The Midway-Sunset Oil Field is located along the southwest edge of the San Joaquin Valley forming a 25 mile long and three mile wide field along the northeast-flank of the Temblor Range by the town of Taft. In 1995, the field’s average production was 163,400 BOPD mostly from steam enhanced recovery making it the largest producing field in the contiguous United States. Cumulative production through 1995 was 2.3 billion barrels of oil and 563 billion cubic feet of gas. The producing reservoirs are upper Miocene, Pliocene and Pleistocene age. The Miocene reservoirs are deep submarine fan sandstones encapsulated in diatomaceous mudstones which act as a source rock, a seal and as a fractured reservoir depending upon depth of burial. Diagenesis alters diatomite (opal-A) to a porcellanite (opal-CT) at 2000± feet and finally to a chert (quartz) at 5600± feet. The submarine fans were sourced by the Gabilan Range on the Salinian block (now 150 miles to the northwest) across the right lateral San Andreas fault. Northeast transport of debris and turbidity flows into the San Joaquin basin created time-transgressive submarine fans stepping to the northwest with migration of the Gabilan Range. Seismic events were probably a major cause of shelf and slope sediment failure creating landslides, debris and turbidite flows. Transpression and wrench tectonics along the San Andreas fault modified the San Joaquin basin margin slope into a series of northwest-trending folds (e.g., Buena Vista, Globe, South Belgian and Midway anticlines) with intraslope basins (synforms) affecting submarine fan morphology. Thick sandstone deposition was restricted to the intervening synclines (e.g., Midway syncline) and anticlinal saddles. Turbidites thinned or pinched out over anticlinal crests. Late Miocene uplift of the Temblor Range accelerated folding and submarine fan deposition. Northeastward-tilt of the Temblor anticlinorium subjected portions of the Miocene to erosion. Several Pliocene transgressions with intervening erosions deposited southwestwardonlapping shallow marine sequences of sandstones, siltstones and mudstones. Continued growth of the Temblors eroded both Miocene and Pliocene rocks depositing northeast-directed alluvial fans, braided stream, and lacustrine shoreline sandstones and mudstones prograding across the Midway syncline and Buena Vista and Globe anticlines. Oil is trapped by anticlinal four way closures as pinch outs across anticlinal noses and flanks, underneath angular unconformities and as fluid level traps. Miocene oil saturated diatomites are trapped anticlinally or under Pliocene unconformities and are sand frac’d (and steamed if heavy oil) to economically produce. The deeper buried naturally fractured opal-CT diatomaceous mudstones produce along anticlines. Pliocene and Pleistocene oil is trapped anticlinally as onlap pinchouts, underneath unconformities and as fluid level traps. The shallow Miocene, Pliocene and Pleistocene sandstone reservoirs contain heavy oil, thermally recovered by cyclic steam, steam drive and fireflood from more than 9600 producing wells. Estimated reserves of 454 million barrels of oil remain to be recovered.

The Temblor Range at the southwestern margin of the San Joaquin Valley is underlain by a diverse suite of Upper Cretaceous to Recent strata that record a complex succession of paleogeographic and paleotectonic events. A series of large, generally anticlinal oil fields, including the Midway-Sunset Oil Field, lie along the northeastern margin or northeast of the range. Because production in these fields is largely from clastic reservoirs that crop out to varying extents in the range, the geology of the Temblor Range has been the subject of a great number of previous published and unpublished studies that include field mapping, structure, stratigraphy, paleontology, sedimentology, and related disciplines. The new geologic map released in conjunction with this field trip guidebook as Plate 1 covers the portion of the range marginal to the northern Midway-Sunset Oil Field and covers only post-lower Miocene strata. These units include the Miocene Monterey Formation, the Pliocene and Pleistocene Tulare Formation, and diverse Quaternary nonmarine deposits. The Monterey Formation includes a succession of diatomaceous shales, altered diagenetically to varying mineralogies, and a number of sandstone bodies that appear to have been deposited as submarine canyons and submarine fans. The Tulare Formation rests with angular unconformity on the Monterey Formation and consists in outcrop of alluvial-fan deposits derived from erosion of the Temblor Range. A large number of generally northwest-trending folds are present in the mapped area and a few minor faults.

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 nonmarine Pliocene and Pleistocene Tulare Formation forms the stratigraphically highest subsurface reservoir in the Midway-Sunset Oil Field (Pl.1) It crops out extensively within and along the western flank of the field in the east-central Temblor Range, where it has been remapped recently by Nilsen (1995; P1.1). In outcrop it generally forms a gently northeast-dipping succession of beds that truncates with angular unconformity all older strata; locally, however, it has been folded and faulted by younger deformation (P1.1). At the northern end of the field and in the McKittrick area, it has been deformed into a series of closely spaced northwest-trending anticlines and synclines associated with the McKittrick fault, a young southwest- dipping thrust; in this area, Farley (1990) has mapped the "Tulare fold belt" in outcrop and subsurface. Santa Fe Energy Resources (SFER) produces significant quantities of heavy oil from the Tulare Formation in the Midway-Sunset Oil Field (Campbell, this volume). This production has been made possible by various enhanced oil recovery (EOR) techniques, including steamflooding and fireflooding. However, because the largely unconsolidated coarse clastic deposits that make up the Tulare Formation include lithologic units and facies that complicate typical EOR techniques, SFER has collected a number of long, continuous cores through the productive intervals of the Tulare Formation. These cores provide a suitable framework for the field trip core workshop. In the late 1980’s, three long conventional cores were taken from SFER Wells 121-25, 363-25, and 443-25 in sec. 25, T. 31S., R. 22E. These cored wells were from a SFER steamflood pilot of the Tulare Formation in the SW/4 sec. 25. These cores were described in detail by Applied Earth Technology, Inc. (AET, 1988) as part of a regional stratigraphic and sedimentologic study of the Tulare Formation. The core descriptions and interpretations presented here in are from the AET report. AET also conducted a supplemental study of the matrix composition and character of the reservoir conglomerates, using scanning-electron microscope techniques; some of these results are also reported herein. In the 1990’s, SFER obtained additional conventional core from the Tulare Formation in five Midway-Sunset wells. Two of these wells were cored across the basal contact of the Tulare Formation into the underlying San Joaquin Formation; core from one of these wells will be shown at the Core Workshop to represent the uppermost San Joaquin Formation (Nilsen, this volume).