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Highlights: • Identify 3 groups of gas migration structures in seismic data from the Danube Fan. • Migration structures related to shallow gas migration and flares at the seafloor. • Gas migration is controlled by lithological heterogeneity and sediment deformation. • Mass transport deposits play a role in controlling vertical migration occurrence. Abstract: A large geophysical dataset, including bathymetry, and 2D and 3D P-cable seismic data, revealed evidence of numerous gas flares near the S2 Canyon in the Danube Fan, northwestern Black Sea. This dataset allows us to investigate potential relationships between gas migration pathways, gas vents observed at the seafloor and submarine slope failures. Vertical gas migration structures as revealed in the seismics appear to be concentrated near submarine slope failure structures. Where these seismically defined features extend upwards to the seafloor, they correlate with the location of gas flares. However, not all these structures reach the seafloor, in some cases because they are capped by overlying sediments. A strong correlation is inferred between gas migration pathways, heterogeneous mass transport deposits and contacts between adjacent units of contrasting lithology. Although missing age constrains prevent a final judgement, we discuss the potential relationship between submarine slope failures and gas migration in order to determine if gas migration is a precursor to failure, or if the presence of slope failures and associated mass transport deposits facilitates the migration of gas. Our observations indicate that lithological heterogeneity, mass transport deposits and minor sediment deformation control gas migration pathways and the formation of gas chimney-like features. Gas migration is focused and gradual, resulting in gas flares where the chimney-like features extend to the seafloor, with no evidence of erosive features such as pockmarks.

Highlights • BSR position does not match BGHS as predicted based on regional TP conditions. • Use steady state and transient models to determine extent of hydrate stability. • Investigate the influence of topographic focusing on hydrate stability. • Variable thermal properties of sediment impact hydrate stability. The Danube Fan in the western Black Sea shows many features indicating the presence of gas and gas hydrates, including a bottom simulating reflection (BSR), high-amplitude anomalies beneath the BSR and the presence of gas flares at the seafloor. The BSR depth derived from 3D P-cable seismic data of an older slope canyon of the fan (the S2 canyon) suggests that the BSR is not in equilibrium with the present-day topography. The Danube Fan was abandoned ∼7.5 ka, and the S2 canyon was likely incised ∼20 ka, suggesting that the gas hydrate system has had at least 7.5 ka years to equilibrate to the present-day conditions. Here we examine the extent and position of the hydrate stability zone through constructing both steady and transient state models of a 2D profile across the S2 canyon. This was done using inputs from mapping of the 3D P-cable seismic data and geochemical analysis of core samples. Using these models, we investigate the effects of different factors including variable thermal properties of heterogeneous sediments in the vicinity of the canyon and, topographic focusing on the geothermal gradient on the extent of the hydrate stability zone. Our results indicate that both factors have a significant effect and that the hydrate system may actually be in, or approaching equilibrium.

Shales spanning a range of thermal maturity (~ 0.6 to > 4.5%Re) were examined by high resolution transmission electron microscopy (HRTEM) in order to characterize the occurrence of turbostratic carbon nanostructures. Such structures, termed Basic Structural Units (BSU), were found in both demineralized and whole-rock samples. Highly aromatic turbostratic nanostructures found in a pre-oil window shale were verified by electron energy loss spectroscopy (EELS). Turbostratic nanostructures increase in abundance with increasing thermal maturity with their greatest frequency occurring in high maturity (> 3.6%R), natural and artificially matured organic rich shales that exhibit anomalously high electrical conductivity in well logs. We hypothesize that these turbostratic carbon nanostructures initially form upon oil generation, increase in frequency with increasing thermal maturation, and eventually become numerous enough to establish mutual contact, either directly or through graphene-like sheets, so that the shale becomes conductive on a macroscopic scale.

