Aplicabilidade dos valores calculados da reflectância da vitrinite e sua implicação na determinação do potencial de geração de hidrocarbonetos

Abstract

Vitrinite reflectance is an organic petrographic technique that consists of measuring the reflective power of vitrinite particles under a reflected white light microscope. This has been considered one of the most important and effective techniques used in the petroleum industry for thermal maturation analysis, delimitation of the petroleum window and in defining the rank of coals. The present work aims to evaluate the applicability of the calculated values ​​of vitrinite reflectance of the source rocks intersected by some boreholes in the Meso-Cenozoic basins of Mozambique and the Karoo. For this purpose, 75 samples were analysed from the Nhanchengue-1 borehole, 20 samples from the Sangussi Marin-1 borehole, 16 samples from the Sofala-1 borehole, 5 samples from the Temane-1 borehole, 1 sample from the Zandamela-1 borehole, 3 samples from the Xai-Xai West-1 borehole, 19 samples from the Mocimboa-1 borehole, 70 samples from the JOG16N-5 borehole, 44 from the Moatize Minjova basin (4 from the ETA65 borehole, 12 from the ETA12 borehole, 6 from the ETA15 borehole, 14 from the ETA72 borehole and 8 from the ETA71 borehole). In the main Karoo basin, 26 samples were analysed from the KZF borehole, 10 from the BH47 borehole and 9 from the KWV borehole. Finally, 10 samples from the Rifugi-Mikumi sub-basin and 19 from the Muchuma-Ngaka sub-basins were analyzed. Fine-grained, dark-colored sedimentary rock samples were collected and subsequently processed following non-oxidative standards proposed by Tyson. In order to achieve the objectives proposed in this study, vitrinite reflectance values ​​were calculated from maximum temperature values ​​measured in the Rock-Eval Pyrolysis process using several vitrinite reflectance models, namely: AGS&TRICAN, Baker1989, Jarvie2001, and Wust2013. According to the standard deviation and the difference between the measured and calculated vitrinite reflectance values, the reflectance model that best suits the evaluation of the hydrocarbon generation potential in the wells of the basins under study is the AGS&TRICAN model. The values ​​obtained from this model were subsequently used to determine the stage of thermal maturation and potential for generating HCs. Based on the results obtained, the organic matter of the rocks intercepted by the boreholes of the marginal basins of Mozambique and Rovuma under study are mostly in the Mature stage, varying to Immature, while the organic matter of the rocks intercepted by the boreholes of the coal sub-basins of Tanzania, by boreholes BH-47, KZF-01, KWV-01, by the drillings of the Moatize-Minjova basin and by borehole JOG16N-5 are abundantly supermature, with a predominance of type-III kerogen and type-IV kerogen, respectively. This type of kerogen allows the organic matter of the marginal basins of Mozambique and Rovuma to be classified as having good potential for the generation of oil and gas, and the organic matter of the coal basins as having good potential for generating gas. The scatter plots of the boreholes from the marginal basins indicate a linear trend, which represents an increase in vitrinite reflectance as a function of depth, whereas the remaining boreholes from the Karoo coal basins do not show any pattern, i.e. the temperatures were not distributed regularly as a function of depth(TRADUÇÃO NOSSA)