Paulo T. L. Menezes

Salt tectonics in the western Gulf of Cadiz, southwest Iberia

This study presents the results from the interpretation of an extensive and recent regional two-dimensional seismic survey focused on the understanding of the salt tectonics in the western Gulf of Cadiz (GoC). Two different salt units were identified: an autochthonous salt unit of the Late Triassic or the Early Jurassic (Hettangian) and an allochthonous unit that originated from the Hettangian salt. Interpretation of the pattern of distribution of the salt in the basin allowed subdivision of the area of study into three distinct salt domains: the eastern domain characterized by the presence of a conspicuous allochthonous salt nappe (Esperanca Salt), the central domain dominated by salt diapirs with mild deformation of Miocene strata and wide salt-withdrawal minibasins, and the southwestern domain where present-day tectonics induces impressive salt deformation affecting the sea floor. This complex pattern is mainly the result of the interaction of inherited basement structure, complex tectonic history, and stress regime of the basin. The intense halokinesis observed has created several salt-related trap geometries and fluid migration pathways. As the focus of worldwide exploration along passive margins is gradually shifting to deep-water regions, the western GoC has the potential to become a deep-water petroleum province in the near future.

Gravity data as a tool for detecting faults: In-depth enhancement of subtle Almada’s basement faults, Brazil

We demonstrate the potential of gravity data to detect and to locate in-depth subtle normal faults in the basement relief of a sedimentary basin. This demonstration is accomplished by inverting the gravity data with the constraint that the estimated basement relief presents local abrupt faults and is smooth elsewhere. We inverted the gravity data from the onshore Almada Basin in northeastern Brazil, and we mapped several normal faults whose locations and plane geometries were already known from seismic imaging. The inversion methoddelineatedwellboththediscontinuitieswithsmallor largeslipsandasequenceofstepfaults.Usingsyntheticdata, weperformedasystematicsearchofnormalfaultslipsversus fault displacement depths to map the fault-detectable region inthisspace.Thismappinghelpstoassesstheabilityofgravityinversiontodetectnormalfaults.Mappingshowsthatnormalfaultswithsmall0.5 km,mediumabout1 km,and large about 2 km vertical slips can be detected if the maximummidpointdepthsofthefaultplanesaresmallerthan1.8, 3.8,and6.8 km,respectively.

High-resolution acoustic impedance inversion to characterize turbidites at Marlim Field, Campos Basin, Brazil

Seismic inversion is routinely used to determine rock properties, such as acoustic impedance and porosity, from seismic data. Nonuniqueness of the solutions is a major issue. A good strategy to reduce this inherent ambiguity of the inversion procedure is to introduce stratigraphic and structural information a priori to better construct the low-frequency background model. This is particularly relevant when studying heterogeneous deepwater turbidite reservoirs that form prolific, but complex, hydrocarbon plays in the Brazilian offshore basins. We evaluated a high-resolution inversion workflow applied to 3D seismic data at Marlim Field, Campos Basin, to recover acoustic impedance and porosity of the turbidites reservoirs. The Marlim sandstones consist of an Oligocene/Miocene deepwater turbidite system forming a series of amalgamated bodies. The main advantage of our workflow is to incorporate the interpreter’s knowledge about the local stratigraphy to construct an enhanced background model, and then extract a higher resolution image from the seismic data. High-porosity zones were associated to the reservoirs facies; meanwhile, the nonreservoir facies were identified as low-porosity zones.

Tectonic framework of the Barra de São João Graben, Campos Basin, Brazil: Insights from gravity data interpretation

The Barra de Sao Joao Graben (BSJG), shallow water Campos Basin, is part of the Cenozoic rift system that runs parallel to the Brazilian continental margin. This system was formed in an event that caused the reactivation of the main Precambrian shear zones of southeastern Brazil in the Paleocene. We proposed a new structural framework of BSJG based on gravity data interpretation. Magnetic data, one available 2D seismic line, and a density well-log of a nearby well were used as constraints to our interpretation. To estimate the top of the basement structure, we separated the gravity effects of deep sources from the shallow basement (residual anomaly). Then, we performed a 2D modeling exercise, in which we kept fixed the basement topography and the density of the sediments, to estimate the density of the basement rocks. Next, we inverted the residual anomaly to recover the depth to the top of the basement. This interpretation strategy allowed the identification of a complex structural framework with three main fault systems: a northeast–southwest-trending normal fault system, a northwest–southeast-trending transfer fault system, and an east–west-trending transfer fault system. These trends divided the graben into several internal highs and lows. Our interpretation was corroborated by the magnetic anomalies. The existence of ultradense and strongly magnetized elongated bodies in the basement was interpreted as ophiolite bodies that were probably obducted by the time of the shutdown of the Proterozoic Adamastor Ocean.

mCSEM data interpretation for hydrocarbon exploration: A fast interpretation workflow for drilling decision

In a hydrocarbon exploration workflow, marine controlled-source electromagnetic (mCSEM) data are usually acquired after seismic interpretation for prospect identification and close-to-the-drilling decision making. Therefore, the mCSEM interpreter must provide quick answers to the asset teams in a way that the EM interpretation can add value to that decision. To achieve that goal, Petrobras developed a fast-track mCSEM interpretation workflow that consists in identifying anomalies in the mCSEM data set by frequency normalization, and then performing 1D CMP inversions followed by 2.5D polygonal inversions. The proposed workflow was successfully applied to several mCSEM surveys offshore Brazil. We evaluated an application in a complex geologic setting where the reservoir dips toward allochthonous salt. The reservoir appears as a flat spot in the seismic section, but with no significant amplitude variation with offset response. The mCSEM analysis confirmed the seismic anomaly and extended it northward. Two drilled wells corroborated the mCSEM interpretation.

