Abstract Microbial carbonates (microbialites) are remarkable sedimentary deposits for four good reasons: they have the longest geological range of any type of biogenic limestones; they form in the greatest range of different sedimentary environments; they oxygenated the Earth’s atmosphere; and they produce and store large volumes of hydrocarbons. However, they are amongst the most intractable of sedimentary rocks to study, as, being formed by the action or influence of microbes, they do not always preserve direct, or diagenetically robust, evidence for their mode of formation. Despite this, the scientific study of microbial carbonates has seen a significant renaissance in recent years, largely because of their importance as petroleum reservoirs, in both the Proterozoic of the Salt Basin of Oman and, more recently, the discoveries in the Lower Cretaceous pre-salt, Santos Basin, offshore Brazil (Figs 1 & 2). Here, production from the pre-salt reservoirs surpassed 500 000 BOPD in June 2014, and the Basin was estimated to have over 50 billion barrels STOIP (Formigli 2014). However, these are in deep-water sites, hundreds of kilometers offshore, hosted in poorly understood carbonate facies and with no nearby outcrop analogues. Many research programmes by academia and the petroleum industry have been spawned in recent years to investigate possible analogues and to further our understanding of these intractable rocks and their complex pore systems. The results from some of this work are contained in this volume together with the first series of scientific papers on the remarkable pre-salt plays of Brazil. This Special Publication provides significant contributions at a pivotal time in our understanding of microbial carbonates, when their economic importance has become established and the results of many research programmes are coming to fruition.

Abstract Carbonate build-ups in lakes, hydrothermal and fluvial settings are characterized by distinctive geometry, spatial distribution, fabrics and geochemical signature but also by some comparable features. Lake margin bioherms form continuous belts for hundreds of metres to kilometres, subparallel to shorelines. Sublacustrine spring mounds are spaced at hundreds of metres to kilometres and aligned along faults. Hydrothermal travertine mounds and aprons with planar clinoforms or terraced slopes are controlled by faults, thermal water discharge and substrate topography. Fluvial tufa barrages, cascades and terraced slopes are controlled by climate, vegetation and substrate gradient. The wide spectrum of carbonate microfabrics ranges from clotted peloidal micrite and laminated boundstone to crystalline dendrite cementstone. Non-marine carbonate microfabrics cannot be linked to specific depositional environments, and are not deterministic proxies for the interpretation of build-up architecture. Microfabric associations can be indicative, but not exclusive, of specific depositional environments and geometry. Stable isotope geochemistry is a useful tool to distinguish between hydrothermal, karstic freshwater and evaporative lake carbonates. Carbonate precipitation results from a continuum of abiotic and biologically influenced/induced processes in settings where carbonate supersaturation is largely driven by physico-chemical mechanisms and microbial biofilms, even if acting as passive low-energy surface sites for nucleation, are widely present.

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).

Abstract Outstanding exposures of Ediacaran-aged thrombolite–stromatolite bioherms and biostromes crop out in the Nama Basin, SW Namibia. Fieldwork, dovetailed with remote sensing and a terrestrial laser scanning (LiDAR) survey, allow the fracture network of this succession to be characterized, and the relative age of fracture sets and families to be determined. The results show that the microbial carbonates were affected by intense syndepositional brittle and ductile deformation. Early brittle fracturing was favoured where early lithification of microbialites took place upon deposition. Such deposits were prone to gravitational collapse due to internal weaknesses during early lithification. Timing of syndepositional fracturing of bioherms and biostromes is demonstrated by contemporaneous microbial overgrowth over brecciated material in open-mode fractures. Ductile deformation occurs preferentially around massive thrombolite domes and columns, represented by folding of mud-dominated sediments in inter-column fill. Secondary fractures developed during the long-lived structural history of the Nama Basin, resulting in a complex fracture network of syndepositional fractures overprinted by secondary fractures. These findings have important implications for carbonate reservoir characterization in microbial reservoirs and subsurface fluid-flow estimations. The observed syndepositional fractures form due to body forces that are intrinsic to the microbial system and thus do not require an external tectonic driver.

