An Overview of Mud Volcanoes Associated to Gas Hydrate System

Among fluid venting structures, mud volcanoes are the most important phenomena related to natural seepage from the earth's surface (Mazurenko and Soloviev, 2003). Mud volcanoes have variable geometry and size, from one to two meters to several hundred meters in height, and are formed as a result of the emission of argillaceous material and fluids (water, brine, gas, oil) (Milkov, 2000; Dimitrov, 2002; Kopf, 2002). They occur globally in terrestrial and submarine geological settings: most terrestrial mud volcanoes are located in convergent plate margin with thick sedimentary sequences within the Alpine–Himalayan, Carribean and Pacific orogenic belts (Hovland et al., 1997; Kopf et al., 2001; Delisle et al., 2002; Etiope et al., 2002; Deville et al., 2003; Yassir, 2003; Shakirov et al., 2004; Stewart and Davies, 2006). Mud volcanoes and mud diapirs are responsible for the genesis of many chaotic deposits, such as mélanges, chaotic breccias and various deformed sediments (Barber et al., 1986; Barber and Brown, 1988; Orange, 1990; Brown and Orange, 1993). The normal activity of mud volcanoes consists of gradual and progressive outflows of semi-liquid material called mud breccia or diapiric mélange. Explosive and paroxysmal activities are interpreted as responsible for ejecting mud, ash, and decimetric to metric clasts. Mud volcano breccias are composed of a mud matrix, which supports a variable quantity of chaotically distributed angular to rounded rock clasts, ranging in diameter from a few millimeters to several meters (Camerlenghi et al., 1992; Dimitrov, 2002; Deville et al., 2003). Clasts are of various lithologies and provenances, derived from the rocks through which the mud passed on its way to the surface or to the sea-floor. Slumps, slides and sedimentary flows can also affect the entire structure of mud volcanoes, even if gradients are very low. The occurrence of mud volcanoes is controlled by several factors, such as tectonic activity, sedimentary loading due to rapid sedimentation, the existence of thick, fine-grained plastic sediments and continuous hydrocarbon accumulation (Treves, 1985; Guliev and Feizullayev, 1996; Ivanov et al., 1996; Limonov et al., 1996; Milkov, 2000; Dimitrov, 2002). A comprehensive study of submarine mud volcanoes is increasing in the last decades due to the wide use of side scan sonar and the increased accuracy of the positioning of bottom samplers. Milkov (2000) presented an up-to-date list of known and inferred submarine mud volcanoes, discuss their distribution, describe the mechanisms by which they form, and characterize associated gas hydrates accumulations (Fig. 1).Research on submarine mud volcanoes is important because:1. they are a source of methane flux from lithosphere to hydrosphere and atmosphere (greenhouse effect and climatic change);2. they may provide evidence of high petroleum potential in the deep subsurface;3. useful data about the sedimentary section in mud volcanic areas can be determined by examination of rock fragments incorporated in mud volcanic sediments (breccia);4. submarine mud volcanic activity may impact drilling operations, ring installations and pipeline routings;5. gas hydrates associated with deep-water mud volcanoes are a potential energy resource.In this Chapter, we focus our attention to mud volcanoes related to gas hydrate system, showing a case history in Antarctic Peninsula, where an important gas hydrate system associated to mud volcanism was studied (Tinivella et al., 1998; Tinivella et al., 2008; Tinivella et al., 2009). Natural gas hydrates are a curious kind of chemical compound called a clathrate. Clathrates consist of two dissimilar molecules mechanically intermingled but not truly chemically bonded. Instead one molecule forms a framework that traps the other molecule. Natural gas hydrates can be considered modified ice structures enclosing methane and other hydrocarbons, but they can melt at temperatures well above normal ice (i.e., Sloan 1998). At 30 atmospheres pressure, methane hydrate begins to be stable at temperatures above 0 °C and at 100 atmospheres it is stable at 15 °C (Smelik and King 1997). This behavior has two important practical implications. First, it is a nuisance to the gas company. They have to dehydrate natural gas thoroughly to prevent methane hydrates from forming in high pressure gas lines. Second, methane hydrates will be stable on the sea floor at depths below a few hundred meters and will be solid within sea floor sediments (Paull and Dillon 2000). Masses of methane hydrate "yellow ice" have been photographed on the sea floor. Chunks occasionally break loose and float to the surface, where