Case study of Elkhorn Slough Field, Solano County, and Grand Island Field, Sacramento County, California: Pre-3-D versus post-3-D field development

Abstract

Elkhorn Slough Field in Solano County, California, is on the eastern flank of the Sacramento Valley, approximately 25 miles southwest of Sacramento. Grand Island Field is in Sacramento County, approximately five miles southeast of Elkhorn Slough Field (Figure 1). Both fields are between the updip edge of the Upper Winters sand pinchout and the beginning of the Winters structural play. Figure 1. Location map. The Winters pinchout play was defined with regional geologic mapping through the basin. It is turbiditic in nature, sands being transported through channels incised into the shelf and deposited into deepwater fans, surrounded by shales. The sands were deposited in massive pulses, one flow possibly cutting into and welding into the previous one. This makes it difficult to distinguish the sands from one another on the seismic because they thin in an updip position. The sands have excellent porosities (18–25%) and thicknesses up to 115 gross ft. In Elkhorn Slough and Grand Island Fields, the depths of these sands are approximately 8500–8600 ft. These elements have made the Winters in this area conducive to AVO analysis. In defining this play regionally, several key mapping tools were used to identify these prospects. They included structural noses due to differential compaction around the sand bodies (which create structure where the sands exist); net sand isopachs to delineate the existence of reservoir; and seismic amplitudes to show potential gas accumulation. In the Elkhorn Slough area, we began with 2-D seismic, mapped the structures, and used them with the regional sand maps to delineate possible fan limits. In 1992, in partnership with EOG Resources (formerly Enron Oil and Gas), Amerada Hess began drilling wells within the fan. The initial well, Enron's 22–1 Nixon (well X-1), was a gas producer from 15 ft of the A sand and had 42 ft of wet …