Abstract The Rasthof Formation is a mid-Cryogenian cap carbonate succession deposited in Namibia following the Sturtian glaciation. It includes a microbial member, typically >100 m thick. This member exhibits contorted intervals, and is divisible into two informally defined units. The lower unit (microbial member 1: MM1) comprises thickly laminated microbialites (1–6 mm); the upper unit (MM2) is characterized by thinly laminated microbialites (sub-millimetre layering). Contortion of the microbialite deposits – a recurrent feature of this succession – is interpreted to result from soft-sediment deformation. Deformed intervals and styles range from metre- to decimetre-scale chaotic folds in MM1 to a few centimetre-scale, localized roll-up structures in MM2. Study of the microfacies of MM1 and MM2 reveals two essentially different architectures. In MM1 the microfacies is dominated by an alternation of thin micritic laminae with thicker cemented intervals; this probably gave less rigidity to the sediments than in MM2 where the laminated fabric is also present but connected vertically as well, forming a continuous framework. We suggest that the continuity of this framework limited the frequency and scale of soft-sediment deformation. In the Rasthof Formation, the microarchitecture is thus suggested to translate into different degrees of rigidity of the macrofacies.

Abstract The 10 m-scale shallowing-up Neoproterozoic carbonate cycles at Qarn Alam (Oman) provide a record of microbial textures and the communities responsible for them. This is documented for four major microbialite facies. Despite their age, these microbialites show extremely fine preservation of microbial fossils and mineral associations (primary calcite and dolomite with minor phosphate, glauconite, palygorskite, hematite and goethite) and they are the record of a suite of microbial communities, from pellicular biofilms (planar laminites) to mats and gels (crinkly laminites, and stromatolitic layered and massive thrombolites) and a more complex microbial community in bushy thrombolites possibly involving a sponge-like form. Mineralized extra-cellular polymeric substances (EPS) resembles that of modern microbial mats. The mineral associations, as well as cathodoluminescence attributes, indicate oxic to suboxic conditions during deposition and early diagenesis for planar laminites and crinkly laminites, but more evaporitic to saline conditions during development of thrombolites of the upper part of a cycle. Early cementation under variable redox conditions sealed the organomineralized phases.

Abstract Extensive facies analysis of the outcrops located near Moscardón (Iberian basin, NE Spain) resulted in a precise reconstruction of the facies architecture of a Bajocian (Middle Jurassic) carbonate platform. Different types of microbialites occurred in the transitional area between the shallow and relatively deep carbonate platform domains, located above and below wave base level, respectively. The most significant volume of the micorbialites is found in the platform slope, located below wave base level, in a depth range of around 30–50 m. In the lower slope, during stages of rapid accommodation gain, the vertical aggradation of individual stratiform building blocks results in the formation of up to 25 m-thick microbial–siliceous sponge buildups. During stages of sea-level highstand the individual building blocks are dominated by lateral (down-slope) progradational accretion, resulting in flatter, lens-shaped buildups. The transition between the upper slope and the shallow platform area is characterized by microbial-encrusted intraclastic–bioclastic packstones. Microbial crust development helped to stabilize the seafloor, allowing the eventual accumulation and preservation of the sand-sized grains above wave base.

Abstract The Sugar Loaf High is an extensive basement structure within the Santos Basin, offshore Brazil, in which all of the elements of the pre-salt hydrocarbon play are present. Following a clastic-dominated early synrift phase, the later synrift and sag phases are characterized by an extensive non-marine carbonate platform over the basement high. The study area within this new 3D seismic dataset contains carbonates stratigraphically equivalent to the reservoirs within the Lula and Sapinhoá fields. Analysis of the 3D seismic data shows that the pre-salt carbonates onlap the basement of the Sugar Loaf High and form aggradational geometries on a platform that has a proximal to distal width of about 100 km and an interpreted relief of some 900 m. The carbonate platform has an aggradational and mounded basinward-facing gullied margin with slope deposits. Progradational units are imaged on the platform top that may be either carbonate or clastic in composition. The platform is preserved beneath evaporites that form the regional seal within the Santos Basin. Platforms of this size are previously unknown from lacustrine settings outside the pre-salt stratigraphy of the Santos and Campos Basins, Brazil.

