ABSTRACT

Yegua sandstones at a depth of 8300 to 8580 ft (2530 to 2615 m) were partly cored in the ARCO No. 1 Jansky dry hole. Total thickness of the sandstone section is approximately 240 ft (73 m). The sandstones are enclosed in thick marine shales and are about 20 mi (32 km) downdip from thicker and more abundant sandstones in the Yegua Formation. The section is similar to reservoirs recently discovered in the area at the Toro Grande (1984), Lost Bridge (1984), and El Torito (1985) Fields. The sandstones are fine to very fine grained and occur in thin beds that are 0.5 to 9 ft (0.15 to 2.7 m) in thickness. Sedimentary structures within the beds range from a lower massive division to a laminated or rippled upper division. Grain size within beds fines upward from 0.18 mm at the base to 0.05 mm at the top. The sandstones are interpreted to be turbidites of the AB type that were deposited within channels. The sandstones contain an average of 50% quartz and are classified as volcanic-arenites to feldspathic litharenites. Carbonate cement is variable from 0 to 27%. Average porosity is 29% and permeabilities are in the range of 60 to 1600 md in the clean sandstones. Much of the porosity is secondary and is the result of the dissolution of cements, volcanic rock fragments, and feldspar grains. Yegua sandstones produce gas and condensate at nearby Toro Grande Field on a gentle, faulted anticline. The local trend of reservoir sandstones may be controlled in part by faulting that was contemporaneous with deposition.