Luiz Fernando De Ros

Infiltrated Clays in Fluvial Jurassic Sandstones of Reconcavo Basin, Northeastern Brazil

ABSTRACT Fluvial sandstones and conglomerates of the Sergi Formation, a Jurassic unit of Reconcavo Basin, northeastern Brazil, contain significant amounts of interstitial detrital clays, which are interpreted as the product of early mechanical infiltration developed in an arid/semi-arid environment. Within such an environment, the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium, concentrating the clays in the upper phreatic zone near the sources of influent seepage. Petrographic textures considered to be typical of mechanically infiltrated (MI) clays are herein presented, using examples from the Sergi Formation. These textures include 1) ridges and bridges, 2) geopetal fabrics, 3) loose aggregates, 4) anisopachous coatings of tangentially-accreted lamellae (cutans), 5) massive aggregates, and 6) shrinkage patterns developed during diagenesis. In addition, their detrital aspect as revealed by SEM observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays. In most hydrocarbon reservoirs in the Sergi Formation, the clays described in this paper are among the main controls on porosity and permeability distribution. Commonly, zones with maximum clay concentration are the main internal permeability barriers within the Sergi Formation reservoirs.

The role of depositional setting and diagenesis on the reservoir quality of Devonian sandstones from the Solimões Basin, Brazilian Amazonia

Abstract Devonian sandstones of the Uere Formation are important oil exploration targets in the Solimoes Basin, western Brazilian Amazonia. The basin fill comprises Ordovician to Permian sedimentary successions, Triassic diabase dykes and sills, and Cretaceous to Recent continental deposits. This study deals with the Upper Devonian interval, which consists of sharp-based, progradational sandstones, attributed to a storm-dominated shelf complex formed during an overall transgressive system tract, overlain by Frasnian-Famennian black shales. In spite of their large lateral extent, the exploration of these sandstones is complicated by intense diagenesis, which strongly affected reservoir quality. The main processes of porosity reduction are mechanical and chemical compaction and cementation by quartz overgrowths, carbonates (siderite and dolomite) and fibrous illite. Porosity (up to 28%) was preserved by the inhibition of quartz overgrowth cementation and pressure dissolution by grain-rimming, eogenetic, microcrystalline quartz and associated chalcedony, or smectite. Early diagenetic silica precipitation is related to the dissolution of sponge spicules, which were concentrated in storm-reworked hybrid arenites and in interbedded spiculite deposits. Locally, massive quartz cementation and recrystallisation occurred as a consequence of hot fluids circulation related to Triassic magmatism.

Origin and impact of authigenic chlorite in the Upper Cretaceous sandstone reservoirs of the Santos Basin, eastern Brazil

Upper Cretaceous sandstones are the main clastic reservoirs of the Santos Basin, eastern Brazil, showing high porosity at depths greater than 4000 m. Porosity preservation in these deep sandstones is ascribed to the inhibition of quartz cementation and pressure dissolution by authigenic pore-lining chlorite. Santos sandstones are mostly fine-grained, lithic arkoses, rich in volcanic rock fragments (VRF). Chlorite, the most abundant diagenetic constituent, occurs as coatings, rims, rosettes and replacing grains. Chlorite precipitation was favoured by the presence of eodiagenetic smectite coatings and by the abundance of VRF. Detrital heavy minerals, biotite and mud intraclasts were also sources and/or substrates for chlorite authigenesis. The chloritization of VRF, biotite and heavy minerals resulted in precipitation of abundant TiO2 minerals. Fluid flow patterns, burial and thermal history also played a role in chlorite authigenesis and reservoir quality evolution. Discontinuous and thin coatings and rims were not effective in inhibiting quartz cementation, while those too thick severely reduced the permeability. The rosette aggregates exerted no inhibition on quartz cementation, while reducing intergranular porosity. Systematic studies are necessary for modelling the diagenetic patterns of Santos Basin sandstones, in order to contribute to the reduction of risks involved in the exploration of these reservoirs.

