Mud and fluid migration in active mud volcanoes in Azerbaijan
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- Planke, S., Svensen, H., Hovland, M. et al. Geo-Mar Lett (2003) 23: 258. doi:10.1007/s00367-003-0152-z
Mud volcanic eruptions in Azerbaijan normally last for less than a few hours, and are characterized by vigorous extrusion of mud breccias, hydrocarbon gases, and waters. Recent fieldwork and mapping on four active mud volcanoes show that dormant period activity ranges from quiet to vigorous flow of mud and fluids. Geochemical analyses of expelled waters show a wide range in solute concentrations, suggesting the existence of a complex plumbing system. The mud and fluids have a deep origin, but are sometimes stored in intermediate-depth mud chambers. A mixing model between deep-seated saline waters and shallow meteoric water is proposed.
Piercement structures, such as mud volcanoes and hydrothermal vent complexes, are common in many sedimentary basins. Hydrothermal vent complexes are numerous in sedimentary basins on the NE Atlantic margins and in the Karoo basin in South Africa (Jamtveit et al. 2003). These vent complexes were formed as a consequence of the intrusion of mafic melts in sedimentary basins in late Paleocene and mid-Jurassic times, respectively. In contrast, mud volcanoes are formed by tectonic processes, e.g., by overpressure buildup in compressional settings, or by maturation and degassing of rapidly buried organic-rich sediments (e.g., Hedberg 1974; Higgins and Saunders 1974; Sjögren 1886; Brown 1990; Milkov 2000; Kopf and Behrmann 2000; Fowler et al. 2000; Kopf 2002).
The piercement structures will clearly have an important impact on fluid migration in sedimentary basins when they are formed (e.g., Dimitrov 2002). However, it has been shown that they may also have an important long-term impact on the fluid-flow history in sedimentary basins (Svensen et al. 2003, this volume). Currently, it is difficult to incorporate such piercement structures in basin modeling theories, and they are thus commonly ignored when studying fluid flow in sedimentary basins.
An important issue regarding mud volcanoes and seeps is if the seep fluids have the same source during the dormant and eruptive phases, and the degree of mixing of deep waters (i.e., from mud reservoir level) with shallow waters. Geochemical data from mud volcanoes suggest that, even during dormant periods, water seeps are sourced from the deep mud reservoir (e.g., Dia et al. 1999). Mixing between deep and shallow waters during ascent is to be expected, and may be related to the presence of intermediate-depth mud chambers (cf. Cooper 2001). Thus, the waters expelled at mud volcanoes may represent complex mixtures of deep and shallow waters, with chemistries affected by processes like mineral dehydration, adsorption and desorption on clay minerals, precipitation and dissolution, redox reactions, and degradation of organic material (e.g., Lagunova 1976; You et al. 1993; Martin et al. 1996; Dia et al. 1999; Kopf and Deyhle 2002).
Mud volcanic processes and terminology
Mud volcanism shows many similarities to magmatic volcanism, and a substantial part of the terminology used to describe volcanic processes and deposits (e.g., Sigurdsson 2000) can be applied to mud volcanoes and mud volcanism. There is, however, also a separate terminology related to mud volcanoes (e.g., Hovland et al. 1997; Milkov 2000; Aliyev et al. 2002; Dimitrov 2002; Kopf 2002). Although the terminology of mud volcanoes has become more rigorous and consistent (e.g., Milkov 2000; Kopf 2002), there is still a need to make a clearer distinction between the active and dormant stages of mud volcanoes. The importance of the active/dormant distinction is crucial when estimating gas and fluid fluxes from mud volcanoes, and when determining their impact on the release of methane to the atmosphere. What are commonly referred to as active mud volcanoes in the literature are in many cases only manifestations of seep activity during the dormant period (e.g., gryphons and salses). Most of the flux estimates from mud volcanoes are calculated from dormant volcanoes (see Dimitrov 2002). The bulk of the fluid and mass is, however, released during eruptions (cf. Kopf and Deyhle 2002; Dimitrov 2002), but quantitative measurements of eruption parameters are scarce. According to Dimitrov (2002), the integrated annual fluxes of methane from mud volcanoes worldwide during eruptions are 7 times (average value) higher than the seep activity in the dormant periods. The estimated value for Azeri mud volcanoes is a 16 times higher annual flux during eruptions (Guliyiev and Feizullayev 1994, in Dimitrov 2002).
