Produção Científica
Artigo em Revista
| Curvelet Denoising for Preconditioning of 2D Poststack Seismic Data Inversion: Application to Data from the Marimbá Oil Field, Offshore Brazil. Seismic inversion methods are crucial for understanding subsurface structures; however, the presence of noise in the data can negatively impact the results of these methods. To address this issue, state-of-art applications employ a variety of techniques to enhance the signal over the noise, with researchers developing increasingly sophisticated denoising methods and combining them into new procedures to further improve the signal-to-noise ratio (SNR). While some methodologies operate on a single scale, the curvelet transform is a multi-scale transform useful for decomposing seismic signals into multi-resolution and multi-direction elements. Here, we evaluate the effectiveness of denoising by means of curvelet thresholding as a preconditioning method for poststack seismic data in a 2D acoustic inversion processing using a Bayesian framework. Our application of the curvelet thresholding method on the Marmousi model and a real dataset from a Brazilian offshore basin demonstrates that it can successfully eliminate random noise. Even the use of a hard global threshold, dependent on curvelet scale and orientation, led to improvements in the deepest parts of the models. However, we observed a decrease in the SNR in the presence of soft rocks with pronounced absorption, which are typical in the shallowest regions. Future work will have to explore alternative methods for selecting coefficients that can robustly incorporate changes in the seismic wavelet with depth. |
|
|
Artigo em Revista
| Impact of fracture set scales and aperture enlargement due to karstic dissolution on the fluid flow behavior of carbonate reservoirs: A workflow to include sub-seismic fractures in 3D simulation model Carbonate reservoirs might contain a large number of critical heterogeneities, such as fractures and karstic features, although some the existing heterogeneities cannot be identified due to the lack of seismic resolution. Our main objective is to evaluate which fracture scales need to be included in the fluid flow simulation models and, in particular, how to model subseismic fractures. We also consider the presence of enlarged fractures in the reservoir models due to karstic dissolution. The fracture network used to compose the simulation models is based on the real karstified fracture sets of the Soledade outcrop, JandaÃra Formation (Potiguar Basin, Brazil). This formation is a large and well-preserved karstified carbonate platform of the Turonian-Campanian age related to the South Atlantic opening. Our work is divided into three phases: in Phase 0, we show how to upscale the fracture network of the Soledade outcrop to in-situ deep conditions. As a result, a semi-synthetic simulation model is composed from the Soledade Outcrop data and of a Brazilian Pre-Salt carbonate reservoir. In Phase 1, we evaluated the impact of the fracture scales and fracture enlargement due to karstic dissolution on the fluid flow behavior of the semi-synthetic reservoir. Finally, in Phase 2, we propose and validate a fluid flow simulation workflow allowing to estimate a statistically equivalent network of subseismic fractures, which might be used on multiple data assimilation methods or in risk analysis of green fields. Results from Phase 1 evidence that subseismic fractures can significantly impact the fluid flow behavior, depending on the producing well location in relation to the fracture network and on the presence of enlarged fractures due to karstic dissolution. On the other hand, the Phase 2 results provide the oil and gas industry with a workflow to better characterize sub-seismic fractures in fluid flow simulation models. |
|
|
Artigo em Revista
| Three-dimensional characterization of karstic dissolution zones, fracture networks, and lithostratigraphic interfaces using GPR cubes, core logs, and petrophysics: Implications for thief zones develop Fractures and karstic dissolution features have been recognized to significantly affect subsurface fluid flow on a range of scales in carbonate rocks. However, due to the information gap between seismic and borehole scales, characterizing these features is a real challenge. We present a three-dimensional multidisciplinary approach to identify the main factors controlling the formation of connected rock volumes composed by permeable karstic dissolution zones. We investigate two outcrops in the JandaÃra Formation, which is an intensely fractured and karstified carbonate platform in the Potiguar Basin, Brazil. Two GPR cubes are jointly interpreted with cored boreholes, borehole images, petrophysical profiles, and thin sections. We verified that attenuated zones of GPR signal are related to secondary carbonate changes caused by fracturing and/or diagenetic alterations, leading to the development of relatively high secondary permeability. At least three types of attenuation/karstic zones might occur: fractured or broken zones affected by fracture enlargement due to dissolution, zones submitted to intense