Tucano Basin Research Articles

Caracterização de microexsudações de hidrocarbonetos na Bacia do Tucano Norte (BA) por geoestatística, classificação hiperespectral e redes neurais

This study focus on the characterization of hydrocarbon microseepages in the northern Tucano Basin (Bahia State, Brazil), using geostatistical analysis of regional hydrocarbon geochemical data yielded from soil samples and digital processing of Enhanced Thematic plus (ETM+/Landsat7 satellite) and Advanced Spaceborne Thermal Emission and Reflection Radiometer imagery (ASTER/Terra satellite). A theoretical detection model was devised for the study in which gas anomalies (seeps) indicated by hydrocarbon geochemistry should spatially match a number of surface expressions such as the presence of bleached soil and rocks (i.e., reduction of Fe 3+ to Fe 2+ ), geobotanical markers and concentration of specific clays (kaolinite) and carbonates (siderite). These indirect evidences were employed for the application of remote sensing data and information extraction techniques in order to locate sites more favorable to host hydrocarbon seeps in the Tucano Basin. The ETM+ data was processed through the pseudo-ratio technique - an adaptation of the classic principal components analysis. The ASTER data was processed through the Spectral Angle Mapper and the Mixture Tuned Matched Filtering (techniques commonly used for the classification of hyperspectral data; here adapted for multispectral data) and via a pioneer approach by means of three different neural network systems (Fuzzy Clustering, Radial Basis Functional Link Network, and Probabilistic Neural Network). The results showed that several sites where the features predicted in the detection model concurred with geochemical anomalies were mapped. Other sites with similar characteristics, but for which no geochemical data were available, were also revealed. These sites are taken as new potential targets for the presence of seeps and oil reservoirs. The research demonstrated the excellent potential of ASTER data and spectral-spatial methodologies for low-cost, onshore exploration of hydrocarbons in Brazil.

Remote detection of hydrocarbon microseepage areas in the Serra do Tonã region, Tucano basin, Brazil

Landsat thematic mapper (TM) images enhanced through principal components analysis allowed the detection of tonal anomalies possibly associated with surface manifestations of hydrocarbon microseepage. Target areas occur mainly surrounding the Serra do Tonã plateau, in accordance with a previous geological model that explains the distribution of the hydrocarbon microseepage in the region. The spatial association between the tonal anomalies and anomalous concentration of hydrocarbon in soil was quantitatively evaluated using weight of evidence (WofE) analysis. Results indicated that tonal anomalies are positive evidence for the occurrence of anomalous concentrations of soil gas data, suggesting a possible geologic origin to the target areas. Field study investigated the terrain characteristics in one of the soil gas anomalies. Ground truth data confirmed anomalous concentrations of gaseous hydrocarbons in soils, accompanied by an increase in the abundance of radioactive and magnetic minerals, a fact that denotes the action of a reducing environment responsible for the precipitation of this hydrocarbon-induced mineralogical assemblage. Results suggest that remote sensing techniques may be used as an auxiliary prospecting tool for similar target areas in the remaining Tucano basin, or even throughout the Recôncavo‐Tucano‐Jatobá rift, in northeastern Brazil.

ABSTRACT: Numerical Modeling of the Thermal Maturation, Petroleum Generation and Migration in the Central Tucano Basin, Brazil

Basin modeling results of the central Tucano basin, including 1D and 2D models, are discussed in this paper. The Tucano basin, located onshore northeast Brazil, is part of an aborted rift system, originated in the Neocomian, and consists of east and west dipping half-grabens. The sedimentary fill in the prerift stage is mainly of Jurassic age and includes the potential reservoir of the Sergi Fm. The synrift development (Berriasian-Aptian) began with the main potential source rock of Candeias Fm, and was followed by a thick sandstone succession of the Ilhas and Massacara Groups. Two regional seismic lines and four wells were modeled. Calibration was achieved considering eroded thickness from the upper to middle synrift succession that increases towards the western border. The models indicate that generation and expulsion could have occurred in the depocenters. Petroleum saturation and migration was mainly restricted to the depocenter. Uncertainties are related to the presence of effective source rocks. The principal pulse of petroleum generation-expulsion for the Candeias Fm could occur during Barremian-Aptian times, coeval with sedimentation of the Massacara Group. Model results indicate that the migration of fluids was influenced by faulting. Compartmentalization is present, especially with an impervious fault model. Vertical leakage is highly effective with pervious faults. Sergi Fm could represent an effective carrier bed in some regions. Petroleum accumulations appear mainly restricted to traps associated to, or near the depocenter.

Terrain characteristics of a tonal anomaly remotely detected in an area of hydrocarbon microseepage, Tucano Basin, north-eastern Brazil

This study reports the geological characteristics of a tonal anomaly in an area of hydrocarbon microseepage, identified in Landsat-5 Thematic Mapper (TM) images. A diversified dataset was gathered, which included soil gas geochemistry determinations, soil radioactivity intensity and soil magnetic susceptibility measurements, as well as laboratory soil spectra. Data indicated that the tonal anomaly holds a higher concentration of gaseous hydrocarbons in soils than the surroundings, accompanied by enrichment in radioactive minerals. However, there is no definitive evidence of magnetic mineral enrichment inside the tonal anomaly compared with the surroundings. An intense silicification process, characterized by a cover of fine-grained silicified cobbles and boulders, seems to be responsible for inhibiting the development of diagenetic clay mineralization, as indicated by variations in the depth of the 2200 nm soil spectra absorption band.

