Petrografía y porosidad de areniscas portadoras de petróleo. Formación Bajo Barreal (Cretácico), Cuenca Golfo San Jorge, Argentina

Abstract

Petrography and porosity of hydrocarbon bearing sandstones. Bajo Barreal Formation (Cretaceous), Golfo San Jorge basin, Argentina”. Petrography and porosity studies were performed to improve the knowledge about fluid migrations and additional hydrocarbon recovery projects throughout Cretaceous fluvial sandstones of the Bajo Barreal Formation Upper section in the Cerro Guadal Norte oil field (Golfo San Jorge Basin, Santa Cruz, Argen- tina). The Upper section of the Bajo Barreal Formation is composed of fine and medium sand- stones which have mainly cross but planar stratification, massive and ripple lamination were also recorded. Occasionally, light brown siltstones with parallel lamination are observed at the top of the sandstone bodies. Extensive fluvial channel belt deposits are well exposed around La Sin Nombre pond, where two stratigraphic logs were described. In addition, sedimentary architectural descriptions and short detailed lateral logs, separated by tens of meters each, were recorded. Forty nine north-oriented medium sandstones were sampled, and two blue epoxy impreg- nated petrographic thin sections were prepared for every sample, one parallel and the other normal to the North. The sandstones are mainly feldspar litharenites although litharenites and feldspar wackes were also identified. Most of them are submature and the mean porosity is around 18 % varying from homogeneous to heterogeneous. Clayey, zeolitic, siliceous and carbonatic ce- ments are present in different proportions. A comparison among petrography, porosity and architectural studies indicates an increase of zeolitic cements towards the top of the sandstone bodies, whereas the clayey cements are predominant at the base of the sandstone deposits. Furthermore, porosity decreases from main channel to adjacent bars and cross over bar-channel. In the bar-channel deposit, porosity decreases in both downstream and upward.