ABSTRACT: Subsurface Structure in the Montezuma Hills, Southwestern Sacramento Basin
KRUG, GENE, Consultant, Shingle Springs, CA, VIC CHERVEN,* Consultant, Cameron Park, CA, JIM ROTH, International Exploration Advisors, Houston, TX, and CHARLIE HATTEN, San Carlos Oil and Gas, Pasadena, CA
The Montezuma Hills occupy approximately 100 sq mi in the southwestern corner of the Sacramento basin between the Kirby Hills on the west and the Sacramento River on the east. The structure in this area is dominated by the deep synclinal axis of the so-called Rio Vista subbasin. The Eocene Domengine Formation reaches depths in excess of 7000 ft along this axis, which trends roughly north-south.
The basin is bounded by the north-trending Kirby Hills fault on the west and the N45 degrees W-trending Sherman Island Fault on the east. Because the Sherman Island structure is a wide zone of subparallel splays, it intersects the Kirby Hills structure over a broad area in the northwest corner of T4N, R1E and the southwest corner of T5N, R1E, in the general vicinity of the Denverton Creek gas field. Closures along the splays of the Sherman Island zone are primarily responsible for gas accumulation at Denverton Creek.
The Sherman Island zone was mapped using a grid of 6-fold, 12-fold, and 24-fold seismic data, supplemented with well control in the Denverton Creek and southwest Rio Vista fields and the available wildcat wells in between. Structure contour maps were constructed on four horizons: Upper Cretaceous H & T shale; upper Paleocene McCormick sand; upper Paleocene Anderson sand; and lower Eocene Domengine Formation. Structure contour maps on the Upper Cretaceous Peterson member of the Starkey Formation and the lower Eocene Hamilton sand were also made in the Denverton Creek area, using well data only. The seismic data terminate along the eastern margin of the Kirby Hills, so the Kirby Hills Fault was not mapped.
Some mapped horizons are cut by as many as eight splays of the Sherman Island fault, forming a zone of faulting at least 2 mi wide. Along the eastern margin of the zone, most blocks are up to the west, in the central and western part of the zone, blocks may be either up or down to the west. Displacements range from 25 to over 400 ft on some horizons. Most faults die out upward and do not offset the Domengine Formation, and appear to be normal, although strike-slip displacement cannot be ruled out.