Downdip Yegua sandstones at a depth of 8300 to 8580 ft (2530 to 2615 m) were partly cored in the ARCO Jansky 1. 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 is controlled in part by faulting that was contemporaneous with deposition.

Correlation of the available cores with the dip log shows that zones of increasing dip (see diagram) are structural, and not depositional, features. The pattern of increasing dip downward forms normal fault patterns that coincide with contorted beds in the core. The faulting apparently caused soft sediment slumping shortly after deposition.

Our interpretation holds that the Yegua stratigraphic and structural relationships in the Toro Grande area demonstrate turbidite transportation across the shelf from the Yegua delta complex to a depositional site on the downthrown flank of an active growth-fault structure.

Figure 1. Diagrammatic illustration of rotated slump block in the downdip Yegua turbidite channel sandstones, Cities Service Dincans 1, Gas Unit 1, Toro Grande field, based on core examination and dip-log interpretation. Slumped section is about 60 ft (18.3 m) in thickness.

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