Mud volcanoes can be classified as active, extinct, or buried, and mud volcanism may represent a major land-building process. A mud volcano eruption can be explosive or effusive, and may occur both in subaerial and subaqueous environments. Hydrocarbon gases are commonly erupted. These gases may self-ignite, and up to 1-km-high fire-columns have been observed during mud volcanic eruptions in Azerbaijan (Aliyev et al. 2000). The erupted solid material is present as fall or flow deposits. Breccia flows are commonly emplaced during mud volcanic eruptions. These flows consist of angular boulders and clasts (‘xenoliths’) derived from the country rocks that are cut by the plumbing system, being embedded in a mud-dominated matrix.
Active mud volcanoes show a variable degree of dormant activity. Many are quiet, with no surface seep activity or ground deformation. However, seep activity is common on many mud volcanoes, expelling muds, liquids, and gases. The seep activity leads to the formation of scenic features such as gryphons (<3 m high, steep-sided cones extruding mud), mud cones (<10 m high cones extruding mud and rock fragments), salses (water-dominated pools with gas seeps), springs (<0.5 m small, water-dominated outlets), burning fires, scoria cones formed by heating of mud deposits during the fires (e.g., Jakubov et al. 1971; Hovland et al. 1997; Guliyev and Feizullayev 1997), and hydrocarbon deposits (Higgins and Saunders 1974; Aliyev et al. 2002). The seep activity is dominantly effusive, but small explosive events do also occur.
South Caspian Basin
The South Caspian Basin is a Tertiary back-arc basin with an up to 25–30 km thick sedimentary package (e.g., Guliyev and Feizullayev 1996; Abrams and Narimanov 1997; Devlin et al. 1999; Guliyev et al. 2002). The South Caspian Basin was a depression from the mid Jurassic, with deposition of marine sediments upon Lower Jurassic oceanic crust formed during back-arc spreading (Abrams and Narimanov 1997). Further collision in the Alpine-Himalayan zone led to regional uplift in the Miocene–Pliocene, with associated rapid deposition of 10 km of deltaic and lacustrine sediments in the South Caspian region. Sedimentation rates during the Quaternary were as high as 2.4 km/106 years (Nadirov et al. 1997), and 5–8 km of sediments has been deposited the last 5×106 years (Tagiyev et al. 1997).
The geothermal gradient in the South Caspian Basin is low (10–18 °C/km), providing hydrocarbon maturation down to great depths (14 km to the onset of the gas window; Abrams and Narimanov 1997; Nadirov et al. 1997). The typically 1–2 km thick Maykop Fm. is regarded as the major source of both the extruded mud and the petroleum (Inan et al. 1997; Fowler et al. 2000). However, mud breccias from the mud volcanoes often contain clasts from formations below the Maikop Fm., suggesting that some of the mud may have an even deeper source (Inan et al. 1997). The Maikop Fm. is locally very deeply buried—it is located between 8.5- and 11-km depth offshore Baku, and at 5.5-km depth underneath the offshore Shah Deniz structure (Fowler et al. 2000). The mud reservoirs for the extrusive volcanism may be as deep as 14 km, with intermediate mud chambers at 2–4 km depth (Cooper 2001).
The rapid Miocene–Pliocene sedimentation and burial led to increased maturation of organic material, created structural traps, and caused initiation of mud volcanism (Abrams and Narimanov 1997). Almost 300 historic small and large mud volcano eruptions are documented in Azerbaijan, occurring on 76 mud volcanoes (Aliyev et al. 2002). The mud volcanoes, and associated hydrocarbon reservoirs, are often found within large anticlines (Jakubov et al. 1971; Guliyev and Feizullayev 1996; Narimanov et al. 1998; Fowler et al. 2000; Guliyev et al. 2002).
