The upper Miocene Potter sandstone reservoir of the Santa Margarita member of the Monterey Formation was deposited in deep water as a coalesced turbidite fan along the west flank of the present day San Joaquin Valley. Contributing over 35 percent of the cumulative production of the giant Midway-Sunset Field, this reservoir is currently producing more than one half of the field’s daily production of 162,000 barrels of oil per day (BOPD). Thermally enhanced oil recovery (TEOR) techniques are routinely used to produce the heavy, viscous oil in these steeply dipping sand packages where gravity drainage is a key factor. Producing since 1910, the field development practices have progressed from conventional primary to cyclic steam to steamflooding and includes some active fire floods. The dynamic continually changing reservoir conditions, due to these recovery processes, add complexity to formation evaluations. With the advent of horizontal drilling this reservoir has been and continues to be one of the most significant at Midway-Sunset Field

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.

Well No. l8R is located in the northwest of the southwest of the northeast quarter of section 21, T31S, R22E, Midway-Sunset field, Kern County, California (refer to the Potter Sandstone paper, this volume). The interval of core that will be on display is shown on the electric log in Figure 1.

Abstract Numerous types of sandstone bodies encountered in the subsurface have two characteristics in common — they are elongate and lenticular in cross section. These characteristics apply to sandstones resulting from fringing-beach, offshore-bar, chenier, strike-valley, sand-wave, tidal-current, and channel deposition, as well as to some turbidites and some component parts of deltas. Of all these types of sandstone, the channel type is very unusual in that it commonly is deposited subaerially and thus can be observed and studied more readily on the surface. In recent years, many pertinent papers dealing with outcropping ancient channel sandstones have been published. The reader is referred particularly to the following authors for basic information: Fisk (1944), Wilson (1948), Lins (1950), Siever (1951), Pepper et al. (1954), Hopkins (1958), Friedman (1960), Schlee and Moench (1961), Andresen (1961, 1962), Potter (1962a), Potter and Pettijohn (1963), and Bernard and Major (1963).

Abstract The subject of secondary porosity in sandstones received little attention in the published literature prior to 1975. Most workers considered only a very small portion of sandstone porosity to be of secondary origin. Except for fractures, most accounts of secondary porosity described outcropping sandstone formations where invading meteoric water had removed soluble constituents (Sedimentation Seminar, 1969; Hraber & Potter, 1969). Krynine (1941) indicated that leaching of carbonate in the Oriskany Sandstone may possibly have occurred in the subsurface. Proshlyakov (1960) was probably the first author The reservoir aspects of fields producing from secondary-sandstone porosity are often more like those of carbonate reservoirs rather than those of fields with primary sandstone porosity. The distribution of secondary porosity in a prospective or productive sandstone formation may not necessarily show a direct relationship with depositional lithofacies or burial history and can be difficult or even impossible to predict with conventional subsurface methods. However, detailed geological and petrological analysis of the factors that control the occurrence of secondary porosity often greatly enhance the understanding and prediction of its distribution. Sandstone reservoirs with secondary porosity have special problems but they also offer special opportunities for both the explorationist and the exploitationist. It is necessary, firstly, to recognize the secondary nature of the porosity of these reservoirs; secondly, to interpret controls and timing of the origin of their porosity; thirdly, to evaluate any factors that may have played a role in preserving this porosity; and, fourthly to survey the existing pore geometry in order to optimize