Re-assessment of the Cerro Toro (Chile) Sandstones in View of Channel-Levee-Overbank Reservoir Continuity Issues

Coleman, Jr., J.L., BP-Amoco Upstream Technology Group, Houston, TX, USA 77079

Abstract

Outcrops of the Cretaceous Cerro Toro sandstones within Torres del Paine National Park (southern Chile) have been described in a submarine channel-levee-overbank context, with the adjacent, thin, levee-overbank beds genetically associated with the mai n channel bodies which are present as post-erosional fill. Examination of key outcrops indicates that a more complex association exists between the thick- to massively bedded channel-fill sandstones and the laterally adjacent thin-bedded sandstones.

The Cerro Toro sandstones are composed of three primary rock types:(1) gravely sandstone, containing clasts ranging from coarse sandstone to small boulders; (2) fine- to medium- (at times coarse-) grained quartzo-feldspathic sandstones and siltstones in s heet sandstones with few or no sedimentary structures, probably deposited in lobe or distal overbank facies; and (3) fine- to medium-grained quartzo-feldspathic sandstones and siltstones in sheet sandstones, with abundant sedimentary structures indicative of the levee/overbank facies.

The gravely sandstones occupy multi-storied, channel-fill positions and rest on erosional surfaces cut into sandstones, siltstones, and/or shales. The gravels rest only in erosional contact with the lobe sandstones and not the overbank sandstones. No thick- to massively bedded gravely sandstones are found to be genetically related to the overbank sandstones.

A reservoir model based on the Cerro Toro sandstones of the Torres del Paine area would be composed of a laterally extensive, thin-bedded lobe sheet sand having a high degree of lateral continuity punctuated by channel-fill sandstones having a highly vari able internal lithologic and potential fluid flow character.