Near‐Surface K‐Feldspar Precipitation in Cretaceous Sandstones from the Potiguar Basin, Northeastern Brazil

The Cretaceous fluvial sandstones of the Açu Formation, Potiguar Basin, northeastern Brazil, contain abundant, very early, authigenic K‐feldspar overgrowths around detrital orthoclase and microcline grains. Detailed optical and cathodoluminescence petrography, scanning electron microscopy, and electron microprobe analyses provided information about the conditions and timing of precipitation of these overgrowths, as well as of the associated diagenetic phases. The overgrowths are optically discontinuous with the detrital grains, untwined, nonluminescing, and were formed clear on grain surfaces under phreatic conditions. The overgrowths commonly show a rhombohedral (sanidine) habit, range in thickness from about 0.4 to 1.0 mm, are chemically pure KAlSi3O8, and constitute up to 6% of the whole rock. The extremely early origin of the K‐feldspar overgrowths is indicated by their coating of mechanically infiltrated smectitic clays. Authigenic smectite rims, booklet and vermicular aggregates of kaolinite, and precompactional calcite cement also cover, and thus postdate, the overgrowths. The K, Si, and Al ions needed to form the K‐feldspar overgrowths were probably derived from the hydrolysis (dissolution and replacement by clay minerals) of detrital feldspars and micas in the coarse, alluvial Açu Formation deposits along the basin margins. Meteoric fluids with high (αK+)/(αH+) and αSiO2aq evolved through evaporative concentration along the groundwater circulation pathways throughout the basin. Relatively high pH and (αK+)/(αH+) ratios were required to precipitate authigenic K‐feldspar within the K2O‐Na2O‐Al2O3‐SiO2‐H2O system at around 30°C and 1 atm. Subsequent smectite and kaolinite authigenesis occurred under decreasing pH and (αK+)/(αH+) ratio, related to K+ consumption, episodic meteoric infiltration, and climatic shifts toward more humid conditions during the deposition of the Açu Formation. The on‐course radiometric dating of the early diagenetic K‐feldspar overgrowths offers the possibility to solve the depositional age of the Açu Formation and other continental clastic sequences, for which biostratigraphic dating is imprecise.

Lithofaciology and Palaeoenvironmental Analysis of Holocene Beachrocks in Northeastern Brazil

Abstract Five lithofacies were identified in beachrock ledges distributed along the eastern and northern coasts of the state of Rio Grande do Norte, northeastern Brazil: (1) massive to weakly stratified conglomerate, (2) low-angle cross-stratified conglomerate and sandstone, (3) medium-scale tabular-planar and trough cross-stratified sandstone, (4) Skolithos-bioturbated conglomeratic sandstone, and (5) massive sandstones. Most of the described beachrocks (53% relative to total thickness) were deposited on the upper shoreface zone, represented by lithofacies 3 and 4. Lithofacies 2, which was deposited on the foreshore zone, represents 31% of the sections described. The remaining 16% are attributed to collapse of overlying material as a result of sea cliff undercutting (massive conglomerate, lithofacies 1), transport as a traction carpet deposit (weakly stratified conglomerate, lithofacies 1), and a high degree of alteration (lithofacies 5). The distribution of 14C-dated beachrock samples in a sea level envelope curve shows that, along the northern coast, framework constituents were predominantly deposited between approximately 4140 and 2190 cal. yr. BP, when sea level dropped, and during low sea level stillstand. On the northern coast, the upper shoreface sedimentation zone is preferentially distributed during sea level drop and low sea level stillstand phases, whereas the foreshore sedimentation zone can be found along the whole sea level curve. Conversely, the beach deposits distributed along the eastern coast were preferentially formed between approximately 7460 and 4240 cal. yr. BP during rapid sea level rise and high sea level stillstand. In the case of the eastern coast, both zones are equally concentrated in the first stages of rapid sea level rise and high sea level stillstand.

Event Ordering Reasoning Ontology Applied to Petrology and Geological Modelling

The inference of temporal information from past event occurrences is relevant in several applications for geological domains. In such applications, the order in which events have happened is imprinted in the domain as visual-spatial relations among its elements. The interpretation of the relative ordering in which events have occurred is essential for understanding the geological evolution in different scales of observation and for various kinds of objects, as in Petrology and Geological Modelling. From the analysis of the cognitive abilities of experts in these domains we propose an ontology for event ordering reasoning within domains whose elements have been modified by past events. We show that the Event Ontology can work as a pattern for domain conceptualization to be applied in distinct domains. It can be used to specify the sequence order of diagenetic paragenesis. It can also be operative for automatic reconstruction of geological surface assemblages.

An Ontology-Based Conceptual Framework to Improve Rock Data Quality in Reservoir Models

In the petroleum industry, a huge amount of data is generated every day by many different sources. The petroleum industry relies on the efficient use of this data to build computational models that represent subsurface petroleum reservoirs. A problem that arises when one is building reservoir models is the fact that the existent data is hardly interoperable. Many companies produce data, but the data is not easily integrated. In this work, we present a novel ontology-based conceptual framework to enhance well logs data quality, in order to improve the construction of reservoir models. Better reservoir models results in a better prediction of the petroleum reservoir quality, supporting a better evaluation of the reservoir economic value. We propose the use of the geological concept of reservoir petrofacies to make explicit the attributes that most contribute to the reservoir porosity and permeability. Thus, with the help of an expert, it is possible identify these attributes in well logs, enabling the possibility to extrapolate these known attributes to other logs in which the expert did not evaluate. The result is the possibility to trace this data along the different modeling activities.

