Carbonate Outcrops Research Articles

Visualization of Acid Treatments With 3D Nuclear-Magnetic-Resonance Imaging

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 168198, “The First Visualization of Acid Treatments on Carbonates With 3D Nuclear-Magnetic-Resonance Imaging,” by M. Krebs, SPE, Clausthal University of Technology; B. Lungwitz, SPE, A. Souza, SPE, A. Pépin, SPE, S. Montoya, P. Schlicht, and A. Boyd, SPE, Schlumberger; and E. Vidoto, R. Polli, and T. Bonagamba, University of Sao Paulo, prepared for the 2014 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, USA, 26–28 February. The paper has been peer reviewed and is scheduled to appear in SPE Journal. Core-flow tests are usually conducted to test and model stimulation treatments at laboratory scale, to predict the performance of such treatments in carbonate reservoirs. The visualization of wormholes created within core-flow tests requires novel technologies for evaluation and pathway-prediction purposes. Unfortunately, past visualization techniques were always associated with the destruction of the core sample. This paper describes the use of nuclear-magnetic-resonance imaging (NMRI) as a nondestructive visualization method in correlation with 3D rock imaging. Introduction The experiments described in the complete paper were conducted with different core samples such as Indiana limestone, Silurian dolomite, Winterset limestone, and carbonate outcrops. The core samples were selected according to their properties, which greatly differ for each type with respect to their origin. The core-flow tests or acidizing tests performed in this study serve the purpose of simulating field treatments at laboratory scale. NMRI applies a gradient static magnetic field such that the strength of the field decreases with increasing distance away from the source. The Larmor frequency becomes dependent on the position of the nuclei, which will be associated with a single value of the gradient magnetic field. Hence, it is possible to locate the nuclei in a region radially around the source. More specifically, NMRI uses mostly pulsed gradient magnetic fields that belong to a group of linear gradient fields in order to link acquired signals to a specific volume element. Therefore, it is possible to locate the nuclei more exactly in the 3D space instead of only defining a region. The gradient fields are used for spatial encoding to acquire a location-dependent signal, and they are the slice-selection gradient, phase-encoding gradient, and frequency-encoding gradient.

Application of handheld energy-dispersive X-ray fluorescence spectrometry to carbonate studies: opportunities and challenges

In situ rapid quantitative geochemical measurements of carbonate using handheld XRF.

GPR investigation of karst guided by comparison with outcrop and unmanned aerial vehicle imagery

The increasing importance of carbonate rocks as aquifers, oil reservoirs, and for urban problems is demanding detailed characterization of karst systems, a demand that can be partially satisfied with GPR imaging. However, the goal of imaging and interpreting karstified carbonate rocks is notoriously difficult due to the complex nature of the geometry of the dissolution and the GPR intrinsic limitations. One way forward is the direct comparison of GPR images with similar outcropping rocks. A joint study involving a 200MHz GPR survey, unmanned aerial vehicle imagery (UAV), and outcrop characterization is presented aiming to improve the interpretation of sedimentary structures, fractures and karst structures in GPR images. The study area is a 500m wide and 1000m long carbonate outcrop of the Jandaíra Formation in Potiguar basin, Brazil, where sedimentary, fracture, and karst features can be directly investigated in both vertical and horizontal plan views. The key elements to interpret GPR images of karstified carbonate rocks are: (1) primary sedimentary structures appear in radargrams as unaltered imaged strata but care must be taken to interpret complex primary sedimentary features, such as those associated with bioturbation; (2) subvertical fractures might appear as consistent discontinuities in the imaged strata, forming complex structures such as negative flowers along strike–slip faults; (3) dissolution may create voids along subhorizontal layers, which appear in radargrams as relatively long amplitude shadow zones; and (4) dissolution may also create voids along subvertical fractures, appearing in radargrams as amplitude shadow zones with relatively large vertical dimensions, which are bounded by fractures.

