Syn-to post-rift tectonic evolution of the Rio do Peixe Basin (Brazil) and implications for siliciclastic reservoirs properties

Abstract

The study of sedimentary basins is crucial to the global economy, as they are sources of underground water and hydrocarbon, strategic resources for the industrial development and human activities. The development of sedimentary basins is linked with the regional stress imparted by geodynamic processes, thus understanding their tectonic evolution help to constrain the structural pattern of the upper crust. This way, the study of sedimentary basins provides a great scientific contribution to global tectonic evolution and the distribution of resources and sites for CO2 storage. In sedimentary basins dominated by porous siliciclastic sequences, the syn-sedimentary basin-boundary faults generally host thin tabular structures known as deformation bands. Such deformational features can develop as single or in clusters in response to tectonic processes, and generally reduce the host rock permeability of up to 4-5 orders of magnitude. Thus, linking the tectonic history with deformation bands evolution in sedimentary basins is important to constrain fluid migration and accumulation in siliciclastic reservoirs. In this PhD thesis, the Rio do Peixe (RdP) basin, located in northeast of Brazil, is used as a case study to understand the intracontinental deformation during the progressive opening of the Atlantic Ocean in the Lower Cretaceous and the later Cenozoic reactivation. The effects of such tectonic evolution on the petrophysical properties of rocks are also documented, and implications for siliciclastic reservoirs are discussed. The greatest challenge of studying the Rio do Peixe basin is the partial lack of quantitative data on the syn-rift stage, which leads to distinct interpretations for the tectonic evolution of the basin. On border faults, only few outcrops are identified, making it quite difficult to constrain the evolution of stretching directions during basin formation based only on field data. The syn-rift siliciclastic sequence of the RdP basin is affected by deformation bands, mm-thick scale with insufficient exposure of kinematic markers such as fault striae and evolved displacements. Additionally, as they are subseismic structures, it is very challenging to study using conventional methods such as seismic interpretation and field structural analysis. In this PhD thesis, multi-scale and multidisciplinary investigation was performed to constrain the tectonic evolution of the basin. In the Rio do Peixe basin, excellent exposures of deformation bands in porous sandstones are common. These structures are very important for the oil industry as they substantially affect the petrophysical properties of siliciclastic reservoirs. In this context, it is very important to understand the deformation processes during the formation and evolution of deformation bands to predict their impacts on reservoir properties. In this PhD thesis we investigated the microstructural and petrophysical properties of deformation bands in connection with the evolutionary stages of the basin. This PhD thesis is organized in 5 chapters, three of which correspond to articles acepted or submitted in peer review journals. The chapter 1 is a brief introduction about the importance of the Rio do Peixe basin in terms of regional tectonics and its relevance for siliciclastic reservoirs study. Further, it is explained the fundamentals of anisotropy of magnetic susceptibility (AMS) and its application on tectonic interpretation of sedimentary basins. A general introduction of deformation bands and its importance to siliciclastic reservoirs is at the end of the chapter. Chapter 2 is an article submitted to the Tectonics journal. In this work we performed AMS and structural analysis to constrain the syn-rift extensional stages of the Rio do Peixe basin and the intracontinental deformation in northeast Brazil during the Early Cretaceous. As main result, we observed that two orthogonal syn-rift stages were responsible for the opening of the Rio do Peixe Basin. The main extension direction of the basin gradually shifted from NNE to NW reflecting the intracontinental deformation of the Borborema Province due to the rotation of the stress field in South America continent during the pre-Pangea Breakup. This contribution is important for better understand of the opening of the South Atlantic Ocean. The chapter 3 is an article submitted to the Journal of Structural Geology. In this contribution, we performed structural/microstructural and petrophysical analysis of deformation bands formed during the syn- and post-rift stage of the basin. This work distinguish the deformation intensity and its effects on fluid flow and mechanical properties on sedimentary units within the basin. As main result, we identified that post-rift deformation bands are likely to form more effective fluid flow barrier and forms more intensely comminuted fault core. It occurs because they form in an already tectonically compacted host rock. This contribution is of crucial importance to the industry, as the progressive deformation observed due to the tectonic reactivation of the basin directly affects the sealing potential of siliciclastic reservoirs. The chapter 4 is an article published in the Journal of Structural Geology. In this work, we emphasized the contribution of the cataclastic matrix generation in the sealing behavior of deformation bands. From high-resolution images, we performed 2D and 3D petrophysical analysis and observed that the amount of cataclastic matrix produced due to shear is either influenced by the mineralogy or the strain intensity of deformation bands. This contribution is of interest to the industry, as cataclastic deformation bands are a frequent problem in siliciclastic reservoirs around the world. The chapter 5 is a general discussion and conclusions at the end of the thesis, emphasizing the influence of the tectonic evolution of the basin in silisciclastic reservoir properties.