Influence of Silt Zones on Steam Drive Performance Upper Conglomerate Zone, Yorba Linda Field, California
Authors: Cook David L. 
Published: December 2007

Influence of Silt Zones on Steam Drive Performance Upper Conglomerate Zone, Performance Upper Conglomerate Zone, Yorba Linda Field, California
This paper describes steam drive and production performance in California's Yorba Linda field where silt layers are discontinuous and create localized barriers to vertical permeability. Improved recovery was obtained by selection of proper injection and production points with respect to the position of silt layers within the reservoir. position of silt layers within the reservoir.

Introduction
The Yorba Linda field is located on the eastern edge of the Los Angeles Basin, approximately 30 miles southeast of central Los Angeles (Fig. 1). It is producing 12 to 15 deg. API gravity oil from Pliocene submarine fan channel turbidites. Production is from five separate reservoirs, ranging in depth from 400 to 2,500 ft (Fig. 2). The Upper Conglomerate, located at the top of the section, has more than 70 percent of the reserves and produces the lowest API gravity oil. Net pay averages 300 ft with porosity approximately 30 percent and permeability 600 md.
The present structure is a gentle homocline with beds dipping from 10 to 15 deg. to the southwest. The beds steepen to the north and outcrop within a short distance from the field. The oil reservoir is bounded by a tilted oil/water transition zone downdip and is overlain by a low-pressure desaturated (air) zone. The reservoir contains essentially no gas and gravity drainage is the producing mechanism.
The Conglomerate zone was discovered in 1954 and developed by primary drilling through 1959. Because of the high viscosity of the oil, ultimate primary recovery was estimated at only 5 percent of the original oil in place. However, with development of steam soak place. However, with development of steam soak operations in 1961, the expected recovery efficiency was increased to 35 percent. During the 1960's, several steam drive pilot tests were conducted. These indicated that conversion from a steam soak to a steam drive operation at an optimum time based on heat communication in the reservoir resulted in a recovery ranging from 45 to 55 percent of the oil in place. In 1971, a pilot Upper percent of the oil in place. In 1971, a pilot Upper Conglomerate steam drive was initiated and was expanded in 1973 and 1974. Today, there are 16 permanent injectors in the Upper Conglomerate.
In contemplation of a drive expansion throughout the entire Upper Conglomerate zone, a study was made to determine better the reservoir continuity. One of the most striking geological features in the field is the discontinuons nature of the silt zones. These zones were mapped and studied in relation to production performance of nearby wells. It became apparent that the silt layers greatly influenced fluid and steam migration and thus well performance. Some guidelines were established to improve well completion practices with relation to the silt zones.

Silt Zone Distribution Within the Reservoir
The Conglomerate zone consists of an assemblage of silts, sands, and conglomerates deposited on the eastern edge of a deep Pliocene submarine basin. A close source rock resulted in a large-size range of sediments, ranging from conglomerates to silts, being brought into the basin. The sediments are interpreted to have been carried and deposited through a submarine fan channel system that trends from northeast to southwest.
The silt layers are believed to be fill deposits in abandoned channels. They are generally lenticular and discontinuous. The major silt breaks range to more than 100 ft thick and extend over one-third of the producing field.
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