Widespread contamination of carbonate-associated sulfate by present-day secondary atmospheric sulfate: Evidence from triple oxygen isotopes

The isotope composition of seawater sulfate is an important tracer of sulfur, carbon, and oxygen cycles in Earth’s deep past. Carbonate-associated sulfate (CAS) extracted by acid digestion is widely used as a proxy for sulfate in paleo-seawater from which the carbonate minerals precipitated. Early and late diagenesis, weathering, and laboratory processing can in some cases compromise original seawater sulfate signals. Here, we report that extracted CAS can also be severely contaminated by recent atmospheric sulfate, especially when the sampled carbonates are from outcrops in arid to semi-arid climates or in heavily polluted regions. Our evidence comes from triple oxygen isotope compositions of sequentially extracted water-leachable sulfate and acid-leachable sulfate from carbonates of diverse ages from northwestern and north-central China and southwestern North America. Independent of the age of the rocks, almost all the water-leachable sulfates and half of the acid-leachable sulfates bear positive 17O anomalies, clearly distinguishable from those of typical seawater sulfate. Because secondary atmospheric sulfate (SAS) is the only source of sulfate known to bear positive 17O anomalies, we conclude that sulfate extracted from carbonate outcrops in these regions has a significant component of SAS. Because SAS generally has a much lower δ34S value than paleo-seawater sulfate, it could shift the δ34S of the extracted CAS to lower values and in some cases even lower than that of the co-occurring pyrite, i.e., the “super-heavy pyrite” enigma reported in geological records. Our findings call for a re-evaluation of many published, outcrop-based CAS data and conclusions.

The Carboniferous Southern Pennine Basin, UK

Many of the Carboniferous outcrops located in the Derbyshire region of the Peak District National Park, UK, have provided sites for both significant and pioneering research relating to the clastic sedimentology of marine palaeoenvironments, particularly so during the 1960s and 1970s when early models describing the sedimentary architecture of fluvio‐deltaic, submarine‐slope and deep‐marine submarine‐fan sedimentation were first developed. The area was subject to hydrocarbon exploration from the 1920s to 1950s, which although unsuccessful in economic terms left a legacy of sub‐surface data. Despite a long‐history of sedimentological research, the deposits exposed at several classic localities in the Pennine Basin continue to broaden and challenge our current understanding of sedimentary processes to this day.

An Experimental Evaluation of Oil Recovery by Steam Alternative CO2 Injection in Naturally Fractured Reservoirs

Among all enhanced oil recovery scenarios, gas and steam injections seem to be promising processes to implement in naturally fractured reservoirs. Major drawbacks of steam injection are availability of water near the injection site and the high cost of steam generation. A new enhanced oil recovery process named “steam alternative CO2” is proposed in this study, which combines the benefits of steam injection and miscible CO2 injection. The experiments aimed to investigate the potential of oil recovery by steam alternative CO2 injection and to optimize design parameters of the proposed method. Gravity drainage experiments have been carried out on a low permeable outcrop carbonate rock, which was surrounded by fracture, established with a novel experimental method. The matrix block was saturated using a recombined mixture of Iranian live oil, while by means of pumping water into the annular space, space between rubber sleeve and outer jacket, high over burden pressure was exerted to obtain the desired homogeneous saturation. Using a back pressure regulator the pressure was kept above bubble point pressure. The inlet was attached to a constant pressure pump injecting CO2 or steam above bubble point pressure, then the over burden pressure was removed gradually and the inlet fluid inflated the rubber sleeve. The amount of produced water from the annular space was recorded to estimate the distance between the rubber sleeve and sand face; this distance creates the fracture surrounding the core. Gas was injected into the fracture at pressures above the bubble point of the oil. Oil recovery, as a function of time, was monitored during the experiments. Results demonstrated that steam alternative CO2 injection produces more oil as compared to both CO2 and steam injections in separate schemes. Gravity drainage experiments indicated that there are optimum values for a number of injected cycles of steam and CO2 and volumetric steam over CO2 ratio; it was proven that it is more efficient to inject CO2 prior to steam slug.