Abstract Microbialite carbonates (e.g. stromatolites, thrombolites, shrubs and spherulites) are sedimentary deposits highly influenced by their environmental settings such as water depth, water chemistry and relative energy. Lower Cretaceous subsalt microbialite carbonates, in the Santos Basin (Brazil), have complex pore systems produced by their growth framework, which are related to carbonate precipitation by biotic and abiotic processes and also influenced by subsequent cementation and dissolution. Complex pore systems and high spatial reservoir heterogeneity result in reservoirs having total porosity ranging from 2 to 27% and permeability from less than 0.01 milidarcys to 4.9 darcys. Differences in textural characteristics such as shrub size, sorting and packing lead to different pore systems that subsequently control the petrophysical properties. Cements and dissolution also modify these texturally controlled pore systems by respectively reducing or enhancing the pore volume and pore-throats. The shrub size is a primary control on changes in the pore size and affects the permeability, whereas the shrub sorting influences the primary porosity, and secondarily the permeability. Packing acts as a secondary control on porosity. As result, a sample with small shrubs, well-sorted and tight packing has lower permeability for the same range of porosity than a sample with the same characteristics, but larger shrubs.

Abstract The cyclic distribution of various types of carbonates and Mg-clays in early Cretaceous rift-sag phase lacustrine carbonates from the subsurface of the South Atlantic provides an insight into how evolving lake chemistries in highly alkaline settings control facies development. The typically subdecametre scale symmetrical and asymmetrical cyclothems exhibit three main components: mud-grade laminated carbonates, millimetre-diameter spherulites with evidence of having been in a matrix of Mg-silicates, and millimetre–centimetre calcitic shrub-like growths. The laminites contain conspicuous numbers of ostracods and vertebrate remains and were produced by short-lived pluvial events, causing expansion of shallow lakes. Later evaporation triggered Mg-silicate precipitation and calcite nucleation within gels to produce spherulitic textures. When the rate of gel precipitation decreased or ceased, calcite growth, now less inhibited, produced shrub-like calcites resembling those produced abiotically in modern travertines, although still with some evidence of the former presence of some Mg-silicates. Physical reworking of these sediments led to the dispersion of the gels and the concentration of detrital carbonate components. Despite earlier proposals, evidence of microbial processes producing carbonates in these Cretaceous lake deposits is rare and the application of facies models based on modern and ancient microbialite analogues maybe be misplaced.

Abstract This study uses a borehole image log, supported by limited sidewall-core samples, and conventional wireline logs to erect facies and stratigraphic models for the Aptian pre-salt microbial carbonates (Macabu Formation (Fm)) of the southern Campos Basin, Brazil. This supports the widely held view that these reservoirs are very significant, unusual, partly microbial, and lacustrine in origin. This single-well study penetrates 220 m of microbialite facies from the pre-salt Lagoa Feia Group non-marine carbonates. Continuous borehole images, available sidewall-core data, and gamma-ray and sonic wireline logs are used to identify and characterize borehole image facies. These facies are interpreted to have formed in four lacustrine depositional environments: deep subaqueous, intermediate subaqueous, shallow subaqueous and subaerial. The borehole image facies commonly show shallowing-upward facies trends topped by emergent surfaces. Such trends are interpreted as metre-scale, high-frequency cycles that are grouped into lower order depositional sequences interpreted from the gamma-ray log. A Fischer plot of the high-frequency cycle suggests that the entire Macabu Fm is represented by a decrease in the accommodation space followed by increase in the accommodation space; consistent with this suggestion are trends in δ 18 O values from an adjacent well that indicate an initial trend of increased evaporation followed by increased freshwater inflow (lighter δ 18 O and thicker cycles).