Materials and methods
Fieldwork was performed at four active mud volcanoes—Bakhar, Dashgil, Koturdag, and Lokbatan, during October 2002 (Fig. 1). Two of the mud volcanoes, Bakhar and Dashgil, are good examples of mud volcanoes with a high seep activity in the dormant period. In contrast, the active Kotyrdag and Lokbatan mud volcanoes are quiet, and recently eruptive deposits and structures are well exposed. Finally, a satellite vent 1 km west of the Bakhar mud volcano was visited.
Detailed mapping of the Dashgil and Lokbatan mud volcanoes was done using a Garmin etrex GPS with a relative accuracy of four meters. Water samples were collected from Dashgil and Bakhar, both from salses, springs, and gryphons. Temperatures were measured in-situ with a thermocouple thermometer. Mud and water was separated by settling (to minimize release of elements adsorbed to clay minerals; cf. You et al. 1996), filtered using a 2-μm mesh, and analyzed in January 2003 at the School of Earth Sciences, Leeds, UK.
Cations and iodide were measured by ICP-MS (Agilent 7500c) on filtered water samples. Anions were measured by ion chromatography using a Dionex 600. Accuracy and precision of both methods was checked by external standards. Precision in all instances was better than 5% RSD (residual standard deviation) and accuracy better than 4%.
The Lokbatan mud volcano
The most recent Lokbatan eruption occurred on 24 October 2001, lasting for about 30 min (Aliyev et al. 2002). It was associated with ground tremors, eruption of mud breccia, and extensive fires lasting for more than one year after the eruption (Figs. 2, 3). A graben extends from the vent along the crest of the anticline (Fig. 2). The vent is flanked by two horst blocks, which were both covered by ejected mud breccia material during the most recent eruption. However, most of the mud breccia was emplaced as a flow within the central part of the graben. The estimated total volume of the flow is approximately 0.0003 km3 (Aliyev et al. 2002).
Extensive faulting is seen along the summit of Lokbatan. Displacement of these faults occurred both during the 2001 eruption (Fig. 3F) and previous eruptions (Jakubov et al. 1971). The subsidence pattern is circular in the summit area, forming a ring fault with gas escape features (burned mud). The displacement on the main graben-forming faults is decreasing away from the summit area, from 10–20 m to zero meters one kilometer to the west.
The westward-trending graben collapse structure suggests the presence of an elongated, shallow mud breccia chamber within the crest of the anticline. During an eruption, mud breccia from this chamber is drained through the vent. We suggest that removal of this material caused subsidence and collapse of the roof rocks, forming the graben structure. Subsequent infilling of the graben by the erupted mud breccia increased the load on the mud chamber, causing further surface subsidence. A shallow mud chamber hypothesis is also consistent with the observation of a negative gravity anomaly above the mud volcano, suggesting the presence of light, buoyant material within the crest of the anticline (Kadirov et al. 2002).
The Kotyrdag mud volcano
The geometry of Koturdag is similar to a simple, cone-shaped composite magmatic volcano of intermediate composition. The morphology of composite volcanoes is controlled by a complex interaction of aggradation (i.e., eruption and emplacement) and degradation (i.e., erosion and gravity flow) processes (e.g., Davidson and De Silva 2000). Whereas composite volcanoes have been extensively studied, no comprehensive studies of mud volcanic eruptions (detailed seismicity surveys, ground deformation measurements, or gas and fluid emission sampling) are available. The understanding of the ascent and eruption processes of composite mud volcanoes is therefore limited.
The most recent breccia flow was erupted from the summit crater and emplaced on the northeastern flank of the volcano. The flow is 1 km long and can be subdivided into two main parts—an upper channelized part on the steep flanks of the volcano, and a lower lobate part on the less steep, lower parts of the volcano. The flow is blocky in the channel, and the presence of red, oxidized mud breccia and scoria shows that escaping gases have ignited and burned during the emplacement (Fig. 4B). The lower lobe is characterized by a smoother surface and the presence of curved pressure ridges. The eruption terminated by slow extrusion of very viscous, plastic mud breccia (Fig. 4D). The emplacement rate of similar extruded mud breccia has been measured to be 2–15 m/year, lasting for several years after the main eruption event (e.g., Guliyev and Feizullayev 1997).
