Mapeamento sísmico de intrusões ígneas na bacia do Amazonas e suas implicações para um modelo não-convencional de geração de hidrocarbonetos

Abstract

Most Brazilian Paleozoic basins have low thermally evolved organic matter in the intervals corresponding to the Devonian. The most appropriate model for understanding the generation, migration and accumulation of HC is related to the phases of diabase intrusion. In the case of the Amazon Basin, although there has been sufficient burial conditions for hydrocarbon generation, the non conventional forms of generation should not be discharged as the one that could lead to commercial accumulations of oil and gas. It is believed that the most suitable range for the generation of hydrocarbons (HC) include only rocks deposited in the Frasniano, although rocks associated with the Llandoveriano interval also should be carefully observed. In order to better understand the role of magmatic activity in the evolution of Amazon Basin, seismic mapping of diabase sills was carried out and geochemical analysis of Rock-Eval pyrolysis and TOC. Thus it was possible to evaluate the generation/migration of hydrocarbon and variation of geothermal parameters in Amazon Basin, caused by the intrusion of diabase sills. Seismic stratigraphy analysis was based on interpretation of 20 2D post-stack seismic lines, in which were recognized and mapped seismic horizons (tops of formations and intrusive igneous bodies), using well data and literature data for correlation. Intrusion of sills are present in the successions of shales/siltstones and anidritas from Andirá and Nova Olinda formations, respectively. It was observed that diabase sills can be intimately linked to oriented dykes, which work as their feeders. These extensive planar sills with transgressive segments occur in shallower stratigraphic levels of the Amazon Basin, and in larger volumes in Nova Olinda and Andirá formations. In some regions sills develop saucer-type morphologies. These bodies can reach up to 500m of thickness. Commonly, the sheet geometry denoted by the parallelism of the reflectors is present on mapping all along the basin. Dome structures were also observed. The thermal effect imposed by the igneous intrusions (dykes and sills) was very important, as without it there would be no heat for the transformation of organic matter. Through the analysis of Rock-Eval pyrolysis and organic carbon content, it was possible to evaluate and correlate the parameters as S2 (potential generation), HI (hydrogen index), S1 (free hydrocarbons) and Tmax (thermal evolution) with depth. Data were collected from 04 wells in which two of them were compiled from published articles and theses. The potential source rocks are usually those that have TOC of not less than 1%. Of the four wells analyzed, two of them had TOC> 1% for Barreirinhas Formation, showing that sedimentary rocks are potentially generating HC. High Tmax values can be justified by the thermal effect caused by diabase intrusions. The results of hydrogen index (HI) had values below 200mgHC / g TOC, indicating this basin s high gas generation potential.