Isostatic constraint for 2D nonlinear gravity inversion on rifted margins

We have developed a nonlinear gravity inversion for simultaneously estimating the basement and Moho geometries, as well as the depth of the reference Moho along a profile crossing a passive rifted margin. To obtain stable solutions, we impose smoothness on basement and Moho, force them to be close to previously estimated depths along the profile and also impose local isostatic equilibrium. Different from previous methods, we evaluate the information of local isostatic equilibrium by imposing smoothness on the lithostatic stress exerted at depth. Our method delimits regions that deviate and those that can be considered in local isostatic equilibrium by varying the weight of the isostatic constraint along the profile. It also allows controlling the degree of equilibrium along the profile, so that the interpreter can obtain a set of candidate models that fit the observed data and exhibit different degrees of isostatic equilibrium. Our method also differs from earlier studies because it attempts to use isostasy for exploring (but not necessarily reducing) the inherent ambiguity of gravity methods. Tests with synthetic data illustrate the effect of our isostatic constraint on the estimated basement and Moho reliefs, especially at regions with pronounced crustal thinning, which are typical of passive volcanic margins. Results obtained by inverting satellite data over the Pelotas Basin, a passive volcanic margin in southern Brazil, agree with previous interpretations obtained independently by combining gravity, magnetic, and seismic data available to the petroleum industry. These results indicate that combined with a priori information, simple isostatic assumptions can be very useful for interpreting gravity data on passive rifted margins.

Integração de parâmetros geomorfológicos e biológicos no desenvolvimento do Índice Integrado de Sensibilidade do Litoral (IISL) (Integration of geomorphological and biological parameters to develop a Coastal Integrated Sensitivity Index (CISI))

No contexto da indústria petrolífera e suas relações com os ecossistemas enquadram-se os instrumentos de políticas públicas, desenvolvidos a fim de prevenir e minimizar os efeitos de acidentes com óleo e derivados, como as Cartas de Sensibilidade Ambiental ao Derramamento de Óleo (Cartas SAO). Dentre as principais informações contidas nas Cartas SAO está o Índice de Sensibilidade do Litoral (ISL). Este índice mede a sensibilidade dos diferentes ambientes costeiros ao contato com óleo de acordo com as características geomorfológicas da região: exposição às forçantes hidrodinâmicas, tipo de substrato e declividade do litoral. Nesse estudo foi desenvolvida uma metodologia específica que integrou dados geomorfológicos e biológicos no desenvolvimento de um Índice Integrado de Sensibilidade do Litoral (IISL). Tal metodologia desenvolveu previamente dois índices distintos, o Índice Geomorfológico de Sensibilidade (IG) e o Índice Biológico de Sensibilidade (IB), os quais tiveram suas variáveis bem definidas permitindo a integração para desenvolvimento do IISL. Três regiões da Bacia de Pelotas, sul do Brasil, foram escolhidas para a aplicação dessa metodologia. Os resultados indicaram alteração nos valores de sensibilidade ao óleo em seis trechos de linha de costa analisados, apurando o mapeamento destas áreas. Assim, o presente estudo buscou apresentar elementos que venham subsidiar o aprimoramento metodológico de mapeamento de sensibilidade ao óleo no país, contribuindo na gestão de incidentes e no gerenciamento costeiro. A B S T R A C TIn the context of oil industry and its relations with ecosystems the instruments of public policy are framed, which are developed to prevent and minimize the effects accidents involving oil and its derivatives, such as the Environmental Sensibility to Oil (ESO charts). The main information in the ESO charts is the Coastal Integrated Sensitivity Index (CISI), which measures the sensitivity of different coastal environments to contact with oil according to geomorphological characteristics, such as exposure to hydrodynamic forces, substrate type and coastal slope. The concept of sensibility used for the characterization of the coastline does not include in its methodological scope biological information, therefore, a new specific methodology was developed, which integrated geomorphological and biological data to develop a Coastal Integrated Sensitivity Index (CISI). This methodology previously developed two different indexes, the Geomorphological Sensitivity Index (GSI) and Biological Sensitivity Index (BSI), which had their well-defined variables allowing the integration of CISI. Three regions of the Pelotas Basin, south of Brazil, were chosen to test this methodology. The results indicated a change in sensibility values in six coastline segments analyzed, improving the mapping of these áreas. This way, the present study effectively contributed to upgrade the methodology for mapping the sensibility to oil spills, also contributing on the management of accidents involving oil and on coastal management.Keyword: Oil spill, ESO charts, coastal sensitivity, biological and geomorphological data.

3D controlled-source electromagnetic imaging of gas hydrates: Insights from the Pelotas Basin offshore Brazil

Mapping of natural gas hydrate systems has been performed successfully in the past using the controlled-source electromagnetic (CSEM) method. This method relies on differentiating resistive highly saturated free gas or hydrate-bearing host sediment from a less resistive low-saturated gas or brine-bearing host sediments. Knowledge of the lateral extent and resistivity variations (and hence the saturation variations) within sediments that host hydrates is crucial to be able to accurately quantify the presence of saturated gas hydrates. A 3D CSEM survey (PUCRS14) was acquired in 2014 in the Pelotas Basin offshore Brazil, with hydrate resistivity mapping as the main objective. The survey was acquired within the context of the CONEGAS research project, which investigated the origin and distribution of gas hydrate deposits in the Pelotas Basin. We have inverted the acquired data using a proprietary 3D CSEM anisotropic inversion algorithm. Inversion was purely CSEM data driven, and we did not include any a priori information in the process. Prior to CSEM, interpretation of near-surface geophysical data including 2D seismic, sub-bottom profiler, and multibeam bathymetry data indicated possible presence of gas hydrates within features identified such as faults, chimneys, and seeps leading to pockmarks, along the bottom simulating reflector and within the gas hydrate stability zone. Upon integration of the same with CSEM-derived resistivity volume, the interpretation revealed excellent spatial correlation with many of these features. The interpretation further revealed new features with possible hydrate presence, which were previously overlooked due to a lack of a clear seismic and/or multibeam backscatter signature. In addition, features that were previously mapped as gas hydrate bearing had to be reinterpreted as residual or low-saturated gas/hydrate features, due to the lack of significant resistivity response associated with them. Furthermore, we used the inverted resistivity volume to derive the saturation volume of the subsurface using Archie’s equation.

Origin and Alteration of Organic Matter in Hydrate-Bearing Sediments of the Rio Grande Cone, Brazil: Evidence from Biological, Physical, and Chemical Factors

ABSTRACTThe Rio Grande Cone is a major fanlike depositional feature in the continental slope of the Pelotas Basin, Southern Brazil. Two representative sediment cores collected in the Cone area were retrieved using a piston core device. In this work, the organic matter (OM) in the sediments was characterized for a continental vs. marine origin using chemical proxies to help constrain the origin of gas in hydrates. The main contribution of OM was from marine organic carbon based on the stable carbon isotope (δ13C-org) and total organic carbon/total nitrogen ratio (TOC:TN) analyses. In addition, the 14C data showed important information about the origin of the OM and we suggest some factors that could modify the original organic matter and therefore mask the “real” 14C ages: (1) biological activity that could modify the carbon isotopic composition of bulk terrestrial organic matter values, (2) the existence of younger sediments from mass wasting deposits unconformably overlying older sediments, and (3) the deep-sediment-sourced methane contribution due to the input of “old” (>50 ka) organic compounds from migrating fluids.