The study of the seep activity during the dormant periods may provide important insight into the mud volcanic process, and is particularly important because no mud volcanic eruptions have been monitored in detail. In addition, studies of seep gases are important because methane and other gases may contribute significantly to the global greenhouse gas budget (e.g., Hovland et al. 1997; Milkov 2000; Dimitrov 2002), and because mud volcanoes are important conduits for petroleum migration (e.g., Katz et al. 2000).
Seep structures on the Dashgil crater field were described by Hovland et al. (1997), and include a gryphon cluster, mud cones, salses, and sinter cones (Figs. 5 and 6). Natural oil seeps are further found on the flanks of the mud volcano. A re-mapping of the crater field in October 2002 (Fig. 5) reveals the same seep features as present in 1995, but some of the gryphons and mud cones are up to 50% larger. The only major difference is the addition of a mud cone field formed during a small eruption in May 2001 (Aliyev et al. 2002).
Active gryphons, mud cones, and springs were also found at the Bakhar mud volcano and a nearby satellite vent. The circular satellite crater is about 10 m deep with a diameter of about 50 m, and is located in an area with natural oil seeps. The crater was formed by an explosive eruption in 1998 on a gryphon field.
Composition of water seeps from mud volcanoes (all values in ppm)
Bakhar s. v.b
The gases seeping through the mud volcanoes in Azerbaijan during the dormant periods are dominated by methane (>90%; Sokolov et al. 1968; Valyaev et al. 1985; Guliev and Feizullayev 1996; Aliyev et al. 2002). The isotopic composition of the seep methane overlaps with the isotopic composition of methane from petroleum fields in the Caucasus (Valyaev et al. 1985; Guliev and Feizullayev 1996), emphasizing the intimate relationship between petroleum formation, secondary migration, and mud volcanism.
Composition of expelled waters
The major element composition of the expelled waters is controlled by the depositional environment (e.g., marine/non-marine, presence of evaporites), diagenetic processes (e.g., Carpenter and Miller 1969; Hanor 1994; Worden 1996), temperature (e.g., Fournier and Truesdell 1973; Hanor 1994), and mixing (e.g., Dia et al. 1999). Elements like Cl, Br, I, and B may give important information both about fluid source, depth, and fluid–rock interactions, and are commonly used as tracers (e.g., Rittenhouse 1967; Harder 1970; Lagunova 1976; You et al. 1993; Worden 1996; Kopf and Deyhle 2002). A full geochemical discussion of all the elements listed in Table 1 is beyond the scope of this contribution, and would need a bigger dataset. Although our dataset is limited, we may draw important conclusions about the main fluid reservoirs that hosted the expelled waters.
The analyzed waters are enriched in B and metals (Al, Cr, Fe, Mn, Ni, Cu, Zn) compared to seawater (Table 1), oilfield brines (Collins 1975), and expelled waters from other mud volcanoes worldwide (e.g., Dia et al. 1999; Kopf and Deyhle 2002). The concentrations of metals like Cu, Ni, and Cr, which are soluble in the reduced state, are very high (84–105 ppm), and independent of the Cl concentration in the waters (Table 1). Metals like Cu, Cr, and Ni are usually present in parts per billion in oilfield brines (Collins 1975). Intriguingly, elements like Na, Cl, Mg, K, and B vary considerably in the analyzed waters, whereas most of the metals show no concentration variations (Table 1). The standard deviation for the average of all analyses in Table 1 is less than 5% for Cr, Mn, Ni, and Cu.