New insights into seismic stratigraphy of shallow-water progradational sequences: Subseismic clinoforms

Seismic clinoforms are the key building blocks for constructing the seismic stratigraphy of progradational depositional sequences. However, not all progradational systems are necessarily represented by seismic clinoforms. We evaluated the definition and interpretation of progradational systems that do not associate with seismic clinoforms. Nonclinoform (or subseismic clinoforms) seismic facies are mainly related to shallow-water deltas where the thickness of a prograding clinoform complex is too thin to be imaged as an offlapping reflection configuration. The clinoform detection limit for clinoform imaging is defined as one wavelength (the thickness of two seismic events) and is related to the predominant frequency of the seismic data and the velocity of the sediments. Three examples from the Songliao Basin of China and Gulf of Mexico illustrated ancient shallow-water deltas with various morphologies in lacustrine and marine environments by integrating the analysis of the core, wireline logs, and amplitude stratal slices made from nonclinoform seismic events. A seismic model of an outcrop carbonate clinoform complex in west Texas further demonstrated the seismic frequency control on clinoform seismic stratigraphy, including transitions between different types of clinoforms and between clinoforms and nonclinoform seismic facies. Ambiguity in interpreting nonclinoform seismic facies can be reduced by high-resolution acquisition, high-frequency enhancement processing, and seismic sedimentology.

Using the chemical composition of carbonate rocks on Mars as a record of secondary interaction with liquid water

In the search for habitable environments on Mars, sites that have been altered by liquid water represent an important target. Quickly dissolving and precipitating carbonates, found in multiple locations on Mars, represent a key potential mineral indicator of locations with a history of aqueous alteration, and therefore possible habitability. Recent liquid surface water on Mars may differ significantly from the fluids, either sedimentary, hydrothermal, or pedogenic, that formed carbonate minerals on Mars. Interactions of carbonate rocks with such recent surface water may therefore cause changes in chemistry and mineralogy with depth that form weathering profiles recording the conditions of alteration. To test the prediction that martian carbonates may record a signature of aqueous alteration on Mars, we performed reactive transport modeling of putative carbonate outcrops on Mars using the reactive transport code CrunchFlow. Model inputs included two parent rocks chosen based on the carbonate-rich rock at Gusev Crater, Comanche, one consisting of 60% magnesite, 20% siderite, and 20% porosity, and one consisting of 50% magnesite, 20% siderite, 9% calcite, 1.6% rhodochrosite, and ~20% porosity. Reacting solutions consisted of four scenarios: acidic, sulfate-containing solutions, perhaps resulting from an acid vapor scenario; dilute solutions resulting from martian analog basalt dissolution; concentrated solutions resulting from modeled evaporation of the dilute solutions; and solutions based on the results of the Phoenix Lander. Modeling was performed under low oxidation and high oxidation conditions. Based on the modeling results, a carbonate rock would become enriched in siderite upon interaction with acidic water under low oxidation conditions; the same rock would become enriched in calcite after interaction with Ca-enriched solutions. A carbonate rock reacting under more oxidizing conditions would preferentially dissolve siderite, leaving a surface enriched in magnesite and ferric oxide. Therefore, the Mg-rich carbonates on Mars are consistent with possible interaction with liquid water under oxidizing conditions, similar to those measured at the Phoenix landing site. Future detailed chemical and mineralogical profiles through weathered rock into the unaltered interior of carbonate rocks may therefore allow the quantitative interpretation of the aqueous history and possible habitability of locations on Mars.