Data integration for a geologic model of hydrocarbon microseepage areas in the Tonã Plateau region, North Tucano basin, Brazil

A diversified database was used for investigating the major factors controlling the distribution of hydrocarbon microseepage in the region of the Tonã Plateau, North Tucano sedimentary basin, Brazil. Visual analysis of this database suggested the existence of spatial relationships between anomalous geochemical soil gas data with rift faults, lithologic contact, and groundwater discharge areas. As a result, a geologic model was proposed. According to this model, rift faults constitute the primary pathways for the upward migration of gaseous hydrocarbons from a deep-seated source rock in the depocenter of the basin. Cherty limestone layers in the Tonã Plateau act as a seal to prevent the escape of the hydrocarbons to the surface. As a result, hydrocarbon microseepage occurs preferentially in the contact zone of the cherty limestones of the Tonã Sequence with porous sandstones of the Marizal Formation, mainly in areas of groundwater discharge. The importance of these three patterns for controlling the near-surface distribution of the anomalous soil gas samples was estimated through Bayesian probability analysis. According to this interpretation, lithologic contact is the spatial pattern more strongly associated with the anomalous soil gas samples, followed by groundwater discharge areas. Rift faults show the weaker spatial correlation with the anomalous near-surface areas of hydrocarbon microseepage.

Remote detection of a tonal anomaly in an area of hydrocarbon microseepage, Tucano basin, north-eastern Brazil

Mapping of a tonal anomaly in an area of hydrocarbon microseepage was possible using a Landsat-Thematic Mapper false-colour composite consisting of TM2/3, TM4/3 and of the difference TM2/3-TM4/3, displayed as red, green and blue, respectively. In this false-colour composite, green and yellow express variations in the vegetation cover, whereas hues of magenta are related to terrain conditions representing bleached materials, which occur close to the centre of a soil gas anomaly. Simultaneous occurrence of anomalous soil gas values with magnetic and radioactive minerals suggests that a reducing environment associated with the microseepage phenomenon created the bleached materials, identified as a tonal anomaly in the enhanced TM image.

Abstract: Data Integration Using GIS in the Tucano Basin, Brazil, as an Aid to Hydrocarbon Exploration

Abstract: Data Integration Using GIS in the Tucano Basin, Brazil, as an Aid to Hydrocarbon Exploration

Abstract: Petroleum System Comparison of the Phitsanulok Basin, Thailand with the Tucano Basin, Brazil 

Abstract: Petroleum System Comparison of the Phitsanulok Basin, Thailand with the Tucano Basin, Brazil 

Modeling the tectonic development of the Tucano and Sergipe-Alagoas rift basins, Brazil

Abstract The Tucano and Sergipe-Alagoas basins of northeast Brazil form part of a regional extensional basin system that was operative during the Mesozoic breakup of South America and Africa with both basins developing on Precambrian crust. The synchronous development of the rift basins suggest that they are genetically linked in space and time. Whereas the offshore Sergipe-Alagoas basin is characterized by a substantial thickness of post-rift sediment, the Tucano basin either failed to develop or at least preserve a significant thickness of post-rift sediment. Observed primary porosities within both pre- and syn-rift Tucano basin sediment imply that significant post-rift sedimentation never occurred. Failure to develop significant post-rift subsidence has important thermal and mechanical implications for the reaction of the lithosphere to rifting and can be explained in terms of: (1) depth-dependent lithospheric extension in which intracrustal detachments allow the extension of the crust to be decoupled from the thinning of the lithospheric mantle, (2) small rates of extension that allow the lithosphere to cool during rifting, and/or (3) lithospheric rifting during which the flexural strength of the lithosphere remains high. With respect to points (2) and (3), forward modeling demonstrates that finite rifting rates over a 20–25 m.y. period are insufficient to cool the lithosphere to the point where post-rift subsidence fails to develop. An interesting complication arises when the flexural strength of the lithosphere remains large during rifting: it tends to suppress the vertical motions of the lithosphere, such as those engendered by the cooling of the lithosphere following rifting, thereby reducing significantly the amplitude of the post-rift subsidence. Thus, the lack of post-rift sedimentation within a basin does not necessarily imply that extension has been limited to the crust. From our kinematic modeling of the Tucano basin, the observed negative free-air and Bouguer gravity anomalies (− 120 mGals) suggest that the flexural strength of the lithosphere has remained sufficiently large to maintain the load of the sediment. We can model successfully the observed Tucano basin architecture and gravity anomaly irrespective of whether we assume depth-dependent or depth-independent lithospheric extension primarily because the flexural strength of the lithosphere tends to “buffer” or suppress the amplitude of the post-rift subsidence. In contrast, the adjacent Sergipe-Alagoas basin is associated with low-amplitude gravity anomalies which may reflect a resetting of flexural strength during rifting. Prior to rifting, however, there was no appreciable elevation difference between the Tucano and Sergipe-Alagoas regions (as might be induced by a hot-spot for example) implying that the flexural strength of the lithosphere was similar. The total thickness of the syn- and post-rift sediments within the Sergipe-Alagoas basin is about the same as the thickness of rift-phase sediments in the Tucano basin. Thus, the amount of crustal extension responsible for each basin was similar. However, since the Sergipe-Alagoas basin contains 4–5 km of post-rift sediments, lithospheric mantle thinning in this region must have been significantly greater than the crustal extension to overcome the effects of “flexural bufferring”. The need for lithospheric mantle thinning to be greater that crustal extension in the Sergipe-Alagoas region was also a result obtained by modeling the development of the ocean/continent boundary between Brazil and Africa. From our coupled kinematic and rheological model of lithospheric extension, we predict that the ocean/continent boundary should form preferentially at the location of greatest crustal and lithospheric mantle thinning, that is, within the region of maximum depth-independent lithospheric extension. The ocean/continent boundary formed east of the Sergipe-Alagoas basin despite the fact that the Tucano basin represents the region of greatest crustal thinning. If extension had been uniform with depth beneath these basins, the ocean/continent boundary should have formed in the Tucano basin. As it did not, some form of intracrustal detachment appears to have been operative to reduce lithospheric mantle thinning beneath the Tucano basin while enhancing it beneath the offshore regions of the Sergipe-Alagoas basin.