they are unstable and effervesce as they decompose.The stability of methane hydrates on the sea floor has a whole raft of implications (i.e., Henriet and Miniert 1998). First, they may constitute a huge energy resource (Holder and Bishnoi 2000). Second, natural disturbances (and man-made ones, if we exploit gas hydrates and aren't careful) might suddenly destabilize sea floor methane hydrates, triggering submarine landslides and huge releases of methane. Finally, methane is an effective greenhouse gas, and large methane releases may explain sudden episodes of climatic warming in the geologic past. The methane would oxidize fairly quickly in the atmosphere, but could cause enough warming that other mechanisms (for example, release of carbon dioxide from carbonate rocks and decaying biomass) could keep the temperatures elevated (Haq 1998).Gas hydrates in marine environments have been mostly detected from analysis of seismic reflection profiles, where they produce remarkable bottom simulating reflectors (BSRs; Cox 1983; Sloan 1998). Generally, the BSR is a very high-amplitude reflector that is associated with a phase reversal (Hyndman & Spence 1992; Max 2003). This phase reversal may indicate that sediments above the BSR are extensively filled with gas hydrates and lower sediments below it are filled with free gas in the pore space (Minshull et al. 1994; Sain et al. 2000). Several studies (e.g. Tinivella et al. 1998) revealed a seismic reflector below the BSR that can be associated with the base of the free gas zone, called BGR (base of the free gas reflector). The scientific community is investing much effort in studying marine sediments containing gas hydrates to characterize the hydrate reservoir and to quantify the gas trapped within sediments from seismic data analysis (Chand & Minshull 2003; Zillmer 2006). Recently, the international community has considered CO2 sequestration as a possible means of offsetting the emission of greenhouse gases into the atmosphere (Ledley et al. 1999). The CO2 storage program is a further reason to assess the feasibility of mapping and monitoring the reservoir by means of an efficient seismic analysis (Chadwick et al. 2002; Arts et al. 2004) and to obtain information about hydrate and free gas concentrations in a time-effective way.The close proximity of mud volcanoes to zones where BSRs crop out on the seafloor deserves particular attention. Seismic records strongly suggest that much of the gas in mud volcanoes originates from levels deeper than that of the gas hydrates as such. Faulting could be responsible for this unique situation. In other areas where gas hydrates are associated with mud volcanoes, such as the Haakon Mosby mud volcano in the Norwegian Sea, or the mud volcanoes in the Anaximander Mountains region in the Eastern Mediterranean (Woodside et al. 1998), the mud volcanoes are characterized by a concentric zonal distribution of gas hydrates. It has been argued that, in these cases, the gas hydrates have probably been formed by gas emanating from the central part of the mud volcano, and transported into solution by diffusion (Milkov 2000). In addition, a strong BSR and the presence of mud volcanoes have recently been detected by means of seismic data along the southwest African margin, which is a passive margin (Ben-Avraham et al. 2002). This region, located in the distal part of the Orange River delta, is also characterized by overpressure which results in active fluid expulsion, as shown by the existence of mud volcanoes, pockmarks, and possibly cold-water corals thriving on methane gas seeps (Jungslager 1999).The global change is particularly amplified in transition zones, such as the peri-Antarctic regions. For this reason, the gas hydrate reservoir present offshore Antarctic Peninsula was studied in the last 20 years acquiring a quite extensive geophysical dataset (Tinivella et al. 1998; Tinivella and Accaino 2000; Tinivella et al. 2011). The analysis of the data allowed us to recognize the presence of four main mud volcano ridges, named Flop, Grauzaria, Sernio and Vualt. The Vualt mud volcano is the highest detected in our study area; its top is at 2,216 m below sea level, with an elevation of about 255 m above the seafloor and an extension of 9.4 km2 (Fig. 2). Interpretation of new data acquired on the South Shetland margin confirms the crucial role of tectonics controlling the extent of the hydrate reservoir, and active venting of fluids and mud through faults bordering and crossing the gas hydrate field. Mud volcanoes and fluid expulsion events are likely located in close association with faults, through which they are connected to the reservoir located beneath the BSR. Their activity is probably episodic.