diagenetic alterations, and void spaces as a final end-member of these two processes. Fractures are preferential routes to fluid flow, thus contributing to increase dissolution processes, particularly in layers having relatively high values of primary matrix porosity and permeability. On the other hand, relevant diagenetic alterations might occur in layers that contain high concentration of stylolites. As a result of interconnecting dissolved stylolites, zones of relatively high secondary permeability can be created regardless of the primary values of matrix porosity and permeability. Geobodies of connected volumes of karstic zones often present tabular shapes and are delimited by subvertical fractures and subhorizontal lithostratigraphic interfaces. In the reservoir scale, these tabular geobodies might compose highly connected permeable volumes of altered carbonate rocks that constitute thief zones or super-K layers. |
|
|
Artigo em Revista
| Combined Nd isotope systematics and geophysical data constrain the crustal evolution of the disrupted Alto Moxotó Terrane, Borborema Province, Brazil The accretionary record of Precambrian orogens is often masked by systematic magmatic, metamorphic and/or structural overprints. The Neoproterozoic Borborema Province presents direct linkage to several orogenic belts along western Gondwana, but its crustal evolution is contentious resulting in a range of geological models. In the region, exotic terranes that might have been docked via thrust- and strike slip structures preserve pre-Gondwana events that provide glimpses for the evolution of orogenic systems more generally. A total of 201 Smsingle bondNd analysis of the Archean-Paleoproterozoic Alto Moxotó Terrane, central Borborema Province reveals source ages in granitic, mafic and metasedimentary rocks that are mostly Paleoarchean to early Paleoproterozoic, which contrast with the adjacent terranes. The successive juvenile inputs and crustal reworking from Neoarchean (ca. 2.6 Ga) to mid-Paleoproterozoic (ca. 2.1 Ga) diverge with the early to late Neoproterozoic dominant crust of the neighboring blocks. 3D geophysical modelling highlights the unique signature of the Alto Moxotó Terrane compared to these blocks, including: i) K-poor and Th-rich crust and ii) higher gravity gradients when compared with the bordering lithospheric record, both in gamma-spectrometric and gravimetric maps. Moreover, prominent magnetic anomalies in all magnetometric maps coincide with the major shear zones, including previously proposed terrane boundaries, suggesting that they might represent cryptic or unmapped suture zones. The isotopic and geophysical data are consistent with derivation of the Alto Moxotó Terrane from older crustal blocks such as the São Francisco and Congo cratons and provide a model to the understanding of the accretionary evolution of western Gondwana. |
|
|
Artigo em Revista
| Deep crustal structure of the Sergipano Belt, NE-Brazil, revealed by integrated modeling of gravity, magnetic, and geological data The Sergipano Belt is located in the Southern Subprovince of Borborema Province in the Northeast of Brazil. Its tectonic framework was consolidated in the Pan-African-Brasiliano Orogeny at the end of the Neoproterozoic. The most recent geological models indicate that its evolution occurred over a complete Wilson Cycle. Gravity and magnetic data profiles that crossed the Sergipano Belt from south to north were modeled jointly by the forward method to provide a 2D view of the deep crustal structure. The modeling process was linked and supported by the use of geological data and models. The result revealed the deep structure of the crust and identified the geometry of the main geological domains to the depth of Moho discontinuity. The folds and thrusts toward the São Francisco Craton are a persistent and deep feature in the Southern crust of the Sergipano Belt. The general tectonic context of the models is compatible with the subduction and collision of the São Francisco Paleoplate under the Pernambuco-Alagoas Superterrane, sutured in the São Miguel do Aleixo Shear Zone. The presence of dense blocks at the base of the crust was interpreted as layers of ophiolites placed by obduction. The metasediments of the Vaza Barris and Macururé domains are tabular bodies with thicknesses lower than 5 km, which dip horizontally or at a low angle on the flanks of the shear zones. Small vertical bodies of lower density within the Macururé Domain have correlation with granitic intrusions. In the Canindé Domain there is a dense layer in the lower crust that was interpreted as the relicts of the oceanic crust that based a back-arc basin. The granitic bodies modeled north of the Canindé Domain and correlated with the Serra do Catu batholith in Pernambuco Alagoas Superterrane, may be the record of subduction of this crust to the north. The data and models revealed geophysical differences between the Jirau do Ponciano and Rio Coruripe domains, and the crust north of the Palmeira dos Ãndios Shear Zone. These terranes are separated by shear zones with evident expression in gravity and magnetic data. However, the existence of Neoproterozoic metasedimentary supracrustal rocks partially covering both the domains and zones and the boundary shear indicates that the junction among these blocks occurred before the deposition of Neoproterozoic sediments. |