A comparative study of authigenic carbonates from mussel and tubeworm environments: Implications for discriminating the effects of tubeworms

The Gulf of Mexico hydrocarbon seeps are often populated by dense mussel beds and tubeworm aggregations, as well as exposed authigenic carbonate outcrops. Previous studies suggest the activity of mussels and tubeworms could influence the sediment geochemistry of their habitats, resulting in variations in the stable carbon isotopes of the associated carbonates. However, this conclusion was based on the analyses of samples from a single site. To better understand whether there are consistent differences in the geochemical environments of mussels and tubeworms, mineralogical and stable isotopic compositions of authigenic carbonates from mussel and tubeworm environments from four seep sites were analyzed. The studied sites span a depth range of 1200m to 2800m on the northern Gulf of Mexico continental slope. We found that carbonate samples from tubeworm environments were more prone to contain aragonite whereas carbonates from mussel environments were more likely to have calcite. This finding supports the hypothesis that vestimentiferans release sulfate across their roots into the pore waters of the surrounding sediments, a process that could generate a locally sulfate-enriched environment that favors the precipitation of aragonite instead of calcite. Moreover, the δ13C values of tubeworm carbonates are generally lighter than that of mussel carbonates from the same site, which is consistent with the fact that tubeworms are fueling extra subsurface methane oxidation through the release of sulfate into the sediment. Such a process, consequently, enriches the subsurface dissolved inorganic carbon pool with light carbon derived from the seeping hydrocarbons. Taken together, our data suggest that tubeworms could produce a carbon isotope shift that is sufficient to influence the sediment geochemistry of their immediate area, and that this impact is reflected in the associated authigenic carbonates.

Gas Injection Into Fractured Reservoirs Above Bubble Point Pressure

Among all enhanced oil recovery (EOR) scenarios, gas injection seems to be promising for implementation in naturally fractured reservoirs. The use of CO2 has received considerable interest as a method of EOR but a major drawback is its availability and increasing cost. Therefore, an alternative gas like CH4 or N2 must be considered to meet the economic considerations. To investigate the efficiency of oil recovery by CO2, N2, and CH4 injection in fractured carbonate rock, a series of experiments was designed. Both miscible and immiscible schemes for gas injection were carried out on a low-permeable outcrop carbonate rock that was surrounded by fracture, established with a novel experimental method. The experiments aimed to investigate the potential of oil recovery by secondary and tertiary gas injection under high-temperature conditions. The matrix block was saturated using a recombined mixture of Iranian live oil, and by pumping water into the annular space, the space between rubber sleeve and outer jacket, high overburden pressure was exerted to obtain the desired homogeneous saturation. Using a back-pressure regulator, the pressure was kept above the bubble point pressure. The inlet was attached to a constant pressure pump injecting gas or water above the bubble point pressure, and the overburden pressure was removed gradually and the inlet fluid inflated the rubber sleeve. The amount of produced water from the annular space was recorded to estimate the distance between the rubber sleeve and sand face. This distance creates the fracture surrounding the core. Gas was injected into the fracture at pressures above the bubble point of the oil. Oil recovery as a function of time was monitored during the experiments. Results from both secondary and tertiary gas injection experiments indicate that CO2 injection at elevated pressure and temperature is more efficient than N2 and CH4 injection.

Lochkovian conodonts from Podolia, Ukraine, and their stratigraphic significance

In the Podolian Dniester Basin (southwestern Ukraine) the Lower Devonian marine deposits are represented by about 530 m thick continuous sequence of interlaminated carbonate and schale outcrops at several localities. Conodonts occur in most of the carbonate layers of the whole Lochkovian but are not abundant and their ramiform elements are mostly broken or lacking. Therefore, only the pectiniform, Pa elements of twenty five stratigraphically important conodont species occurring in the region are discussed and two new species, Caudicriodus schoenlaubi and Pandorinellina? parva are proposed. The hypothetical phyletic relationships within the main representatives of the icriodontid and spathognathodontid genera, Caudicriodus, Zieglerodina, and Pandorinellina? are traced. Comparison of the previously published and newly obtained data revealed discrepancies in the hitherto used interpretation of some of the conodont taxa and their stratigraphic ranges. Contrary to the earlier reports, Caudicriodus postwoschmid...

