inundation by the sea occurs. Well known examples are Wilmington field in California and Ekofisk field in the North Sea. In South Belridge field the Belridge diatomite member of the late Miocene Reef Ridge Shale has proven prone to compaction during production. The reservoir, a high porosity, low permeability, highly compressive rock composed largely of diatomite and mudstone, is about 1000 ft thick and lies at an average depth of 1600 ft. Reservoir compaction within the Belridge diatomite due to withdrawals of oil and water in section 12, T28S, R20E, MDB&M, was noticed after casing failures in producing wells began occurring and tension cracks, enlarged by hydrocompaction after a heavy rainstorm, were observed. Surface subsidence in section 12 has been monitored since April, 1987, through the surveying of benchmark monuments. The average annualized subsidence rate during 1987 was -1.86 ft/yr, -0.92 ft/yr during 1988, and - 0.65 ft/yr during 1989; the estimated peak subsidence rate reached -7.50 ft/yr in July 1985, after l½ yrs of production from the Belridge diatomite reservoir. Since production from the Belridge diatomite reservoir commenced in February, 1984, the surface of the 160 acres producing area has subsided about 12.50 ft. This equates to an estimated reservoir compaction of 30 ft in the Belridge diatomite and an average loss of reservoir porosity of 2.4% from 55.2% to 52.8%. Injection of water for reservoir pressure maintenance in the Belridge diatomite began in June, 1987, and has been effective in mitigating subsidence and repressurizing the reservoir to near-initial pressure. An added benefit of water injection has been improved recovery of oil from the Belridge diatomite by waterflood. The operation of four other water injection projects in South Belridge Field in the Belridge diatomite reservoir suggests that reservoir compaction and surface subsidence is a common occurrence in the field.

ABSTRACT During late Cenozoic time, the San Joaquin basin was an inland sea that was connected to the Pacific Ocean through a relatively narrow strait west of Coalinga, California. The San Joaquin sea became progressively restricted in late Miocene and Pliocene time due to the northwestward migration of the Salinian block and uplift of the Temblor and Diablo Ranges, and by the beginning of Pleistocene time, the sea had completely withdrawn from the San Joaquin basin. The upper Miocene and Pliocene (~7 to ~2.2 Ma) Etchegoin Group forms the basis for the interpretation of the late Neogene regressive history of the San Joaquin basin. The Etchegoin Group is comprised of the Jacalitos, Etchegoin, and San Joaquin Formations, that together attain a maximum thickness of 8100 ft (2470 m). These formations consist of siltstone, sandstone, conglomerate, and rare tuff beds that are arranged in the following facies associations: low-energy tide-dominated, high-energy tidedominated, low-energy shelf, and storm-dominated shelf facies associations. These facies associations include middle to outer shelf deposits, and tidal channel, tidal flat, intertidal shoal, and sand-wave deposits formed in estuarine environments. The Jacalitos Formation (maximum thickness of 2477 ft; 755 m) is composed of sandstone that was derived dominantly from the Coast Ranges and a granitic terrane west of the San Andreas fault. The Jacalitos Formation records the shallowing of the San Joaquin basin that began during deposition of the underlying Reef Ridge Shale in the late Miocene. Marine invertebrate macrofaunal assemblages suggest that water depths during deposition of the upper part of the Jacalitos Formation reached a maximum of 121 ft (37 m). The Etchegoin Formation (maximum thickness of 5485 ft; 1672 m) consists of sandstone that is rich in andesitic detritus, and was mainly derived from the Sierran magmatic arc, the Quien Sabe volcanics, and uplifted Franciscan-Coast Range source terranes. Deposition of this formation occurred largely in bay and estuarine environments. Sandstone of the San Joaquin Formation (maximum thickness of 2232 ft; 680 m) was derived from the dissected Sierran magmatic arc, the Coast Ranges, and possibly the Gabilan Range west of the San Andreas fault. Brackish-water conditions periodically existed during deposition of the San Joaquin Formation and were associated with restriction of the San Joaquin sea. Well-log cross sections of the lower Etchegoin Group that are correlated with measured surface sections are presented in order to illustrate typical well-log signatures of the Etchegoin Group and to show that the depositional environments and geometry of the sandstone beds interpreted from outcrop exposures are manifested in the subsurface. The Jacalitos Formation is characterized by relatively flat Spontaneous Potential (SP) curves, representing siltstone, and by serrated SP and resistivity curves, representing interbedded siltstone and sandstone. The Etchegoin Formation in the study area is marked by a sharp-based, fining-upward sandstone unit at its base, and is dominated by fining-upward sandstone units, interpreted as tidal channel deposits, and blocky to coarsening-upward sandstone units, interpreted as intertidal shoal and sand-wave deposits. The channels and shoals are generally elongate in the northwest-southeast direction. Many of the sandstone units are lenticular and encased in finer grained sedimentary deposits, suggesting that the Etchegoin Formation sandstone units could form good petroleum reservoirs. The principal factor controlling the late Miocene and Pliocene sedimentary history of the San Joaquin basin was tectonism. Tectonic subsidence of the San Joaquin basin clearly kept pace with sedimentation in order to allow for the accumulation of up to 8100 ft (2470 m) of sediment deposited at intertidal to neritic water depths. During the middle to late Pliocene, rates of sediment accumulation exceeded rates of basin subsidence, leading to the regression of the San Joaquin sea.