Leonardo Borghi

Lower Devonian fish remains from the Falkland Islands

We report the discovery of fossil vertebrate remains from the Lower and Middle Devonian of West Falkland (Fig. 1). Acanthodian remains were found in the marine Fox Bay formation (Lower Devonian; Emsian?) near Roy Cove, Port North. Although this material is fragmentary, it represents the first record of any Lower Devonian vertebrate from the Falkland Islands. An incomplete placoderm central plate was also recovered from the Port Philomel formation (Middle Devonian, Givetian) at Dunnose Head, West Falkland.

Facies and depositional environments for the coquinas of the Morro do Chaves Formation, Sergipe-Alagoas Basin, defined by taphonomic and compositional criteria

Lacustrine carbonate rocks form important hydrocarbon accumulations along the Brazilian continental margin, some of which are contained in oil fields in which coquinas are one of the main reservoirs (viz. Campos Basin). The complexity and heterogeneity of these deposits make them a challenge in terms of reservoir description. For the necessary classification and paleoenvironmental interpretation of the coquinas, it is essential to evaluate many aspects including biological (such as carbonate productivity), sedimentological (energy regime in the depositional environment, transport of bioclasts, terrigenous supply), taphonomic (fragmentation of shells, abrasion) and diagenetic processes. The facies analysis applied in this study is considered a more appropriate classification approach to understand these coquinas, since it is more flexible and comprehensive than the existing classifications for carbonate rocks. The material investigated here consists of rock samples of the coquinas from the Atol Quarry of the Morro do Chaves Formation (Barremian/Aptian), Sergipe-Alagoas Basin. These rocks that crop out in the Atol quarry complex can be considered as a case study for similar coquinas reservoirs found in the Brazilian continental margin basins. Six sedimentary facies were described, using the main taphonomic (fragmentation of shells) and compositional (presence of micrite and siliciclastic matrix) features as a diagnostic criteria. Two carbonate facies, two mixed carbonate-siliciclastic facies and two siliciclastic facies (mudstones) were identified. From the facies succession, combined with a review of the literature on the subject, the following depositional paleoenvironments were defined: high-energy lake platform, lacustrine delta in a high-energy lake platform and lake-centre. In this paper, a new facies model for the studied coquinas succession is proposed.

Microbial carbonates: a sampling and measurement challenge for petrophysics addressed by capturing the bioarchitectural components

Abstract Ancient and modern stromatolites are potentially a challenge for petrophysicists when characterizing biosediments of microbial origin. Because of the heterogeneity, sometimes very cemented and lacking porosity, sometimes highly porous, these widely differing states can be used to develop techniques that can have wider application to addressing the representative elementary volume (REV – single or multiple REVs) challenge in microbial carbonates. Effective media properties – like porosity – need to be defined on REV scales and the challenge is that this scale is often close to or significantly larger than the traditional core plugs on which properties are traditionally measured. A combination of outcrop images, image analysis techniques, micro-computed tomography (CT) and modelling have been used to capture the porosity (or in some cases, precursor porosity) architecture and provide a framework for estimating petrophysical property sensitivities in a range of situations that can be subjected to further calibration by measurements in relevant microbial reservoir rocks. This work will help guide the sampling approach along with the interpretation and use of petrophysical measurements from microbial carbonates. The bioarchitectural component, when controlling porosity in microbial carbonates, presents a significant challenge as the REV scale is often much larger than core plugs, requiring careful screening of existing data and measurement and additional geostatistical model-based approaches (with further calibration).