Carbonate rocks in the Mojave Desert as an analogue for Martian carbonates

AbstractCarbonate rocks in the Mojave Desert are presented as potential analogues for the carbonates on Mars. Rocks collected from the Little Red Hill site contain iron oxide-bearing coatings that greatly suppress the spectral features due to carbonate of the underlying material and impart a spectral slope. The Mojave Desert was formerly a lush pedogenic soil environment that, over time, transformed into the current arid climate with abundant rock varnish. One niche for microbes in the current desolate environment is inside and underneath the rocks where the microbes profit from solar protection by the iron oxide rock coatings. Carbonates were long predicted to be present on Mars and have recently been detected by instruments on Phoenix and MER and using hyperspectral orbiters such as the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), the Planetary Fourier Spectrometer (PFS) and the Thermal Emission Spectrometer (TES). We describe here the results of a study of carbonate rocks from the Little Red Hill site of the Mojave Desert that includes X-ray diffraction (XRD), chemistry and visible-infrared reflectance spectroscopy. Coatings on the carbonate rocks greatly reduced the strength of the carbonate bands and caused changes in the shape of some bands. We compare these data with a carbonate outcrop at Nili Fossae, Mars. If microbes once inhabited Mars, similar carbonate rocks with iron oxide coatings could have provided a UV-protected niche there as well. Thus, analysis of carbonate-bearing regions on Mars by future landers would be useful sites to search for biosignatures.

Surface and subsurface drainage evolution of the Corfino and Soraggio karst areas (Tuscany, Italy)

The Pania di Corfino and Ripa di Soraggio are two minor karst areas in Tuscany, having a surface of only 11 km2, but contain more than 100 known caves. Some caves are old epi-phreatic passages testifying to a discontinuous lowering of base level in the two major valleys that cross the carbonate outcrops: the Serchio di Soraggio and the Fiume rivers, respectively located along the NWand SE borders of the massif. The spatial-altimetric distribution of major caves, whichare found on a vertical range of a few hundreds of meters, and their relationships withthe position of surface alluvial deposits have allowed to infer a first evolutionary framework of karst during the late Quaternary. If we refer to a simple model, where fluvial deposition occurs mainly during cold stages and incision during warm stages, the discrete distribution of cave passages suggests that the different epi-phreatic phases are the responses to the alternation of cold and warm periods. In any case, the re-organization of the river network induced by the tectonic uplift had a relevant effect on cave systems. First, the underground diversion of surface drainage enhanced the downcutting of NWand SE peripheral streams, whichreceived a larger quantity of water throughkarst springs due to the favored morpho-structural setting. Successively, the backward piracy of the allogenic catchments of the karst systems by surface tributaries led to the dewatering of caves and to the present situation.

NMR relaxation of neritic carbonates: An integrated petrophysical and petrographical approach

Abstract A set of carbonate outcrop samples, covering a wide range of the sedimentary textures and depositional environments existing on carbonate systems, was studied through an integrated petrographical and petrophysical approach. With the aim of improving the understanding of the NMR (Nuclear Magnetic Resonance) signal of carbonates, this work is: 1) providing an atlas for various carbonate reservoir rock-types, 2) providing a workflow for integrating geological and petrophysical data and, 3) documenting common shortfalls in NMR/MICP analyses in carbonates. The petrographical investigation includes thin section and SEM (Secondary Electron Microscope) observations, whereas petrophysical investigation includes porosity (Φ), permeability (K), NMR, MICP (Mercury Injection Capillary Pressure), and specific surface area (BET) measurements. On the basis of NMR and MICP data, 4 groups of samples were identified: (1) microporous samples, (2) micro–mesoporous samples, (3) wide multimodal samples, and (4) atypical samples. The microporous samples allow us to define a maximum NMR threshold for microporosity at a T 2 of 200 ms. NMR and MICP response of the investigated carbonates are often comparable in terms of modal distribution (microporous, micro–mesoporous and wide multimodal samples). In particular, micritization, a well known but underestimated early diagenetic process, tends to homogenize the NMR signal of primarily different sedimentary facies. A grainstone with heavily micritized grains can display well sorted unimodal NMR and MICP signatures very similar, even identical, to a mudstone–wackestone. Their signatures are comparable to that of a simple sphere packing model. On the contrary, several samples (labeled atypical samples) show a discrepancy between NMR and MICP response. This discrepancy is explained by the fact that MICP can be affected by the physical connectivity of the pore network, in case of disseminated and isolated molds in a micrite matrix for instance. Similarly, NMR can differentiate pore classes but with less resolution. It does not rely on connectivity but can be affected by diffusional pore coupling, i.e. the diffusion of water molecules carrying the magnetization between micropores and macropores. The pore-coupling phenomenon, through its impact on the T 2 distribution, may disturb the permeability calculations from NMR data. For core plug characterization, NMR appears to be a complementary tool to MICP and should be used to complete rock-typing analysis in carbonates. For reservoir rock-typing, the obvious advantage of developing a NMR based approach is the use of NMR logging data providing continuous records of pore size distributions.

