Potencial gerador dos folhelhos cretáceos e modelagem 1D na Sub-bacia Tucano Sul, Bahia, Brasil

Abstract

The Tucano Basin (TB) comprises the central portion of the Recôncavo-Tucano-Jatobá (RTJ) intracontinental rift that was aborted during the Eo-Aptian. Its origin is related to the crustal extension that occurred in the Early Cretaceous, with the fragmentation of the Gondwana supercontinent and the formation of the Atlantic Ocean. There are few published studies on the Tucano Basin regarding the generation of hydrocarbons. Thus, the present work aims to contribute to the knowledge of the generating potential of the southern compartment of this basin, through the evaluation of the shales of the Ilhas Group (which is undivided) and of the Candeias Formation in a well (TBO-1-BA) and the modeling 1D for hydrocarbon generation conditions. The data from this well were provided by the ANP, and then, the geochemical characterization of samples of these shales was carried out with data on TOC, pyrolysis and macerals containing kerogen. Well 1-TBO-1-BA is in the central-west portion of the basin with a depth of 3,598 m. The Ilhas Group has the highest TOC contents, reaching values of 2.6%, while the Candeias Formation reaches 1.4%. Regarding thermal maturity for the generation of hydrocarbons the samples of the Candeias Formation present greater maturity in relation to the Ilhas Group, which would make it a more interesting exploratory target, with Tmax values between 448 ºC and 451 ºC and %Ro between 0.84 and 0. 94%. Samples from the Ilhas Group range from immature at its top (Tmax = 432 ºC, Ro =0.55%) to mature at its base (Tmax = 450 ºC, Ro = 0.77%). The Van Krevelen type diagram classifies the kerogen contained in the shale samples from the two units as predominantly type II and III but evaluating itself with the set of maturity data and macerals made available by the ANP, it is suggested that there is a mixture of types I and II. For the construction of the basin's thermal and subsidence history, Schlumberger's Petromod software was used, inserting geological and geochemical data and the crust (4.0) and mantle (1.1) stretching factors in the Mckenzie module. As a result, the heat flow and subsidence curve in geological time is obtained. The heat flow variation for Well 1-TBO-1-BA in the initial phase of the rift was 32 mW/m2 (Barremian) reaching 57 mW/m2 at the end of the rift phase (Aptian), with an erosion thickness of the well of 500 meters (350 meters at the top of the São Sebastião Formation and 150 meters at the top of the Marizal Formation). The modeling results reinforced that the base shales of the Candeias Formation have greater potential for the generation of hydrocarbons than those of the Ilhas Group, with its greater potential for oil generation, since the Candeias Formation has reached greater maturity (paleo-temperatures reaching 148 ºC and Ro to approximately 1%), reaching a transformation rate of 48%, while in the Ilhas Group there was no transformation. It was evident that the generation process was predominant during the synrift phase in theBarremian-Aptian (128 to 124 Ma), demonstrating that the heat flow from the basin opening process is the predominant factor for the maturation of organic matter. To verify which conditions would be ideal for the shales of these units to generate high potential conditions, a hypothetical well (HP-1) mirrored in well 1-TBO-1-BA was modeled, modifying the depth conditions of the well(5.600 metros) and the mean values of TOC(Ilhas Group 2,0 %Ro, Candeias Formation 1,7 %Ro) and IH(Ilhas Group 650 mgHC/gCOT, Candeias Formation 550 mgHC/gCOT). With these changes, there were significant changes in the thermal maturation of the units and, consequently, in the transformation rates with gás generation. The Candeias Formation reached 2.3 %Ro and the Ilhas Group 1.4 %Ro, with the two units reaching a 100% transformation rate and this condition may be feasible, since the basin can reach a depth of 7,000 meters, indicating an important prospective potential for the low blocks of this sub-basin.