A geoquímica orgânica na Formação Irati, com ênfase na influência de intrusões de diabásio na geração de óleo e gás: Área de Forquilhinha (SC)

Abstract

The deposition of Irati Formation in the Paraná Basin occurred during the Eopermian and Neopermian, which is composed by shales and siltstones of Taquaral Member and by interbedded bituminous shales and carbonates of the Assistência Member. A detailed organic geochemical study of Irati Formation was made in samples collected in the PVD-359 well situated in the Forquilhinha city of the Santa Catarina State (SC), aiming a better comprehension of principal transformations occurred in organic matter during the increase of thermal maturity by two diabase sills. For this, six quimiostratigraphic units (A to F) initially were defined using the total organic carbon (%COT), total sulfur (%S) and insoluble residue (%RI), being subsequently analyzed by pyrolysis Rock-Eval and liquid and gas chromatography. All samples from all units exhibit high ratios homohopane C31 (S/(S+R), homohopane C32 (S/(S+R), sterane C29 (S/(S+R) and sterane C29 (ββ/(ββ+αα)), characterizing thermal evolution from the intrusions. Obviously, because of the higher proximities with the diabase sills the units B, C and F are the intervals more affected by the thermal halo. This effect is much more pronounced in samples located at depths up to 1,5 times the intrusion thickness, when occur drastic decreases mainly in total sulfur contents (%S) and in the values of S1, S2 and IH that reaches practically zero at the beginning of the gas generation phase. The increase of this effect also reflects in the liquid chromatography results, which the proportions of saturated and aromatic hydrocarbons considerably decreases in relation to the proportions of resins and asphaltenes (non-hydrocarbons). Biomarkers analyses show principally increases in the ratios of tricyclic/pentacyclic and methylphenanthrene index. The units C and F being siliciclastics and having low organic facies variations, also showed more notable rises in the ratios Ts/(Ts+Tm), C29Ts/(C29Ts+C29), diahopane C30/hopane, homohopane C32 (S/(S+R)), diasterane C27/sterane C27 due to the increase of thermal evolution. At depths about 1 time the thickness of the intrusive, the greater hopane destruction causes significant rises in the ratios tetracyclic C24/pentacyclic, hopane C29/hopane C30 and gammacerane/hopane, indicating greater thermal stabilities of this compounds in high temperatures. The total organic carbon is also affected by the heat of diabases, but in closer regions from the intrusive body (~0,5 times of the diabase thickness). In that regions, only the methylphenanthrene index continued to rise, indicating greater utility of aromatic hydrocarbons comparatively to the saturated in the evolution evaluation because their isomers stabilizes at higher temperatures.