Relations between geological characteristics and satellite‐derived infrared and microwave emissivities over deserts in northern Africa and the Arabian Peninsula

Infrared and microwave emissivities have been previously calculated over the globe from satellite observations to estimate the longwave radiative budget of the planet in global circulation models (GCM) or to facilitate the assimilation of surface‐sensitive satellite observations over land in numerical weather prediction (NWP) models. This study examines the relationship between these emissivities and the soil lithology over the deserts in northern Africa and the Arabian Peninsula at large scales. For this purpose, a lithology map of the area has been carefully derived from combining several geological maps and in situ lithostratigraphic columns. Infrared and microwave emissivities show large spatial variability, much more than often assumed in GCM or in NWP models, that relate well to the surface properties, especially the lithology. Changes in the lithology explain ∼40% and up to 50% of the emissivities variability at the considered frequencies in the infrared and in the microwaves. In particular, siliceous and carbonate outcrops have specific signatures that can be identify in these wavelength ranges, with a surprisingly good sensitivity of the microwaves to the presence of carbonates. This study clearly indicates that geological information is necessary for emissivity modeling, not only in the infrared but also in the microwaves, for GCM and NWP applications. The lithology maps created from the geological maps for this study is available under request. Our study also suggests that the combination of infrared and microwave emissivities has some potential for lithology mapping at large scales, especially for land surface models.

Megafauna community composition associated with Lophelia pertusa colonies in the Gulf of Mexico

The deep-water coral Lophelia pertusa provides habitat for diverse communities in the Gulf of Mexico. Photomosaics and analyses within a Geographic Information System (GIS) were used as non-destructive sampling tools to examine megafauna community composition associated with L. pertusa colonies on authigenic carbonate outcrops in two regions of the Gulf of Mexico. Megafauna communities associated with L. pertusa were more similar within a region than between regions. Within regions, the amount of dead coral, number of abiotic and biotic substrata, and percentage of live L. pertusa influenced the diversity, composition, and structure of the coral-associated communities. Elevated diversity levels in the communities associated with L. pertusa structure indicate that L. pertusa provides a distinct, localized habitat source. Outcrops with high proportions of dead L. pertusa harbored more higher order consumers than outcrops with primarily live coral framework.

From fractures to flow: A field-based quantitative analysis of an outcropping carbonate reservoir

Fractures can play an important role in the fluid storage–migration properties of fault damage zones. In this present contribution, we document the role exerted by fractures on fluid flow in carbonate damage zones of hydrocarbon-bearing, km-long, oblique-slip normal faults with 10's of m-throw. The carbonate fault damage zones were analysed by mean of scan line surveys conducted in both tar-free and tar-rich outcrops. In this way, the relationships among the individual fracture characteristics (length, spacing, aperture, orientation, connectivity and distance from slip surfaces pertaining to small faults of the fault damage zones) and hydrocarbons have been established. Data obtained by scan line surveys were also used to compute the amount of fracture porosity, the degree of fracture connectivity and, based upon simple assumptions, the orientation of the local σ hmax at times of faulting. Additionally, scan line surveys were also carried out along outcrops exposing unfaulted carbonate host rocks. The results of our computation are consistent with a carbonate host rock made up of a quite isotropic fracture array comprised of isolated and coupled fractures, in which individual fracture sets have negative exponential spacing distributions. In terms of fluid flow, the fracture array of the carbonate host rock enhances the fluid storage. Conversely, the fracture array of the fault damage zones is characterized by a pronounced anisotropy due to interconnected fractures, which enhance the fluid migration. Fractures in the fault damage zones include those inherited from background deformation and others related to the faulting processes. The latter fracture sets are characterized by power law spacing distributions. In conclusion, counter-intuitively, both fracture length and fracture spacing do not have any correlation with hydrocarbons in the fault damage zones. On the contrary, fracture anisotropy, fracture spread and fracture orientation are positively correlated with hydrocarbons.

