Relationship between diagenesis and the emplacement of bitumen in the Lower Triassic Piramboia Formation sandstones, Parana Basin, SW Brazil

摘要

Outcrops of eolian sandstones of the Lower Triassic Piramboia Formation in the eastern margin of the intracratonic Parana Basin are known as the main occurrence of tar sandstones in Brazil because contain heavy oil ("tar"). In order to investigate the migration of oil into the sandstones petrographic analyses of dune facies were performed. Optical (transmitted, reflected and epifluorescence lights) and Field Emission Scanning Electron (FE-SEM) microscope studies together with X-Ray Diffraction (XRD) analyses showed that the Piramboia Formation sandstones have not been affected by intense diagenesis. Eogenetic features includes mainly thin, continuous inherited ferruginous clay coatings and minor mechanically infiltrated clay coatings around detrital mineral grains. The authigenesis of smectite and K-feldspar overgrowths precipitation begin to develop and during shallow mesodiagenesis, authigenic smectite was gradually transformed into mixed-layer illite-smectite; K-feldspar overgrowths, quartz overgrowths continuous to develop, and pore-filling calcite cement were formed locally. In the late mesodiagenesis the emplacement of oil occurred, filling pores and covering grains, clay coatings and calcite. During later surface exposure, water washing by meteoric fluids and biodegradation transformed the oil into a heavy oil rich in asphaltenes. The influx of acid meteoric waters caused dissolution of detrital K-feldspars as well as meteoric waters generate organic acids which also caused the dissolution of K-feldspars and the consequent generation of secondary porosity with the precipitation of inter- and intragranular vermicular kaolinite. However, kaolinite can be also a product of eodiagenetic process similar to the occur in the telodiagenesis. To evaluate the porosity of the sandstones microtomography analysis were preformed revealing high porosity volume (ca. 26%) and high volume of porous with connectivity (ca. 26%). Although not currently commercially exploited nowadays the knowledge of the Piramboia tar sandstones diagenetic evolution contributes for a better understanding of the Parana Basin sedimentary fill during the Lower Triassic.