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Abstract: Fracture Analysis of Paleozoic Sandstones in Ramos field, Northwest Argentina

Clivio, Jose and Jose Luquez - Pluspetrol; Marco Sanguinetti* - Schlumberger

The Ramos field is located in Salta province, northwestern Argentina. The sequence stratigraphy in this area is composed mainly by Paleozoic sediments (Silurian-Carboniferous) and by a lower Cenozoic cover, all affected by a Cenozoic tectonic. The main structure is given by a north-northeast elongated anticline that belongs to a part of the called Subandina Fold Trust Belt.

The main reservoir is made up of highly siliceous cemented quarzitic sandstones that form the Huamampampa formation (Devonian). This formation was deposited in a shallow marine environment (shoreface facies). These sandstones contain a great number of natural fractures as a product of the superposition of several compressional events. Most of these fractures are tensional open or partially open, and confer to this reservoir a very good fracture porosity.

One of the early problems in the history of this field was to characterize the fracture system and explain the differences in hydrocarbon production, from the Huamampampa fractured sandstones, in some wells drilled at different locations or with different orientations. Outcrops information was too distant to be directly applied. Core samples were scarce and there was little recuperation on the surface. Conventional logs helped but they did not define exactly the problem. Consequently, borehole images turned out to be the most accurate tool to define the fracture features, such as orientation, morphology, distribution (density), aperture and porosity.

Borehole images showed two different scales of natural open fractures: macro-and mesoscale (Fig. 1). These consist of a dominant regional conjugated set that strikes west-northwest and west-southwest (Type I), that is related to the principal compressional stress direction (west-east), and two subordinate north and north-northeast-striking sets (Type II), that are related to the local structure bending. The Type I fracture set is generally steeply dipping toward north-northeast and north-northwest, normal to bedding, and evenly and widely spaced. Fractures of the Type II set also dip steeply toward east and east-southeast, but are more closely spaced than the west-northwest and west-southwest-striking fractures, parallel to bedding. The Type II fracture set has a higher aperture and porosity values than the Type I. A fluid transmissibility given by interference tests made on some continuous wells was found to be parallel to the Type II fracture set direction.

The identification of different fracture sets together with their properties, are the most important parameters in a fractured reservoir. These are able to find out the most productive fracture sets, the highest hydrocarbon potential zones and to place future wells at optimum locations, in particular horizontal wells.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil