G.G. Akhmanov

Mud volcanism in the Gulf of Cadiz: results from the TTR-10 cruise

A new deep water mud volcano field (between 2000 and 3500 m water depth) was discovered in the deep South Portuguese margin, as well as several new mud volcanoes in the South Spanish and Western Moroccan margins of the Gulf of Cadiz, during the TTR-10 (Training Through Research, UNESCO/IOC) cruise, in July/August 2000. This work followed the discovery of a large mud volcano field in the Gulf of Cadiz, first investigated during the TTR-9 cruise [Gardner (2001) Geophys. Res. Lett. 28, 339–342; Kenyon et al. (2000) IOC, Technical series no. 56]. The discoveries were made based on a SEAMAP side-scan sonar mosaic and multibeam bathymetry (SEABEAM) collected in the area by the Naval Research Laboratory (NRL), Washington, DC, USA, in 1992, kindly released for this purpose. Single-channel seismics, long-range side-scan sonar (OKEAN), TV-controlled grab, hull-mounted 3.5-kHz profiler and coring were used to investigate several seafloor features observed on the side-scan sonar imagery, in the South Iberia, Spanish and Moroccan margins of the Gulf of Cadiz, which were confirmed to be mud volcanoes. The typical structures related to fluid venting in the Gulf of Cadiz are essentially represented by conical mud volcanoes with diameters ranging from several tens of meters to 4 km and heights that can reach 200 m. Some of these structures appear to be aligned along major conjugate NE–SW and NW–SE trending faults that can be identified on the side-scan sonar imagery. The new field discovered in the South Portuguese margin is the deepest in the Gulf of Cadiz area and includes three new mud volcanoes – Bonjardim, Olenin and Carlos Ribeiro – which seem to be quite active, with near-surface gas hydrate occurrence and a high saturation in H2S and hydrocarbon gases (mainly methane) in the mud breccia and overlying pelagic sediments. Gas hydrates were recovered from the Bonjardim mud volcano. An intensely gassified mud breccia, with one fragment of semi-consolidated claystone with a thin bituminous veneer at the surface, was recovered from the Carlos Ribeiro mud volcano. The fauna recovered consists mainly of pogonophoran worms belonging to several species and undetermined species of Foraminifera. Three new mud volcanoes were also discovered in the NW Moroccan margin: Rabat, Student and Jesus Baraza. These show a richer fauna that includes several species of molluscs, polychaetes, pogonophoran worms, crustaceans, echinoderms and some fragments of dead coral (Madrepora and Lophelia). Carbonate crusts were recovered from the Student mud volcano. The Ginsburg mud volcano, discovered during the previous TTR-9 cruise, was revisited and gas hydrates recovered once again. A new mud volcano was also discovered in the Spanish margin, Tasyo, where evidence was found of coral build-ups on the hard substratum of the mud volcanic edifice.

Integrated petrographic and geochemical record of hydrocarbon seepage on the Voring Plateau

Authigenic carbonate crusts, nodules and chemoherms were sampled from pockmarks and mud diapirs on the southern part of the Vøring Plateau during the TTR-8 and TTR-10 marine expeditions. A petrographic and geochemical study was carried out to investigate their possible relationship with the seepage of hydrocarbon fluids. All authigenic carbonates are depleted in 13C (−31.6‰ < δ13C < −52‰) indicating that methane is the primary source of the carbonate carbon. Furthermore, pyrite framboids are often associated with these samples, indicating that sulphate reduction is spatially coupled with methane oxidation and implying that the carbonates are formed through the anaerobic oxidation of methane. The oxygen stable isotope composition of the near-subsurface carbonates (3.1‰ < δ18O < 4.9‰) suggests a precipitation temperature very close to the one recorded on the sea floor (between −1 and 2 °C), which is consistent with their stratigraphic position, and a recent (Holocene?) age of formation. Carbonates sampled from greater depths (up to 5.5 m below the sea floor) are richer in 18O (4.6‰ < δ18O < 6.2‰), which is interpreted as a result of precipitation from an 18O-rich fluid. The occurrence of different carbonate mineral phases (aragonite, calcite, dolomite) is possibly related to varying dissolved sulphate concentrations in the diagenetic environment. Fluid inclusion microthermometry and Raman spectroscopy indicate the presence of an aqueous + hydrocarbon mixture inside the inclusions. This seepage mixture was almost certainly immiscible, resulting in heterogeneous trapping.

Lithology of mud breccia clasts from the Mediterranean Ridge

Abstract Clasts from mud breccia extruded by mud volcanoes, discovered during the TTR-3 Cruise of the R/V Gelendzhik (1993) on the Mediterranean Ridge, are represented by a number of different source rocks. They provide important information about the composition and genesis of the deep-seated deposits of the Mediterranean Ridge. Detailed descriptions of the main rock types obtained as clasts from the Mediterranean Ridge mud breccia were made on the basis of a study of more than three hundred thin sections and X-ray data. The genetic features of the rock types indicate deep-sea conditions during their accumulation, and the presence of turbidities in the sedimentary sequence of the Mediterranean Ridge. This suggests that turbidity currents from the African margin were capable of reaching the area presently forming the modern Mediterranean Ridge crest, and that the Mediterranean Ridge was not a positive morphologic structure during the accumulation of lithologies which were extruded from depth by mud volcanoes. The dating of the clasts should shed some light on the history of the Mediterranean Ridge.

New discoveries of mud volcanoes on the Eastern Mediterranean Ridge

Data from two mud volcanoes discovered on the eastern Mediterranean Ridge are presented, showing that these features have erupted episodically within historical time. Both volcanoes mark the site of active gas and fluid venting from deep within the sedimentary pile. Large volumes of mud breccia have been extruded onto the volcano slopes. For the first time, clasts up to c. 1 m in size have been recovered from sea-floor mud volcanoes. The submarine clasts show lithologies ranging from shallow water bioclastic sandstones to marls and limestones, dated using microfauna as Oligocene, Miocene and perhaps Cretaceous in age. Deep-towed video footage of one of the volcanoes shows clasts up to several metres across within the crater area. Deep-towed, high resolution side-scan sonar data and core data are presented to compare the two examples. The paper documents the discovery of the two volcanoes: the Dublin mud volcano and the Stoke-on-Trent mud volcano, the clasts that have been recovered from them, the intermittence of the eruptions as interpreted from gravity-core data, and how the volcanoes contrast with other mud volcanoes further to the west.

Sedimentary succession and evolution of the Mediterranean Ridge western sector as derived from lithology of mud breccia clasts

Abstract Rock fragments from mud breccia extruded by mud volcanoes, discovered during the TTR-4 cruise of the R/V Gelendzhik on the western Mediterranean Ridge, are represented by a number of different lithologies. They provide important information on the composition, genesis, and age of the deep-seated sedimentary series of the basin. Lithological description and age determination of the main rock types obtained as clasts from mud breccia were made on the basis of a study of ninety-seven samples. Depositional processes and environments of their accumulation were determined using sedimentological criteria. The sedimentary section through which mud volcanoes erupted on the western Mediterranean Ridge includes Aptian–Albian claystones, Late Cretaceous limestones, Paleogene siliciclastic rocks, and a variety of Miocene limestones and mudstones. The rock clasts studied throw insight into the evolution of the western sector of the Eastern Mediterranean Basin from, at least, Aptian time. The Aptian–Albian clayey series have been deposited in the marine conditions of a relatively narrow basin with a high input of terrigenous clayey material. During the Late Cretaceous the basin enlarged, terrigenous input into deep parts of the basin decreased, and carbonate pelagic sedimentation became prevalent. Regression in Paleogene time led to the deposition of thick terrigenous siliciclastic series of deltaic–prodeltaic sediments. Extensive turbidite systems were emplaced in the western sector of the Eastern Mediterranean during the Miocene, along with predominantly carbonate pelagic sedimentation.

Pore space in source rock during the generation of hydrocarbons

Modern technical facilities for laboratory experiments allow carrying out the step-by-step description of HC generation from the organic rich rocks and its reflection in rock structure in an undisturbed rock sample. A clayeycarbonate rock sample of the Domanik Horizon (D3fr) with high content of organic matter and its low maturation grade was heated in nitrogenatmosphere and alterations in its pore space structure was monitored. That was shown that pores in the initial rock are small and isolated. When the rock is heated, the pores are first bound with each other due to the propagation of fractures along bedding, resulting in the formation of isolated filtration intervals. At a certain stage of organic matter maturity further heating provokes the formation of bedding-perpendicular fractures that connect the isolated filtration intervals into a single system, promoting filtration in the low permeable media. Experimental works described in the paper can serve as a basis for the development of new methods for the successful prediction of nontraditional reservoir zones.

Sills and gas generation in the Siberian Traps

On its way to the surface, the Siberian Traps magma created a complex sub-volcanic plumbing system. This resulted in a large-scale sill emplacement within the Tunguska Basin and subsequent release of sediment-derived volatiles during contact metamorphism. The distribution of sills and the released sediment-stored gas volume is, however, poorly constrained. In this paper, results from a study of nearly 300 deep boreholes intersecting sills are presented. The results show that sills with thicknesses above 100 m are abundant throughout the upper part of the sedimentary succession. A high proportion of the sills was emplaced within the Cambrian evaporites with average thicknesses in the 115–130 m range and a maximum thickness of 428 m. Thermal modelling of the cooling of the sills shows that the contact metamorphic aureoles are capable of generating 52–80 tonnes of CO2 m−2 with contributions from both marine and terrestrial carbon. When up-scaling these borehole results, an area of 12–19 000 km2 is required to generate 1000 Gt CO2. This represents only 0.7–1.2% of the total area in the Tunguska Basin affected by sills, emphasizing the importance of metamorphic gas generation in the Siberian Traps. These results strengthen the hypothesis of a sub-volcanic trigger and driver for the environmental perturbations during the End-Permian crisis. This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.

Late Vendian-Early Cambrian formations of the north-west Siberian Craton

The composition and structure of principal key-sections for the Tira (Late Vendian) and Danilovo (Late Vendian-Early Cambrian) Horizons were characterized on the basis of deep-drilling data and studies of natural outcrops along the peripheries of the Kureika syneclise. The typical sedimentary formations/associations were recognized, their vertical and lateral successions were distinguished, and their deposition environments were interpreted. A distribution pattern scheme of thicknesses and depositional environments of Upper Vendian-Lower Cambrian deposits was compiled at the 1: 1000000 scale. The reservoir characteristics of the formations are discussed and forecasted for poorly studied areas of the northwestern Siberian platform.

Neoproterozoic and Lower Cambrian Rock Complexes in Central Areas of the Siberian Craton: Their Structure and Petroleum Prospects

The paper summarizes data on the geology, lithology, and geochemistry of petroliferous Riphean, Vendian, and Lower Cambrian rocks in the central parts of the Siberian Craton. The petrological-geological properties of these sediments have been assessed based on results of paleogeographic analysis of these rocks, discrimination of oil reservoirs and oil-source successions, determination of secondary alterations of the rocks, and sources of oil generation and regional migration of hydrocarbons into various traps in zones of possible oiland-gas accumulation.

Cenozoic complex in the Upper Telekai oil-and-gas field in the Chukchi Peninsula (results of petrographic examination of the succession penetrated by well 10)

The paper discusses new data resulting from petrographic examination of the composition and structure of Cenozoic sediments penetrated by Well 10 to a depth of 2350 m in the Upper Telekai oil-and-gas field within the Anadyr Basin. An alluvial-deltaic sedimentation environment of tuffaceous-sedimentary and terrigenous producing Lower Miocene rocks was found and their secondary alterations examined, including formation of clay minerals, carbonates, and zeolite. The vitrinite reflectance indices allowed determination of the catagenic zoning of the Miocene succession, and a significant catagenic unconformity related to pre-Pliocene erosion was recorded at its top.