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).

Although the Etchegoin Formation is considered to be Pliocene in age, K-Ar and 87 Sr/86 dating indicates that the lower part of the formation may be of late Miocene age. Molluscan fauna and foraminiferal assemblages indicate it was deposited in shallow-marine to intertidal and estuarine environments. Trapping of oil in the Etchegoin is caused by the pinchout or truncation of individual sandstone members often against the flanks of anticlines. The Etchegoin Formation produced most of the first one billion barrels of Midway-Sunset’s 2.4 billion barrels of cumulative oil production indicating much future potential remains.

The San Joaquin Formation is the youngest unit of the upper Miocene and Pliocene Etchegoin Group and provides a record of the late Neogene history of the San Joaquin basin. During Pliocene time, the San Joaquin basin had become an inland sea that was connected to the Pacific Ocean through a narrow strait along the western side of the basin. The Pliocene San Joaquin Formation unconformably overlies the Etchegoin Formation and underlies conformably to unconformably the Tulare Formation. At Midway-Sunset, the San Joaquin Formation ranges in thickness from 150 ft in the north to 1100 ft in the south. It is divided into two main zones, the Mya (Mya Tar) and Top Oil (Scalez). The reservoirs deposits are clay-rich, very fine to fine grained sandstones alternating with siltstone and claystone. Fossil assemblages indicate shallow water deposition with alternating brackish and freshwater conditions. Production from the San Joaquin Formation, at the Midway-Sunset field, is minor with production figures not available because it has been commingled with production from the Etchegoin and Tulare Formations. The oil is 12° to 28° API gravity and is found in both stratigraphic and structural traps. San Joaquin Formation oil production at Midway-Sunset field has been maintained in part by use of waterflooding and steamflooding since 1965.

In 1991 and 1993 two seismic surveys were acquired across and in the vicinity of the Midway-Sunset Oil Field. High quality imaging has been difficult to obtain due to noise from the thousands of pumping wells, and steamflood operations associated with the field. Complex subsurface structure has also contributed to the problem. This paper discusses the acquisition and processing history of these surveys and the numerous methods applied to analyze and to reduce the effects of "production" noise. Also, discussed is the application of a mini-3D processing technique on one test line. This technique has provided results indicating that imaging quality of a 3D survey may be substantially su- perior to that of comparable 2D data.

The Pliocene Etchegoin Formation forms a distinctive subsurface reservoir unit in the Midway-Sunset field. Because it does not crop out in the adjacent Temblor Range to the west (P1.1), conventional core data is critical for informed interpretations of its reservoir character, facies, and depositional systems. Santa Fe Energy Resources (SFER) recently obtained a 50.6-foot-long conventional core of the Etchegoin Formation in well 504 in section 23, T31S, R22E (see Wylie and Huntoon, Fig. 3, this volume) that is included herein for examination. The cored interval was described quickly and not in great detail by Nilsen in early 1996.