The Marcellus Formation of Pennsylvania represents an outstanding example of an organic matter (OM)–hosted pore system; most pores detectable by field-emission scanning electron microscopy (FE-SEM) are associated with OM instead of mineral matrix. In the two wells studied here, total organic carbon (TOC) content is a stronger control on OM-hosted porosity than is thermal maturity. The two study wells span a maturity from late wet gas (vitrinite reflectance [Ro], 1.0%) to dry gas (Ro, 2.1%). Samples with a TOC less than 5.5 wt. % display a positive correlation between TOC and porosity, but samples with a TOC greater than 5.5 wt. % display little or no increase in porosity with a further increasing TOC. In a subset of samples (14) across a range of TOC (2.3–13.6 wt. %), the pore volume detectable by FE-SEM is a small fraction of total porosity, ranging from 2 to 32% of the helium porosity. Importantly, the FE-SEM–visible porosity in OM decreases significantly with increasing TOC, diminishing from 30% of OM volume to less than 1% of OM volume across the range of TOC. The morphology and size of OM-hosted pores also vary systematically with TOC. The interpretation of this anticorrelation between OM content and SEM-visible pores remains uncertain. Samples with the lowest OM porosity (higher TOC) may represent gas expulsion (pore collapse) that was more complete as a consequence of greater OM connectivity and framework compaction, whereas samples with higher OM porosity (lower TOC) correspond to rigid mineral frameworks that inhibited compactional expulsion of methane-filled bubbles. Alternatively, higher TOC samples may contain OM (low initial hydrogen index, relatively unreactive) that is less prone to development of FE-SEM–detectable pores. In this interpretation, OM type, controlled by sequence-stratigraphic position, is a factor in determining pore-size distribution.

Three macroscopic diagenetic features can be recognized in the sandstones of the Upper Cretaceous Desert Member of the Blackhawk Formation and Castlegate Sandstone of the Mesaverde Group exposed in the Book Cliffs, Utah, each of which have distinctive form, geometry, and stratigraphic distribution. Diagenetic alterations are: (1) leached zones ("whitecaps"), up to 10 m thick, beneath coal beds; (2) large (up to 8 m) concretionary carbonate-cemented bodies in amalgamated shoreface and thin fluvial sandstones; and (3) thin (up to 2 m), laterally extensive carbonate-cemented horizons beneath major marine flooding surfaces. Each feature has distinct petrographic and geochemical signatures, and formed through discrete diagenetic processes. Large isolated carbonate-cemented bodies are composed of ferroan dolomite, most of which precipitated during early diagenesis. Field and petrographic data, coupled with stable-isotope data (early cements, 13C = -2.5 to +3.4o/oo VPDB; 18O = -7.8 to -12.0o/oo VPDB; 87Sr/86Sr = 0.7078; later cements, 13C = -3.1 to -5.7o/oo VPDB; 18O = -12.0 to -15.1o/oo VPDB; 87Sr/86Sr = 0.7093) suggest precipitation from meteoric fluids, input into sediments during times of relative sea-level fall. The source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids. Carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events (relative sea-level rise). Cement precipitation in these horizons continued through sediment burial as a result of organic-matter oxidation reactions in overlying organic-rich mudstones. The results of this study show a link between sedimentation (related to changes in relative sea level) and diagenesis, leading to the potential for the development of process-based, predictive models of early diagenesis in depositional successions.

Whole-rock shale and

Mudrock samples from a well penetrating Eocene and Paleocene strata (1235-4455 m) of the Texas Gulf Coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis, and the mechanism by which detrital smectitic illite/smectite (I/S) is transformed into diagenetic illitic I/S. Idealized smectite and illite end-member compositions for interstratified I/S are estimated to be [K0.00X+10.56Mg0.39Fe0.57Al1.13Si3.90O10(OH)2] and [K0.53X+10.18Mg0.17Fe0.16Al2.28 Si3.40O10(OH)2], respectively. There is no obvious difference in the amount of K addition to I/S in random- and ordered-interstratified I/S. Aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite-to-illite reaction is a complete dissolution-precipitation reaction rather than a solid-state [K+1 + Al+3] for Si+4 substitution reaction. Depth-relat d mineral trends and mass-balance calculations suggest that illitic I/S may form from kaolinite, and possibly illite or mica, as well as from smectitic I/S. Although most whole-rock element abundances are invariant over the sampled depth interval, K2O content increases from ca. 2.0 weight percent at 1500 m of burial to ca. 3.8 weight percent at 4000 m. Some of this increase is probably caused by import of K into the shale via fluids derived from interbedded sandstones, implying that Gulf Coast mudrocks behave as open chemical systems during burial diagenesis. If this is the case, a minimum of 103 pore volumes of fluid must have passed through the most K-enriched shales to have introduced the added potassium, provided that the K content of present-day formation water is representative of that of ancient formation water.