A contribution to the reconstruction of Miocene seepage from authigenic carbonates of the northern Apennines (Italy)

Authigenic carbonates from outcrops of the northern Apennines consist of small and irregular lenses and exhibit numerous features indicative of cold-seep settings. Detailed petrographic, mineralogical and geochemical studies from two Miocene deposits are presented. The first carbonate outcrop, named Fosso Riconi, is located in the foredeep basin of the Apenninic chain, whereas the second deposit represents a satellite basin called Sarsetta. The stable isotope data from specific carbonate minerals show a wide range of values well known from other palaeoseeps of the Apennine Mountains. The majority of seep carbonates are formed by low-Mg calcite and ankerite. Those minerals have δ13C values between −7 and −23‰ V-PDB, suggesting variable amounts of carbonate derived from oxidized methane, seawater (dissolved inorganic carbon) and sedimentary organic matter. Dolomite samples have the lowest δ13C values (−30.8 to −39.0‰ V-PDB), indicating methane as the main carbon source. The findings suggest an evolutionary formation of the seeps and development of authigenic carbonates influenced by the activity of chemosynthetic organisms, of which large lucinid clams are preserved. Bioirrigation by the clams controlled the sediment–water exchange, and is here considered as an explanation for the anomalous Mg content of the calcite. We hypothesize that the seep carbonates were formed during periods of active methane-rich seepage, whereas during periods of slow seepage carbonate formation was reduced. Despite different geological settings, the two examined deposits of Sarsetta and Fosso Riconi show similar features, suggesting that a common pattern of fluid circulation played a major role in carbonate formation at both seep sites.

Improving fractured carbonate-reservoir characterization with remote sensing of beds, fractures, and vugs

Many key aquifers and oil reservoirs are in carbonate rocks. Understanding the flow behavior within this commonly complex pore space requires new perspectives and technology in order to improve carbonate aquifer and reservoir characterization. Dissolution of carbonates is related to flow; hence, quantifying the size of dissolution vugs on carbonate outcrops can help characterize controls on flow, namely matrix permeability and fracture connectivity. LIDAR (light detection and ranging) scans, combined with high-resolution photography, enable us to both measure vugs' areas and assess spatial relationships between vugs, beds, and fractures. We developed a method of obtaining and interpreting necessary vug, bed, and fracture data on the basis of these technologies. Application of this method on a Cretaceous Edwards Group outcrop in Texas (United States) revealed a significant correlation between the relative vug area of beds obtained remotely and air permeability measured in plugs extracted from these beds (R2 = 0.94, P = 0.001). The total area of vugs intersected by fractures was used to establish a probability density function of fracture lengths that can improve flow modeling of the reservoir. These findings show the potential of using LIDAR and photo images in reservoir characterization by data analysis of geological features, in addition to their use for accurate mapping.

Climatic and lithologic controls on the temporal and spatial variability of CO2 consumption via chemical weathering: An example from the Australian Victorian Alps

A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO2 consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ34S values of 16 to 20‰CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS2. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~1.1 to ~4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO2 fluxes from silicate weathering range from ~0.012×106 to 0.039×106 mol/km2/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO2 consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO2 consumption rates in particular, based on major ion abundances, should be treated with skepticism.