Microfácies e diagênese dos carbonatos da Formação Itaituba, borda norte da Bacia do Amazonas, município de Urucará (AM)

Abstract

Carbonate rocks are considered of great importance in petroleum systems around the world, since they are commonly associated with reservoir intervals, for example in the Brazilian pre-salt. Nevertheless, such characteristics are not applied to petroleum systems in the intracratonic basin of Amazonas, since the carbonate rocks found in this context are often associated with sealing intervals. This scenario has generated an increase of the investigations to understand how the diagenesis influenced in the generation, preservation of porosity and in the quality of the reservoir. The Itaituba Formation, Tapajós Group, Carboniferous of the Amazon Basin, consists of carbonate rocks, intercalate with shales, with sedimentation attributed to a shallow marine lagoon environment, considered as the seal of the Barreirinha-Monte Alegre oil system. Much of the studies in these carbonates are concentrated on the southern border of the basin, mainly involving microfacies evaluation and paleoenvironmental reconstructions, while information on the diagenetic evolution are still scarce. Therewith, 80 thin sheets were selected from a mining front of the company Itautinga Agro Indústrial S / A, municipality of Urucará (AM), north border of the basin, with a profile of 30.75 m thickness, with the objective of performing a petrography and microfaciology, as well as punctuating the main diagenetics processes that acted in this unit. In this sense, ten microfacies (MF) were identified: MF1- Recrystallized mudstone, MF2- Intraclastics Mudstone, MF3- Silicilcastics mudstone / wackestone, MF4- Dolomudstone / wackestone, MF5- Peloidal dolowackestone / packstone, MF6- Foraminiferal and bioclastic wackestone / packstone, MF7- Oolitic packstone / grainstone, MF8- Grainstone with peloids, MF9- Recrystallized ooids grainstone, MF10 - Bioclastic grainstone. These microfacies were grouped into four paleo depositional associations: Association 1, referring to the sabkha domain, refers to a deposition area in the upper portion of the tidal flat predominant exposed and subject to inudantion periodics represented by the microfacies MF1, MF3 and MF4; Association 2, referring to the tidal flat, a predominantly micritical precipitation area, with a low gradient and regularly influenced by the tidal level oscillation, represented by the microfacies MF1, MF2 and MF5; Association 3, semi-restricted lagoon, deposition in lowered zones permanently flooded in inframare, of calm waters with restricted to semi-restricted circulation composed of microfacies MF5 and MF6 and; Association 4, formed by the bioclastic and oolitic bars, area of deposition between the inframare submitted to currents where precipitation predominate of carbonates supported by ooids, peloids and bioclasts represented by the microfacies MF7, MF8, MF9 and MF10 . In relation to the diagenetics processes identified, these include: micritization, cementation, neomorphism, dolomitization, physical compaction, silicification, piritization, dissolution, desdolomitization and chemical compaction, acting mainly in the shallow diagenetic context (meteoric and marine). The main diagenetic processes responsible for the generation of secondary porosity are associated with meteoric selective dissolution in ooids of calcitic composition that can increase the porous volume by up to 20% when associated with microfacies MF7 and MF9, which opens a perspective on unit heterogeneity and behavior double with sealant and reservoir.

Keywords: Amazon Basin, Itaituba Formation, Microfacies, Diagenetics process.