In January, 1991, Santa Fe Energy Resources conventionally cored 660 ft (201 m) of the upper Miocene Spellacy sandstone in well 495-17, located in section 17, T32S, R32E, Midway-Sunset field, Kern County, California (Fig. 1). Section 17 is a Santa Fe Energy fee property that has produced heavy oil (10-14° API) since 1906, and currently has approximately 165 active wells producing an average of about 7 barrels of oil per day (BOPD). Since the mid-1960’s, thermal enhanced oil recovery (TEOR) methods have been used on this property in the form of cyclic steam to increase the production of the high-viscosity crude oil (1,200-10,000 cp at 90°F (32°C)). Well 495-17 was cored to document the basic reservoir characteristics for a four-pattern steamflood pilot, which began operation in 1991.

The Midway-Sunset Oil Field lies along the tectonically active western margin of the southern San Joaquin basin, only 6-7 mi (9.6-11.2 km) east of the western edge of the North American plate. Upper Miocene Spellacy coarse clastics were derived from a granitic source located nearby to the southwest. Poorly sorted conglomerates and arkosic sands were transported through canyons cutting the upper slope and were deposited by sediment gravity flows in a deep water, intra-slope environment that was actively folding. The flows typically deposited normally graded and amalgamated beds that form hydrocarbon-bearing sandstone bodies with high porosities (23-34%) and high horizontal permeabilities (500-4,000 md). Spellacy reservoirs at Midway-Sunset field produce significant quantities of typically heavy (11-14° API) oil from shallow depths. After more than 90 years of production, many Spellacy reservoirs are still in the process of being developed by delineation and infill drilling. Well spacings of 5/8 acre (165 ft / 50 m apart) are common in spite of the high reservoir quality because of both highviscosity oil and low reservoir pressures. Thermal stimulation has been used extensively since the mid-1960’s to increase production rates.

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.

Abstract Seacliff outcrops in the Paleogene Talara forearc basin of northwestern Peru provide a nearly continuous exposure of a 290-m (951-ft)-thick succession of coarse-grained turbidites of upper Eocene age. Lithofacies include thick, amalgamated conglomerate layers, thick-bedded sandstone layers, and interbedded sandstone and mudstone layers. The basal 210 m (690 ft) is a coarse-grained succession composed of fining-upward packages, averaging 20 m (66 ft) thick, with each package including conglomerates at the base and sandstones at the top. The top 80 m (262 ft) is a finer grained succession of fining-upward packages, each composed of amalgamated sandstone layers at the base and interbedded thin-bedded sandstones and mudstones at the top. Coarse-grained units range from pebble conglomerate to coarse-grained sandstone, are up to 7 m (>3 ft) thick, and often pinch out laterally as a result of erosion by subsequent flows. Unlike most thick-bedded, coarse-grained turbidites, these units are dominated by current-structured, SI turbidite divisions that are often capped by intervals of large-scale cross-stratification (Tt turbidite divisions). The abundance of traction structures, coarse grain size, and abundant scoured contacts indicate that the depositing high-density flows were highly erosive and long-lived, and that large amounts of finer grained sediment probably bypassed to more basinward settings. Overall, the succession fines upward and is interpreted to represent multistory, aggradational, coarse-grained channel-fill deposits overlain by finer grained, smaller channel and associated out-of-channel deposits.

This field trip is an overview and reappraisal of the prolific oil basins of southern California (Fig. 1A) using exploration methods now commonly used in international exploration. As a result of the dramatic decline in oil and gas exploration in California during the last decade these mature and well known basins have received limited modern hydrocarbon research and it is hoped that our field trip and guidebook will outline some of the important aspects and questions of these intriguing petroleum systems. We have used balanced cross sections and other types of structural analyses integrated with basin modeling, geochemical and geophysical data to gain new insights into the structure, trapping mechanisms, and petroleum systems (Magoon and Dow, 1994) in a setting combining strike-slip and convergence (transpression). Southern California geology also has the scientific advantage, but societal disadvantage, of earthquakes (Fig. IB) which provide useful data about the deeper structure which will be presented during the trip. Our field examples are in the eastern Ventura basin, Ridge Basin, southern San Joaquin basin, Cuyama basin and western Ventura basin as well as a transect of the western Transverse Ranges (Fig. 1A).