Geochemical modeling of diagenetic reactions in Snorre Field reservoir sandstones: a comparative study of computer codes

Diagenetic reactions, characterized by the dissolution and precipitation of minerals at low temperatures, control the quality of sedimentary rocks as hydrocarbon reservoirs. Geochemical modeling, a tool used to understand diagenetic processes, is performed through computer codes based on thermodynamic and kinetic parameters. In a comparative study, we reproduced the diagenetic reactions observed in Snorre Field reservoir sandstones, Norwegian North Sea. These reactions had been previously modeled in the literature using DISSOL-THERMAL code. In this study, we modeled the diagenetic reactions in the reservoirs using Geochemists Workbench (GWB) and TOUGHREACT software, based on a convective-diffusive-reactive model and on the thermodynamic and kinetic parameters compiled for each reaction. TOUGHREACT and DISSOL-THERMAL modeling showed dissolution of quartz, K-feldspar and plagioclase in a similar temperature range from 25 to 80°C. In contrast, GWB modeling showed dissolution of albite, plagioclase and illite, as well as precipitation of quartz, K-feldspar and kaolinite in the same temperature range. The modeling generated by the different software for temperatures of 100, 120 and 140°C showed similarly the dissolution of quartz, K-feldspar, plagioclase and kaolinite, but differed in the precipitation of albite and illite. At temperatures of 150 and 160°C, GWB and TOUGHREACT produced different results from the DISSOL-THERMAL, except for the dissolution of quartz, plagioclase and kaolinite. The comparative study allows choosing the numerical modeling software whose results are closer to the diagenetic reactions observed in the petrographic analysis of the modeled reservoirs.

Calcretes pedogenéticos da Bacia de Itaboraí, Estado do Rio de Janeiro

The Itaborai Basin, located in Rio de Janeiro State, Brazil, is considered the first segment from the Southeastern Brazil Continental Rift opened, during the Paleocene. The occurrence of calcretes was previously reported in the literature, although studies dealing specifically with such calcretes and their distribution were not performed. Furthermore, there is limited research involving the petrographic characterization of the sedimentary deposits of the basin. This study developed a characterization on both macro and micro scale of the vadose calcretes that occur along the northern and western margins of the basin and constitute its initial infilling. The compositional, textural and fabric features of 27 thin sections prepared from calcrete samples were described and interpreted. The pedogenetic calcretes were separated in different facies (chalky, nodular, massive, platy, incipient laminar and brecciated), and classified according to morphogenetic stages. The main microscopic features include nodules, contraction cracks, vadose pisoliths, rhizocretions, infiltrated clays, corroded grains, expanded micas and clay aggregates. The pedogenetic calcretes were developed on weathered alluvial deposits during tectonic stability periods. They are typically generated on continental environments under arid to semiarid climates and occur intercalated with travertines (carbonate crusts of hydrothermal origin), which deposition is favored by fault activity during humid climate periods. The results indicate both pedogenetic and hydrothermal processes have been extremely active and have played important role during the early evolution of Itaborai Basin. Thus, the characterization of such processes is essential to the understanding of the initial evolution of Southeastern Brazil Continental Rift.

A Conceptual Framework for Rock Data Integration in Reservoir Models Based on Ontologies

InthedomainofE&Ppetroleumchain,theauthorshavedevelopedastudyandproposedaconceptual frameworkforinsertingdirectrockdataintoreservoirmodels,throughcalibrationofwelllogs.This typeofdataisoftenignoredormanualprocessedinactualpetroleumreservoirmodelingactivities, duetoitshighcostofacquisition,orduetothehighcomplexityforitsmodeling,interpretationand extrapolation.Directrockdata is thedata that isacquiredbydirectobservationof therock, like descriptionsofwellcores,insteadofindirectdata,likeseismic,welllogs,etc.Directlyobserveddata isimportantbecauseitallowscalibrationofindirectinterpretationmethods,detectionoferrorsin theirevaluationsandalsotoverifyrockpropertiesthatisnotpossiblewhenusingindirectdata.The authorsclaimthatawelldefinedontologycanhelpindescribingreservoirpetrofacies,whichare aggregatesofrockdatathatcanbedetectedthroughaspecialsignatureintheregistersofindirectly collectedrockdataand,therefore,theycanusetointegraterockdatainreservoirmodels. KeywORDS Data Integration, Data Interoperability, Direct Rock Data, Ontology, Petroleum Data, Reservoir Modeling