Abstract For many years, sedimentary dolomite rocks have been considered to be primarily a replacement product of the calcium carbonate components comprising the original limestone, a process known as secondary replacement dolomitization. Although numerous dolomite formations in the geological record are composed of fine-grained crystals of micritic dolomite, an alternative process, that is, direct precipitation, is often excluded because of the absence of visible or geochemical indicators supporting primary precipitation. In this research, we present a study of a modern coastal hypersaline lagoon, Brejo do Espinho, Rio de Janeiro State, Brazil, which is located in a special climatic regime where a well-defined seasonal cycle of wet and dry conditions occur. The direct precipitation of modern high-Mg calcite and Ca-dolomite mud from the lagoonal waters under low-temperature hypersaline conditions is associated with the activity of microbial organisms living in this restricted environment. The mud undergoes an early diagenetic transformation into a 100% dolomite crust on the margins of the lagoon. The biomineralization process, characterized by the variations of the physico-chemical conditions in this environment during the annual hydrological cycle, is integrated with isotopic analysis to define the early diagenetic processes responsible for the formation of both dolomitic mud and crust. The carbon isotope values indicate a contribution of respired organic carbon, which is greater for the crust (δ 13 C=−9.5‰ Vienna Pee Dee Belemnite (VPDB)) than mud (δ 13 C=−1.2‰ VPDB). The oxygen isotope values reflect a moderate degree of evaporation during mud formation (δ 18 O=1.1‰ VPDB), whereas it is greatly enhanced during early diagenetic crust formation (δ 18 O=4.2‰ VPDB). The clumped isotope formation temperature derived for the Brejo do Espinho mud is 34 °C, whereas it is 32 °C for the crust. These temperatures are consistent with the upper range of measured values during the dry season when the lagoon experiences the most hypersaline conditions.

Abstract Utah contains unique analogues of microbial hydrocarbon reservoirs in the modern Great Salt Lake and the lacustrine Eocene Green River Formation within the Uinta Basin. Characteristics of both lake environments include shallow-water, ramp margins that are susceptible to rapid widespread shoreline changes, as well as comparable water chemistry and temperature that were ideal for microbial growth and formation/deposition of associated carbonate grains. Thus, microbialites in Great Salt Lake and cores from the Green River Formation exhibit similarities in terms of microbial textures and fabrics. A detailed petrographic analysis provides unique insights into these modern and ancient deposits that can be used to determine reservoir characteristics in other microbial carbonate petroleum plays. Great Salt Lake is a hypersaline lake and carbonate ‘factory’, containing actively forming microbial mats, stromatolites, thrombolites and associated carbonate grains. Open constructional pores are common within a spectrum of microbial structures. Green River Formation cores display excellent examples of stromatolites and thrombolites that contain primary megascopic pore and microporosity, as well as carbonate grainstones composed of ooids, peloids and skeletal material with abundant interparticle and intraparticle porosity. West Willow Creek oil field produces from a Green River microbial buildup/mound, a feature not currently recognized in Great Salt Lake.

Abstract Integrated Ocean Drilling Program (IODP) Expedition 310 (Tahiti Sea Level) offered an opportunity to study the geomicrobiology of a reef framework. Offshore drilling was conducted on the coastal reefs of Tahiti (French Polynesia) at 22 sites in water depths of up to 117 m. Up to 80% of the retrieved core material comprises authigenic grey microbial carbonates with laminated or thrombolitic morphologies, which are associated with corals. Microbialites infilled the cavities during reef development and stabilized the coral reef framework. Rock-surface analyses were performed to track ongoing microbial activity in biofilms that could represent a modern counterpart of the processes at the origin of the formation of fossil microbialites. Significant concentrations of adenosine 5′-triphosphate, indicative of the presence of living microorganisms, were detected at relatively shallow depths, 0–6 m below the seafloor. Exoenzyme activities confirmed the presence of an active metabolizing microbiota forming biofilms in reef cavities. Onshore investigations of the recovered microbes and biofilms completed our picture that the rapid postglacial formation of carbonate microbialites was mediated by the activity of anaerobic microbes, such as sulphate-reducing bacteria and iron-respiring organisms, stimulated by the highly productive reef environment.

Abstract Microbial carbonates (microbialites) are remarkable sedimentary deposits. They have the longest geological range of any type of biogenic limestones, form in the greatest range of different sedimentary environments, oxygenated the Earth’s atmosphere and produce and, furthermore, store large volumes of hydrocarbons. This Special Publication provides significant contributions at a pivotal time in our understanding of microbial carbonates when their economic importance has become established and the results of many research programmes are coming to fruition. It is the first book to focus on the economic aspects of microbialites and in particular the giant pre-salt discoveries offshore Brazil. The volume contains papers on the processes involved in the formation of both ancient and modern microbialites and the diversity of style in microbial carbonate build-ups. Structures and fabrics from both marine and non-marine settings are discussed from throughout the geological record.