3D inversion of total field mCSEM data: The Santos Basin case study

We present a marine Controlled Source Electromagnetic (mCSEM) 3D interpretation workflow based on anisotropic inversion applied to a dataset acquired in the Santos Basin offshore Brazil as part of a co-operation project between Petrobras and Schlumberger to evaluate the integration of deep reading Electromagnetic (EM) technologies into the full cycle of oil field exploration and development. The mCSEM dataset was acquired to demonstrate the improved detection and delineation of challenging EMtargets such as “smaller and deeper” hydrocarbon filled reservoir zones in complex background using 3D total field data. The project area hosting a proven reservoir was covered by a receiver grid and an orthogonal source lines grid extending beyond the receivers in the in line and cross line directions with all receivers active throughout the acquisition of both the orthogonal sets of source lines. We show that 3D inversion of the mCSEM total field data embedded in an advanced integrated workflow improves our ability to delineate hydrocarbon, their position and thickness resolution and increases our confidence about the resistivity at the reservoir(s) level. This increased resolution can provide in subsequent integrated interpretation workflows detailed information about reservoir volume distributions.

Kimberlite exploration at Serra da Canastra province, Brazil

Kimberlite and lamproite pipes are the only economically significant source rocks forming primary diamond deposits. Brazil, the world’s seventh largest diamond producer, has all of its production from alluvial deposits. The quest for a primary deposit has continued over several decades with very few positive results. To that end, airborne magnetic data from the Serra da Canastra diamondiferous province was used to identify kimberlite signatures. Serra da Canastra is located in the central portion of the Brazilian province in a low-magnetic-latitude region. The main tectonic feature of the area is a northwest-southeast major crustal fracture zone that extends for more than 3000 km within the territory. The interpretation strategy was based on joint analysis of analytic signal and Euler deconvolution. A selected kimberlite target should typically have a roughly circular analytic signal anomaly coincident with a depth (0–200 m) and structural (N=2) constrained Euler solution. The proposed approach led to rec...

Connecting onshore-offshore Campos Basin structures: Interpretation of high-resolution airborne magnetic data

We interpreted northwest-trending transfer faults whose extensions are not entirely mapped in the Precambrian basement of the onshore and offshore Campos Basin. To enhance the subtle northwest–southeast lineaments not clearly seen in the total-field data, we reprocessed and merged two airborne magnetic data sets aiming at producing a single merged magnetic data set. Next, we applied a directional filter to these integrated magnetic data. Finally, we applied a multiscale edge detection method to these filtered data. This combination allowed the detection of edges and ridges that are used to produce several northwest–southeast lineations. We interpreted these northwest-trending lineations as magnetic expressions of transfer faults that cut across the onshore adjacent basement of the Campos Basin to the shallow and deep Campos Basin waters. These interpreted northwest-trending faults suggested the continuity of the known northwest-striking transfer faults in the deep Campos Basin waters toward the shallow Campos Basin waters and the adjacent continent. Moreover, our interpreted northwest-trending faults revealed the control of several known oilfields in the Campos Basin. This result supported the hypothesis of the influence of the northwest–southeast-trending transfer faults on the petroleum system of Campos Basin, which were reactivated in the Tertiary providing a pathway for the turbidite sedimentation, reworking, and redistribution of several deepwater reservoirs. In addition, it was hypothesized that this faulting system controlled the hydrocarbon migration paths from the presalt source rocks through salt windows into basal suprasalt layers.

Magnetotellurics as a modeling tool in the extensive magmatic context of Paraná Basin, Brazil

Parana Basin is a huge oval-shaped basin covering the southern portion of Brazil and parts of Argentina, Paraguay, and Uruguay, with an area of more than 1,700,000 km2 (Figure 1). Hydrocarbon exploration in Parana Basin has been a challenge due to the worlds second-most-extensive inshore flood basalt complex overlying the Paleozoic sedimentary rocks (Figure 1), which in turn are extensively intruded by swarms of diabase dikes and sills, making seismic imaging a difficult task. The basin itself represents approximately one-half of the sedimentary rock volume of onshore Brazilian basins and is still underexplored. No major discovery for oil or gas has been made to date.

Broadband marine controlled-source electromagnetic for subsalt and around salt exploration

AbstractSalt basins, mainly Tertiary basins with mobilized salt, are notoriously difficult places to explore because of the traditionally poor seismic images typically obtained around and below salt bodies. In areas where the salt structures are extremely complex, the seismic signal-to-noise ratio may still be limited and, therefore, complicate the estimation of the velocity field variations that could be used to migrate the seismic data correctly and recover a good image suitable for prospect generation. We have evaluated the results of an integrated seismic-electromagnetic (EM) two-step interpretation workflow that we applied to a broadband marine controlled-source EM (mCSEM) research survey acquired over a selected ultra-deepwater area of Espirito Santo Basin, Brazil. The presence of shallow allochthonous salt structures makes around salt and subsalt seismic depth imaging remarkably challenging. To illustrate the proposed workflow, we have concentrated on a subdomain of the mCSEM data set, in which a s...