The majority of Tertiary volcanic rocks of the western United States and Mexico are alkaline in composition and may contain as much as 50 wt% equivalent K-feldspar. Emplacement of these volcanic strata is coeval with Tertiary shale deposition in the Texas Gulf Coast, and they previously have been identified as likely sources of sediment for Gulf Coast shales. Evaluation of chemical trends in Gulf Coast shales, particularly K 2 O, indicates changes in sediment composition in the lower Eocene, Oligocene, and near the Oligocene-Miocene boundary. In particular, there is a 250% increase in K 2 O content from ∼2 wt% to ∼5 wt% from the late Eocene to the early Oligocene. Gulf Coast shale bulk-rock compositions are consistent with a Tertiary volcanic source. Estimates of erosion and mass balance calculations suggest that in the south Texas Gulf Coast, the Oligocene Frio Formation may contain between 60% and 85% volcanic detritus, and coeval Frio shales to the north contain ∼25%. Vertical and lateral compositional variations highlight variable abundances of source detritus and the effects of weathering and depositional processes on Gulf Coast shale composition. Trends of increasing K 2 O content with depth in Gulf Coast shales previously have been interpreted to result from open-system diagenesis and K-metasomatism at depth. The data presented herein suggest instead that these trends result from variable provenance and the influx of large volumes of Tertiary alkaline volcanic material. Therefore, diagenetic models that invoke a homogeneous initial shale composition and open-system behavior may be invalid.

Sm-Nd model ages of sandstones and shales from the Oligocene Frio Formation of south Texas and the Paleocene-Eocene Wilcox Group of south-central Texas increase with increasing depth of burial. Depleted-mantle model ages (Tdm) of Frio shales increase from ca. 800 Ma at 1 km of burial to ca. 1000 Ma at ∼4km; in Frio sandstones, Tdm increases from ca. 750 Ma at 1 km to over 1200 Ma at 4 km of burial. In Wilcox shales, Tdm increases from ca. 1300 Ma to ca.1500 Ma over a burial depth of 1.5 to 4.5 km; Tdm of Wilcox sandstones increases from ca. 1450 Ma at 2.5 km of burial to as much as ca. 2000 Ma at 4.0 km. The sampling scheme employed minimizes, but does not eliminate, the possibility that these trends in Tdm are due to detrital variability. Close correlation of these trends with diagenetic changes in the rock and evidence of rare earth element mobility preserved in the diagenetic products suggests that diagenesis has modified Tdm. Diagenetic modification of Tdm in Frio sandstone and Wilcox shale is due primarily to loss of radiogenic Nd, whereas increased 147Sm/144Nd produces elevated Tdm in deeper Wilcox sandstones. The Tdm of Frio shales does not change appreciably with depth, owing to the opposite effects on Tdm of decreased 147Sm/144Nd and decreased ϵNd(0). Loss of radiogenic Nd is due primarily to reactions involving unstable volcanic detritus, which is more abundant in the Frio than in the Wilcox. Decreased 147Sm/144Nd in authigenic illite, and consequent increased 147SM/144Nd in pore fluids, results in elevated Tdm in Wilcox and Frio sandstones containing late-diagenetic cement, and in decreased 147Sm/144Nd in deeper Frio shale. The similar Tdm trends observed in both units, which have different detrital characteristics, suggest that modification of Sm-Nd model ages in siliciclastic rocks may be commonplace during burial diagenesis.

Sm-Nd analyses of sandstones and shales from the Wilcox Group (Upper Paleocene-Lower Eocene) suggest that the whole rock Sm-Nd signature is changed by burial diagenesis. The samples are from the San Marcos arch, south-central Texas, at depths ranging from outcrop to 15,000 feet. Sandstones from above the depth of intense feldspar reactions (ca. 10,000 feet) have whole rock Nd model ages of 1400 - 1500 Ma, whereas sandstones buried deeper than ca. 10,000 feet have Nd model ages of 1500 - 2000 Ma. Subsurface shale samples have whole rock model ages that increase from 1300 Ma at 5,000 feet of burial to 1500 Ma at 15,000 feet. Model ages increase most between 5,000 and 10,000 feet, within the zone of intense smectite illitization. The similar depositional age of all samples makes change in provenance an unlikely cause of the observed Nd model age variation. Sm and Nd reservoirs within the shale change considerably with progressive diagenesis. Outcrop samples contain subequal concentrations of Sm and Nd in the acid-soluble and silicate fractions. The acid-soluble reservoir in deeper samples contains progressively less Sm and Nd, and higher Sm/Nd. In all samples, the acid-soluble Nd is more radiogenic than the Nd in the corresponding silicate fraction, and the difference between the two reservoirs increases with depth. Diagenetic enrichment of Sm relative to Nd in the authigenic phases is the most likely explanation for the observed increase in Nd model ages in the sandstones. Decrease in Nd causes the increase in model age in the shales.