Integration of Cretaceous Morro do Chaves rock properties (NE Brazil) with the Holocene Hamelin Coquina architecture (Shark Bay, Western Australia) to model effective permeability

Coquinas are significant producing facies in the pre-salt reservoirs of Brazil and Angola. This paper describes an analogue model study to help understand the reservoir characterization of coquina reservoirs. Porosity and permeability measured in 39 core plugs from a single coquina bed in a Lower Cretaceous outcrop in NE Brazil have been combined in a three-dimensional geological model for use in reservoir simulation. A training image derived from the geometry of a Holocene coquina analogue in Western Australia controlled the distribution of the bed-scale petrophysical properties from the outcrop. A synthetic well test showed that the effective permeability of the system lies between the geometrical and harmonic averages of the plug data in the layer. There is also moderate flow anisotropy, with preferential fluid flow aligned along the beach ridges. This paper demonstrates how the combination of outcrop data and an appropriate modern environment might be used to improve our understanding of the behaviour of coquina reservoirs and to guide future reservoir studies.

Carbonate Coquina Reservoirs - New Insights for Petrophysical Reservoir Characterisation

With the recent discoveries of Pre-Salt oil fields in lacustrine carbonate reservoirs of Barremian-Aptian age, on both African and Brazilian Margins of the South Atlantic, interest has increased in the petrophysical aspects of these complex reservoirs. Some of these reservoirs are composed of coquinas which are limestones composed of shells or shell fragments, in this case bivalves. The petrophysical challenges in coquina pore systems result from wide range of textures, fabrics and diagenetic processes showing; a. Primary shells preserved in fine-grained carbonate and/or mixed carbonate-clastic matrix b. Dissolved or partially dissolved and corroded shells in a fine-grained matrix c. Recrystallised shells and matrix d. Dissolution of fine-grained matrix and preferential preservation of shells e. Gradations between, and combinations of, all of the above We have investigated methods to quantify porosity, measure geometry, scale and distribution of pore space and its connectivity in plugs from coquina samples from the Sergipe-Alagoas Basin (onshore NE Brazil). These are the results of our initial investigation, where we could relate m to the type of porosity, connectivity of pores, pore throat size distribution and permeability. In general, the increase of porosity in these samples correlates with an increase in m.

Formalização estratigráfica do Membro Fundão, Formação Rio da Batateira, Cretáceo Inferior da Bacia do Araripe, Nordeste do Brasil

O intervalo litoestratigrafico conhecido como “camadas Batateira” e bastante citado na literatura sobre a Bacia do Araripe, tendo sido reconhecido tanto na Sub-bacia do Cariri, quanto na Sub-bacia de Feitoria. Sua ampla distribuicao e, aparentemente, pouca espessura, levaram-no a ser considerado um marco de correlacao regional. No entanto, apesar da importância que lhe e atribuida, o intervalo nunca foi alvo de uma descricao formal, o que levou o nome “camadas Batateira” a ser utilizado com abrangencia variada entre os diversos autores, tornando urgente sua discussao e formalizacao. Analises litoestratigraficas, palinologicas e do conteudo de carbono orgânico total do furo de sondagem 1-PS-06-CE e sua correlacao com o afloramento no Rio Batateira (citado originalmente como secao de referencia do intervalo) permitiram uma caracterizacao mais precisa do intervalo e uma proposta de formalizacao sob a denominacao de “Membro Fundao”, com a definicao de holoestratotipo, representado pelo afloramento no Rio Batateira (localidade do Sitio Fundao, municipio do Crato, Ceara), e de um estratotipo suplementar, constituido por testemunhos do furo de sondagem 1-PS-06-CE (localidade de Salobro, municipio de Abaiara, Ceara).

Microbialitos da Sequência Balbuena IV (Daniano), Bacia de Salta, Argentina: caracterização de intrabioarquiteturas e de microciclos