Basin-forming processes and the deep structure of the Campos Basin, offshore Brazil

This paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins. The events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors: shear zones, thin layers of intrusive rocks, underplated material, gneissic banding, fluids and heterogeneities in the crust and mantle. Several types of sedimentary basins have been recognized with the aid of deep seismic reflection data: basins formed along active master faults; basins formed along inactive faults; basins with no fault control; basins formed by low angle detachment faults; and basins formed by pervasive pure shear, or an approximately pure shear, where the lower crust has been locally extended by a different amount than the upper crust. The mechanisms of basin forming processes recognized in deep seismic profiles are: (a) simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle; (b) thinning of the lower crust and mantle by ductile processes, convective mechanisms or flow in the mantle and the upper crust by faulting, resulting in an overall pure shear; and (c) thinning of the lower crust associated with phase changes in the mantle or non-conservation of mass. Two end-member models of basin formation involving lithospheric stretching may be recognized in the South Atlantic: simple shear stretching, where only the upper crust is involved in the process and mantle uplift is offset relative to the basin; and pure shear stretching, where uplift of the mantle vertically balances the basin fill. The first mechanism has been suggested for the onshore Tucano Basin. The data obtained for the offshore Campos Basin suggests regional lithospheric stretching and crustal thinning with Moho uplift compensating for sediment accumulation in the basin depocentre. Complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the Campos basin.

A new technique of locating turbidite fans: Candeias formation (lower cretaceous), Recôncavo Basin, Brazil

The reservoir package in the Candeias oil field, the largest stratigraphic accumulation of hydrocarbons in the Recôncavo Basin, has been reinterpreted as consisting of sublacustrine turbidite fans interbedded among Candeias shales, with the best quartzose arenite reservoirs corresponding to channeled mid-fan lobes. The occurrence in the turbidites of reworked lacustrine carbonate clasts with subaerial exposure features indicates that the Candeias submarine fans originated from an intrabasinal shallow platform and followed NNW-SSE fault-controlled channels on their way to final deposition. Thick and irregularly shaped lobes of massive fine sandstones (Pitanga Member) are always located immediately behind (that is, updip from) the relatively thin and coarse turbidite fans and overlap them diachronically. The petrographic composition of the massive sandstone expresses an extrabasinal origin from low-grade metamorphics of the Precambrian basement, and their sedimentary structures indicate large-scale fluidized sediment flow depositional processes. However, the admixture of a small amount of shallow water lacustrine carbonate clasts reveals that Pitanga sediments also crossed the carbonate platform before being deposited as isolated or coalescent lobes. Candeias turbidite fans had a stationary source area, the carbonate platform, and reached the distal portion of the shale slope. Pitanga fluidized flows were deposited in a proximal position of the shale slope and had a deltaic source that gradually prograded from NNW to SSE across the Tucano Basin. The constant geometric relationship between Pitanga lobes and Candeias turbidite fans originates from the fact that both depositional systems followed the same NNW-SEE fault-controlled channels; this becomes, therefore, an exploration tool for the prediction of oil-producing Candeias fans that are at the limit of resolution of seismic reflection techniques. Three operational steps are required: structurally analyzing the carbonate platform to locate potential transverse faults to which feeder channels might correspond; outlining the lobes of Pitanga massive sandstones related to such channels using seismic reflection profiles; and drilling exploratory wells for Candeias turbidite fans beneath the frontal portions of Pitanga lobes.