Fluid sources and mixing trends
Figure 8 shows the Br and Cl relations of the analyzed waters, revealing a good linear correlation (r2=0.95). The slope of the trend line for the Cl–Br correlation is less steep than that of seawater evaporation, making it unlikely that the waters acquired their salinity by surface evaporation of parent water in the vents. Although Cl/Br versus Na/Br plot close to the seawater evaporation trend, the salinities are far too low to correspond to possible surface evaporation (~146,000 ppm Cl at the onset of halite precipitation; Fontes and Matray 1993).
The high B contents of mud volcano waters in Azerbaijan suggest that the waters have a deep origin from strata affected by the smectite–illite transformation (e.g., You et al. 1996; Kopf and Deyhle 2002), where the B could originate from marine clays or organic material (e.g., Ishikawa and Nakamura 1993; Williams et al. 2001). The high Br concentration and lower Cl/Br ratio in the type 1 component do not support a Br source related to organic material, as the I concentrations are too low (cf. Worden 1996). Interestingly, the B and Li concentrations in expelled waters from a Kerch-Taman mud volcano was increased following an eruption (Lagunova 1974), suggesting that these elements are enriched in the source region. However, our data do not show any correlation between B and Li as described from the Kerch-Taman area (Lagunova 1976). In the case of B release during the smectite–illite transformation, an accompanying salinity reduction due to dehydration should occur as well (e.g., Moore and Vrolijk 1992; Fitts and Brown 1999). However, the B and Cl concentrations are positively correlated (Fig. 7), suggesting a more complex relationship between Cl–B concentration and smectite dehydration.
The low Cl/Br ratios of the waters (160–236 by mass) suggest that their salinity was not derived from dissolution of evaporates in the source strata or during ascent, which would have resulted in higher Cl/Br ratios. The narrow Cl/Br ratio range of the waters suggests that all waters represent dilution of one, high-salinity water reservoir by low-salinity waters. The low B concentrations in the type 2 water component suggests that it has undergone different processes than type 1 waters, possibly originating from shallow reservoir levels with fluids not significantly affected by clay mineral diagenesis (cf. the Guneshli water composition), or from meteoric water. The latter is more likely, considering the narrow Cl/Br range of the waters.
The metal concentrations in the analyzed waters do not vary with the chlorine concentration, and may be explained by enrichment after mixing. Both metal desorption from clay minerals (e.g., Helios Rybicka et al. 1995) and redox-controlled precipitation–dissolution are possible mechanisms that may occur in the upper part of mud volcanoes. High Fe/Mn ratios (6.7–11.8) do not suggest loss of iron due to oxidation, and can be considered as indicative of a reduced fluid (Bottrell and Yardley 1991); hence, the waters may carry transition metals soluble in the reduced state, as shown by the water analyses.
Type 2 waters have the lowest salinity and possibly the shallowest origin, and represent the most water-dominated parts of the mud volcanoes (i.e., the salses). The salses apparently have higher runoff rates than the springs and the gryphons. The waters plotting along the mixing line away from the type 2 component were all sampled from mud-dominated systems (gryphons and springs). These observations can be explained if the flux from the shallow reservoirs (meteoric water) is high in the salses, whereas the gryphons and springs represent a low-flux system originating from the deep-seated mud source.
New data on water geochemistry, integrated with published gas data (Guliev and Feizullayev 1996; Katz et al. 2000), support the hypothesis of a complex plumbing system beneath the mud volcanoes during the dormant periods. We propose that the mud volcano seep fluids represent leakage from a deep source mixed with a component of meteoric waters. This is consistent with the recognition of the deep-seated Maikop Fm. as the main source of the extruded mud, and the existence of intermediate mud chambers at 2–4 km depth (Cooper 2001).
Dormant mud volcanoes show a large variation in activity, from mainly quiet (e.g., Koturdag, Lokbatan) to very active (e.g., Dashgil, Bakhar). We suggest that the water-dominated salses represent expulsions of shallow waters, whereas deep-seated fluids percolate slowly to the surface and are expelled as mud–water mixtures in gryphons and springs.
This work was partly financed by the Norwegian Research Council grant 120897 to B. Jamtveit/H. Svensen. We would like to thank Statoil for logistic assistance during fieldwork, and Achim Kopf for a constructive review.