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Geology of the Midway-Sunset Oil Field
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 southwestward-onlapping 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.
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