Mirador Formation in SW Tachira State, Venezuela: Potential Reservoir for the Paleogene Sequence: ABSTRACT

The Mirador Formation, in SW Tachira State, displays its best outcrops along the Rubio-San Antonio and Rubio-Santa Ana highway. Measured thicknesses are from 120 m (La Alquitrana) to 69 m (Rubio-San Antonio). Lithologic proportions are: heterolithic facies with undulated/flaser/lenticular bedding, 0-20%: pure sandy facies, 66-94%: non-carbonaceous shales and siltstones, 6-16%; carbonaceous siltstones and coal, 0-6%. For the region San Pedro del Rio-San Antonio del Tachira-Rubio-La Alquitrana, the Mirador Formation always consists of three intervals, the upper and lower being purely sandy. The intermediate interval, generally with prograding geometry at mesoscopic scale, shows intercalations of sandstones and pelitic rocks. The upper and lower intervals correspond to sandy transgressive environments during two separate events, following eustatic regressions, and overlying surface type SBI. Recent porosity and permeability data (UCV theses) indicate that surficial porosity varies from 1 to 8% and render permeabilities between 3 and 9000 millidarcys. Our research indicates petrographic porosity (point counting method) between 1 and 20% (average 8%). Because we can expect a statistical relation between petrographic porosity and experimental permeability, it is advisable for modelling purposes for Mirador in the subsurface to assume lower limits for average porosity as 8% and average permeability between 2000 and 9000 millidarcys.