|
|
Artigo em Revista
| A review of the geophysical knowledge of the Borborema Province, NE-Brazil, and tectonic implications This review summarizes the results of nearly 50 years of application of geophysical data to study tectonic problems in Borborema Province (BP), Northeastern Brazil. Much information has been obtained from the interpretation of magnetic, gamma-ray spectrometry, gravity, seismological, magnetotelluric, deep seismic and heat flow data. The resulting picture is that, in comparison with other Brazilian geological domains, BP has a thin crust, a lithosphere with small effective elastic thickness, the presence of predominantly positive free-air gravity anomalies, besides the most expressive geoid anomaly, in addition to relatively high heat flow and intraplate seismicity. These facts reinforce the hypothesis that the BP lithosphere suffered a strong impact in Gondwana fragmentation. Predominantly positive free-air gravity anomalies reveal that part of the province has not yet reached isostatic equilibrium. Gravity data established the boundaries of the BP crustal block with neighboring blocks; in particular, the crustal boundary with the São Francisco Craton is located in the innermost regions of the marginal fold belt, tens of kilometers at the north of the conventional limit. Magnetic data allowed to infer the continuity of basement structures beneath sedimentary basins, in particular that BP continues to the Transbrasiliano Lineament, underneath the sedimentary of the ParnaÃba Basin. In addition, enhanced magnetic maps made it possible assessing the amount of deformation associated with the major shear zones and identified a continental-scale Cretaceous dike system crossing the BP. Gamma-ray spectrometry data allowed extensive geological mapping of metasedimentary sequences, granitic intrusions and basement complexes. Seismological data allowed to recognize that major shear zones in BP have lithospheric significance. Deep seismic data evidenced a well-defined contrast between the upper and lower crust in BP (Conrad discontinuity). Magnetotelluric data revealed several deep geoelectrical anomalies in BP, some of them interpreted as relics of subduction zones. Intraplate seismicity studies allowed inferring that the current stress state in the BP upper crust is predominantly transcurrent. The list of open problems in BP is also expressive. Notable examples are: a) it has not yet been possible to define which type of orogeny (accretionary or intracontinental?) was dominant in the Neoproterozoic; b) the cause of the expressive geoid anomaly is unknown; c) the tectonic significance of the Transbrasiliano Lineament (collisional suture? Just an intracontinental shear zone?) is also unknown. Regarding economic applications, the use of geophysical data to prospect new ore deposits is still relatively poor. We hope that this review can contribute to increase the familiarity and acceptance of geophysical results by the geological community. |
|
|
Artigo em Revista
| Review of tectonic inversion of sedimentary basins in NE and N Brazil: Analysis of mechanisms, timing and effects on structures and relief Most intraplate areas worldwide are under compression characterized by thrust and strike-slip deformations. The sources of the stresses in these regions are a combination of far-field (regional) stresses (e.g., ridge push, slab pull, lithospheric drag) and near-field (local) stresses (e.g., topography, lateral and vertical anisotropy of strength or elastic properties, erosion, flexural bending, overloading, buoyancy of low-density regions). These intraplate regions are sites favorable for the tectonic inversion of sedimentary basins. Here, we review the tectonic inversions of the sedimentary basins in NE and N Brazil, which comprise a significant part of the South American intraplate (the South American Platform). These regions have been under compression since the Late Cretaceous. Reports of post-rift basin inversions derived from seismic profiles, fault-slip data, and topography have been described in the Sergipe-Alagoas, ParaÃba, Potiguar, Ceara, ´ Rio do Peixe, Araripe (NE Brazil), and Solimoes Ëœ (N Brazil) basins. Reports of inversions in the rift phase have also been described in the Araripe Basin. The presentday stress field indicates that inversions in NE Brazil can mainly be characterized as strike-slip and thrust fault regimes, whereas in N Brazil, inversions have occurred under reverse stress regimes. The most frequent inversion structures are reactivated faults, folds, and dome structures affecting the topography and the consequent uplift of rift and post-rift sedimentary units. Inversions have led to the reactivation of continental-scale structures, such as the strike-slip dextral shearing of the Transbrasiliano lineament, which affected rift and post-rift units in the Equatorial Margin of Brazil. The small crustal thickness of these areas favors inversion, as indicated by the most recent Moho depth map of South America. The interplay between far-field and near-field stresses explains the stress pattern in NE and N Brazil, where inversions have been observed. |
