The contribution of brittle deformation processes on improving hydrocarbon carbonate reservoirs : the Jandaìra Formation (Turonian-Campanian) as analogue

Abstract

Nowadays, hydrocarbons are the most important source of energy. In 2008 they represented 60% of the main types of fuel consumed worldwide (oil – 36%; natural gas – 24%; coal – 29%; nuclear energy – 5%; hydroelectric – 6%). In this context carbonate reservoirs are significant for the global oil and gas supply, as long as more than 60% of oil and 40% of gas reserves are found in this type of rock. But the knowledge about the key elements controlling the porosity and permeability on these reservoirs is incomplete. What is especially true regarding the understanding of fracturing contribution to secondary porosity creation and permeability increases. The goal of this thesis was to contribute for the investigation of the actual role of fractures on carbonate reservoirs. To accomplish this commitment were studied a great exposition of Jandaíra Formation carbonates (Potiguar Basin, NE Brasil) where are located a well-developed cavern system named Furna Feia (linear development of 739m, maximum depth of 30m) and other karstic features (e.g. sinkholes; minor caverns), all of them presenting strong evidences of structural control. Such karstic environment was used as analog to investigate the relation between fracturing and high permeability corridors on carbonates, based on the generally accepted concept that caverns on carbonates are mostly formed by erosion due to natural acid in groundwater, which seeps through bedding-planes, faults, joints, etc. The methods used in this study were: compilation of public data; lineament analysis (regional and local scales); outcrops structural analysis; structural measurements analysis; numerical modeling. Below, the sequence of analysis performed and theirs main results are described. 1. Information about the Potiguar Basin evolution and available data about the nowadays stresses were compiled from the literature. The state of stress operating today on the Basin was understood from earthquakes focal mechanisms, borehole breakouts analysis, and fracturing testes results from wellbores. These data demonstrated that: (i) the nowadays state of stress predominant through the Basin is strike-slip, with some reverse or normal mechanisms; (ii) the orientation of the maximum horizontal stress component ranges from NW-SE to E–W. Given such stress regime, it was considered that the NE-SW faults through the Potiguar Basin are favorably oriented to be reactivated as inverted faults, while the NW-SE, E-W and N-S faults are favorably oriented to be reactivated as strike-slip faults. 2. Lineament analysis were performed at regional-scale (using SRTM3 data) and at local-scale (using GoogleProTM high resolution satellite images). From the regionalscale analysis, the main azimuth frequency of lineaments obtained that for the Potiguar Basin is oriented NE-SW (N65oE). From the local-scale analysis, the main azimuth frequency of lineaments is oriented NW-SE (N49oW). These two directions are nearly orthogonal (114o divergent), putting into evidence that at the study area some particular structural evolution exists and it could be related or not to the regional tectonicstructural history. 3. From the analysis of outcrops and structural measurements, the NW-SE direction was confirmed as the principal structural trend at the study area. By using specific fault inversion methods included into the Daisy software, two stress tensors that possibly generated the observed deformation were calculated: (i) using the “inversion by rotax analysis” method it was obtained a stress tensor oriented NW-SE (N330oE), compatible with a strike-slip fault regime, which could only explain the observed faults; (ii) using the “Direct Inversion” method it was obtained a stress tensor also oriented NW-SE (N318oE), but compatible with a reverse fault regime, which could explain the whole deformational elements observed through the area (i.e. faults; joints; bedding). Both stress tensors are in conformity with the present day regional state of stress given by the seismology and the borehole data analysis. But the second stress solution was assumed to be the most reliable because the Monte Carlo test included on the algorithm presented a well constrained distribution of the possible stress tensors. 4. A comparative analysis between the azimuth distribution of the Furna Feia principal galleries axis and the statistical analysis (Azimuth by Frequency distribution) of the local-scale lineaments and structural elements (i.e. faults; joints) was done. Such comparison evidenced a clear coincidence between the principal trends obtained for the study area and the cavern’s galleries axis. 5. On the basis of all available information the following tectonic-structural model for the study-area was proposed: a. A major not outcropping NE-SW normal fault exists somewhere toward NW from the study area; b. Given the estimated nowadays stress regime, the hypothetical NE-SW normal fault was reactivated as an inverted fault; c. This inversion movement caused the exhumation of the Jandaíra Formation, on the returning hanging wall of the main NE-SW fault; d. Minor NW-SE strike-slip faults result from the accommodation movements due to the major NE-SW fault activity; e. Minor NE-SW strike-slip faults constitute conjugated pairs with the NW-SE strike-slip faults, or are related to oblique movements of the main NE-SW fault. f. The fracturing associated to this deformation history was responsible for the creation of oriented high fractured corridors. These corridors control the meteoric water circulation through the Jandaíra carbonates, conducting the dissolution process and the formation of the Furna Feia caverns system. 6. Afterwards a sequence of numerical models was run in order to examine the reasonability of the theory proposed for the permeability increase in the Jandaíra carbonates. Such numerical models are based on mechanical analysis by finite element method for 2D simulation, comprised in the TECTOS program. At the end of this work it was accepted that the faults and joints observed through the whole study area, associated to a dilatancy effect, led to a porosity increase in the Jandaíra carbonate rocks and controlled the creation of high permeability corridors. Such high permeability zones focalized the water percolation into the carbonate rocks and conditioned the growth of the Furna Feia cavern system