Microbialites produced in lacustrine environment exhibit a wide range of internal structures and textures. The organization and the characterization of the arrangement formed by these attributes are denominated as intrabioarchitectures. This work sought to identify the main types of intrabioarchitectural patterns in the microbialites of Balbuena IV Sequence (Danian, Salta Basin, Argentina) in mesoscopic scale (10 0 – 10 -2 m) and the cyclical interpretation for the recurrence of the components that build these intrabioarchitectures, in microscopic scale. Seven main intrabioarchitectural types were recognized, and denominated as: coarse grain agglutinated microbialite (MA-gg); fine grain agglutinated and sparite fan crust, banded microbialite (MBFA-gf); micrite agglutinated, pseudo-microcolumnar microbialite (MPMA-m); fine grain agglutinated, grumous, shrubby microbialite (MArbMA-gf); fine grain agglutinated grumous, dendriform microbialite (MDMA-gf); fine grain agglutinated with esferulites, sparite fan crust, banded microbialite (MBFA-gf,esf); coarse grain agglutinated, grumous, stromatolite (EMA-gg). These intrabioarchitectures were grouped and related to three types of microcycles. The microcycles allow organizing the microbial record in a meso- and microscopic scale. That cyclic organization is related to climatic variations that affect the base level, the input of siliciclastics sediments, the biotic and geochemical processes in lake system. Therefore, it was possible to associate the formation of microbialites to phases of higher or lower aridity in the depositional environment.

Caracterização Sedimentar e Tafonômica de Depósitos Bioclásticos na Reserva Tauá e Marina Búzios (Holoceno do Estado do Rio de Janeiro)

O estudo sobre acumulacoes bioclasticas tem aumentado nos ultimos anos, devido a sua grande fonte de informacoes sedimentar, tafonomica e paleoambiental. Entretanto o conhecimento sobre as caracteristicas de feicoes associadas com ambiente meteorico em acumulacoes bioclasticas nao e completamente compreendido. O objetivo deste estudo e o reconhecimento de feicoes de alteracao superficial, integrando a identificacao taxonomica com analises sedimentologicas e tafonomicas, que ocorrem em conchas de moluscos de depositos holocenicos da planicie costeira do estado do Rio de Janeiro. O deposito Reserva Taua e composto por uma matriz lamosa com biotrama densa e com 54% das conchas com concavidades voltadas para baixo. A Marina Buzios e composta por uma matriz de areia com biotrama densa a frouxa e 65% das concavidades voltadas para cima. As analises tafonomicas mostraram que os depositos possuem fracas alteracoes tanto fisicas e quimicas, entretanto o pavimento exposto da Reserva Taua propiciou uma alteracao de carater microbiana, observado pelas microperfuracoes nas conchas. Apesar da bioestabilizacao do deposito da Reserva Taua ja ocorre ha algumas centenas de anos, a evolucao dos processos pedogeneticos e limitada pela proximidade com o lencol freatico.

Gênese e Morfologia de Estruturas Sedimentares Induzidas por Atividade Microbiana (MISS) em Sedimentos da Lagoa Vermelha (Região dos Lagos – Rio de Janeiro)

MISS are primary sedimentary structures formed by interactions between biofilms and microbial mats with physical and chemical dynamics of sediments. Recognizing these structures in the geologic record are difficult because there are no criteria of identification, and sometimes they can be confused with other depositional or erosional physical structures. During the last decades, lots of biological, mineralogical, geochemicals and isotopic studies related to stromatolites and microbial mats were performed in the Lagoa Vermelha. However, the sedimentary structures formed by microbial induction (MISS) have never been the subject of these studies. The aim of this study is the description and characterization of MISS that occur in sediments of the Lagoa Vermelha, and recognize the main biosedimentological factors involved on their genesis. MISS were recognized macroscopically in the field and cores. The main processes recognized are microbial growth (that form laminated leveling structures), bioestabilization (that form desiccation cracks, mat curls, mat chips), metabolism (that form biolaminite, stromatolite, biscuit stromatolite, thrombolytic nodule) and binding, baffling & trapping. The combination of processes can also produce structures (wrinkles, detachment structure, petees). Petrographic analysis supported the identification of binding, baffling & trapping processes and others microscopic microbial features. The results allow a better understanding of the relationship between microbial mats and physical and chemical processes in a lagoon sedimentary environment. The recognition of these structures formed by microbial induction in modern environments helps their recognition in the geologic record of continental and marine successions.