|
|
Artigo em Revista
| Can Full-Waveform Inversion Compensate for the Lack of Illumination in Crosswell Tomography? Seismic illumination can be defined as the maximum angle between rays that pass through a point. Only interfaces completely contained in the angular apertures can be correctly imaged with first arrival traveltime tomography (FATTT). We investigate if 2D acoustic full-waveform inversion (FWI) can compensate for the lack of illumination of a crosshole tomography experiment. We use synthetic data generated with Ricker wavelets with peak frequencies at 100 or 500 Hz, resulting in small overlapping in the frequency bandwidths, allowing the use of a multiscale FWI approach. We investigate two FWI cases: in the first case (FWI T), just the waves recorded at the opposite borehole are used whilst, in the second case (FWI T+R), the waves recorded at the two boreholes are used. For a single interface, the shape of the transmitted waveform only varies significantly when it is contained in the angular apertures. Accordingly, shot gathers for layered models with interfaces outside the angular apertures can be roughly reproduced with equivalent homogeneous media. Thus, in comparison with FATTT, both FWI cases give mild improvements for models with interfaces inside the angular coverage, and cannot compensate for the lack of illumination of the experiment. However, in the mixed condition where layers with interfaces inside the angular coverage are cut by a fault, FWI offers substantial improvements over FATTT, even if the fault plane is outside the angular coverage. In this mixed situation, resolution increases when FWI T+R is used. However, for the studied cases, no significant improvements were obtained in the FWI stage where the 500 Hz peak frequency was used. |
|
|
Artigo em Revista
| Correlation analysis: application of DFA and DCCA in well log profiles Detrended fluctuation analysis and detrended cross correlation analysis are used in this work to identify and characterize correlated well log data. This is performed by first separating the different fluctuations due to external trends, and evaluating the autocorrelation and cross-correlation exponents to determine whether scale properties persist as the size of the series changes. Two new methodologies were developed to identify optimal values of the cross-correlation coefficients and graphically display them, which we call the automatic search procedure and correlation map. The methodologies were applied to well logs from the Jequitinhonha Basin, Brazil, to verify the existence of scale property in these data. For practical purposes, our goal is to use a local analysis framework to detect all points of high cross-correlation among different physical parameters in the same well, and among one same physical parameter in different wells. The correlated events suggested the continuity of geological features, including the vertical displacements of rock layers. In particular, it was possible to identify layers of calcilutites in a specific depth range. These rocks are of particular importance to the study of stratigraphic correlations due to their great regional extent and regular layering. |
|
|
Artigo em Revista
| Controls of fracturing on porosity in pre-salt carbonate reservoirs This work aims to improve the understanding of how fracture zones affect carbonate reservoir properties based on observations of a pre-salt well located in the Santos Basin, Brazil. The identification of fracture zones allowed for the observation of a relationship between the occurrence of rock fractures and the silicification, as the latter plays an important role in determining porosity (higher silica content may increase brittleness of the rocks therefore increasing the likelihood of creating fractures zones and fractures may be filled up reducing the total porosity). To support the proposed observation, an inte-grated study was conducted using borehole imaging, spectroscopy logs, and sidewall core samples. The porosities were defined using nuclear magnetic resonance log analysis, alongside sidewall core samples, and thin sections. The integration of rock samples and well data with seismic analysis was performed to analyze the presence of a regional fault system that could explain high fracture densities as well as observed silica content characteristics. The results show how different types of cement filling up the formation pores affect fracture densities and total porosity. Furthermore, it was possible to infer that the amount of silica content observed in well logs and thin sections relates to hydrothermal fluids reaching out the reservoir through regional fault systems detected in the seismic section. Therefore, this paper supports the comprehension of how diagenetic processes can significantly affect the properties of pre-salt reservoirs. |
|
|
<< < 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 > >>
