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<p>This work presents electromagnetic (EM) responses using both marine magnetotelluric (MMT) and controlled source electromagnetic (MCSEM) methods applied to a resistivity model typical of the Campos basin. The Campos basin is located in the Brazilian east margin, with origin in the Neocomian stage of the Cretaceous period 145–130 million years ago during the breakup of the supercontinent Gondwana (South America and Africa split). The clastic reservoirs in this basin have been the largest oil producer in Brazil for the past three decades and the present challenge moves to deeper waters, well known for great challenges in exploration enforced by the complex geology posed by the tectonics associated with giant saline bodies. The seismic reflection is the highest resolution geophysical method and the most used tool in hydrocarbon (HC) exploration; however, it finds difficulties in generating good images in complex geological environments (i.e., associated with the presence of salt and volcanic rocks) and is not a direct hydrocarbon indicator. EM methods, in contrast, are sensitive to resistive variations, and can therefore aid in indicating a reservoir HC-filled and/or in the obtaining subsurface images that can be integrated in joint approaches with seismic for producing less ambiguous interpretations. A set of geophysical data available in the study area includes: CSEM stations, 2D seismic lines, and 3D seismic cube, in addition to 33 well logs. The studies are still in an early stage and the main objective now is to evaluate the responses of the MMT and MCSEM methods in different scenarios involving hydrocarbon accumulations in a thin post-salt Maastrichtian reservoir. The EM forward modeling responses was performed using the modified version of Modular System for EM inversion (ModEM code) within a research project at Observatório Nacional – Brazil. The resistivity model used in this study incorporates vertical transverse isotropy (VTI) in resistivity and is derived from the interpretation of the seismic data and well logs present in the study area. The obtained EM responses demonstrate the effectiveness of the method for detecting thin reservoirs HC-filled, when compared to an environment without accumulation, and the exercise is valuable to compare and understand the real data collected in the study area.</p>

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RESUMONeste trabalho foram utilizados dados BHT - Bottom Hole Temperature ou temperatura de fundo de poço na inversão de dados com o objetivo de obter a distribuição 1-D do gradiente geotérmico. Antes da inversão propriamente dita, foi utilizado o método de correção de Horner, para determinar a temperatura correta da formação. A inversão foi realizada em um modelo sintético inspirado em dados reais do Campo de Pineview (Utah, EUA), no caso, com o objetivo de obter gradientes geotérmicos de nove formações utilizando dados BHT de 32 poços. A matriz  do problema geotérmico contém os elementos , ou seja, a espessura da i-ésima camada perfilada no j-ésimo poço. O método dos mínimos quadrados foi utilizado, e devido à existência de ruído foi necessário o amortecimento. A implementação numérica da inversão, ou seja, a determinação do operador inverso  ou  foi através da decomposição em valores singulares. As inversões iniciais não geraram resultados satisfatórios, melhorando bastante com a introdução do amortecimento. A melhoria dos resultados é explicada quantitativamente pelo fato do número de condição da matriz a ser inversa reduziu bastante com a utilização do amortecimento. Por seu turno, o amortecimento demanda a escolha de um parâmetro ótimo, sendo que foi utilizada a curva L para esse fim. Palavras-chaves: problemas inversos; gradiente geotérmico, bottom hole temperature. ABSTRACTThis study consists in obtain the 1-D distribution of the geothermal gradient from the inversion of Bottom Hole Temperature (BHT) data. Before the inversion procedure, Horner correction method was used to determine the correct formation temperature. The inversion was performed in a synthetic model based on real data from Pineview Field (Utah, USA), in this case, to obtain geothermal gradients from nine formations using BHT data from 32 wells. The  matrix of the geothermal problem contains the elements , i.e., the thickness of the i-th layer logged in the j-th well. The least squares method was used, and, because of the occurrence of noise, damping was required. The numerical implementation of the inversion, i.e., the determination of the inverse operator  or  was performed by singular value decomposition. Initial inversions did not produce satisfactory results, but they significantly improved with the introduction of damping. The improvement of the results is quantitatively explained by the fact that the condition number of the matrix to be inverted greatly reduced with the use of the damping. In turn, damping requires the choice of an optimal parameter, and the L-curve was used for this purpose.Keywords: inverse problems; geothermal gradient, bottom hole temperature.