THE GUAVIO ANTICLINE, A SURFACE ANALOG MODEL OF A SUBSURFACE STRUCTURE, LLANOS FOOTHILL, COLOMBIAN ANDES

The Guavio Anticline is located in The Colombian Llanos Foreland Basin. The structure is composed by units ranging in age from Paleozoic to recent. In this area, geologists have discovered, important oil fields in the last decade, an a interest for this portion of the Andean basin, has been growing in the last years. The Guavio Anticline is an structure composed by rocks ranging in age from Paleozoic times to recent deposits. At surface tertiary rocks are outcropping, and cretaceous and Paleozoic rocks are buried. As oil reservoirs, several units had been recognized; into the cretaceous system there are three oil source units, and one more, of paleogene age is also very important, being these: Albian age (Une Formation Ksu), Upper cretaceous age (Guadalupe Formation Ksg), lower Paleogene (Barco Formation), and the most important of Miocene age Mirador Formation , which is recognized extensively along the llanos foothills. Furtheremore the Carbonera Formation (of Oligocene age) is considered as, as reservoir, anyway, it's primary porosity is not well developed, but secondary porosities had shown remarkable values, these derived of the development of fracture systems, in this way it's now known that it`s secondary porosity allows the fluid flow, in some areas like the Mirador Formation. In the Guavio Anticline we had found three fracture sets. The mean fracture system has two sets, the first is parallel to the fold axis and the second it's almost orthogonal to the first, the third set, appears to be apart of the principal fracture systems, and it shows at outcrop cutting obliquely the former described systems, other systems described, could be present due fault presence. Outcrop data shows that the obliquous set it's the youngest, probably due the last Andean orogenic pulse, while the age of the mean fracture system results to be pre-folding or at least synfolding. The analysis of calculated parameters as fracture density or intensity, allows to characterize the fracture systems present in the Guavio Anticline mostly in areas with a low fracturing degree. This tectonic discrimination will allow, to define zones with presence of well developed fracture systems, and in consecuence, of more well developed secondary porosities, which in late investigations can contribute to find of new fractured oil reservoirs, since this structure in surface, reflects analogiacally a subsurface behavior.