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1 Mud volcanism: an updated review
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3 Adriano Mazzini
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, Giuseppe Etiope
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4
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Centre for Earth Evolution and Dynamics, University of Oslo, Norway
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2
Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma 2, Italy, and Faculty of Environmental Science
7 and Engineering, Babes Bolyai University, Cluj-Napoca, Romania
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10 ABSTRACT
11 Mud volcanism, or sedimentary volcanism, represents one of the most intriguing phenomena
12 of the Earth’s crust, with important implications in energy resource exploration, seismicity,
13 geo-hazard and atmospheric budget of greenhouse gases. Since the first review papers were
14 issued at the beginning of 2000s, a large amount of new geological, geophysical and
15 geochemical data has been acquired, which clarified ambiguous concepts and significantly
16 improved our knowledge of mud volcanism. Here, we offer an updated review of the
17 knowledge and implications of mud volcanoes, with emphasis on: the terminology used to
18 describe different processes and structures; the physical, chemical and morphological
19 characteristics of the several fluid emission structures; the chemical properties of the released
20 fluids, in particular the molecular and isotopic composition of the gas; the mud volcano
21 formation dynamics; and the several implications for petroleum exploration, geo-hazards and
22 global atmospheric methane budget. This review integrates new fluids data collected in
23 Azerbaijan and is complemented with field observations from various mud volcano provinces
24 worldwide.
25 Although the total number of mud volcanoes on Earth is still uncertain, more than 600 main
26 onshore structures, with a large variety in shapes and sizes, are documented in recent global
27 data-sets, and several thousand are assumed to exist in the oceans. It is clear that: (a) mud
28 volcanoes are broadly distributed throughout the globe in active margins, compressional
29 zones of accretionary complexes, thrust and overthrust belts, passive margins, deep
30 sedimentary basins related to active plate boundaries, as well as delta regions; (b) they are
31 specifically located in hydrocarbon bearing basins, along anticline axes, strike slips and
32 normal faults, and fault-related folds in Petroleum Systems; (c) they represent a specific
33 category of natural gas/oil seepage manifestation, often related to deep and pressurised
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34 reservoirs; (d) the main engine driving mud volcanism is given by a combination of
35 gravitative instability of shales and fluid overpressure build-up, followed by hydrofracturing;
36 (e) hydrocarbons are generally of thermogenic origin, while microbial gas is released in only
37 a few cases. Mud volcanism on other planets (e.g. Mars and Titan), and microbial activity
38 associated with gas seepage represent emerging issues and opportunities for future research.
39 Keywords: Mud volcanoes; gas seepage; diapirism; mobilised shales; morphology; sedimentary basins;
40 hydrocarbons; hydrofracturing; methane; petroleum; seismicity.
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42 Contents
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44 1 Introduction
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46 2 Fundamentals: terminology, distribution and morphologies of mud volcanoes.........
47 2.1 Definitions and terminology ................................................................................
48 2.2 Main characteristics
49 2.3 Global distribution and settings............................................................
50 2.4 Morphologies……………….
51 2.5 Internal structure: feeder channel and roots
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53 3 Mud and fluid emission structures
54 3.1 Plumbing system and cone structures.........................................
55 3.2 Gryphons.................................................................................................................
56 3.3 Pools .......................................................................................................................
57 3.4 Salsa lakes ..............................................................................................................
58 3.5 Sinter structures ......................................................................................................
59 3.6 Mud density vs height .....................................................................................................
60 3.7 Diffuse degassing ………………
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62 4 Fluid temperature and geochemistry
63 4.1 Temperature ...........................................................................................................
64 4.1.1 Insights from temperature readings......
65 4.2 Molecular and isotopic composition of gas ………….
66 4.3 Water chemistry ........................................................................................................
67 4.4 Learning from seasonal sampling and temporal variability ......................................................
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69 5 MV formation dynamics ......................................................................................................
70 5.1 Gravitative instability, fluid overpressure and hydrofracturing....................................................................
71 5.2 Constraints in modelling
72 5.3 MVs and seismicity
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73
74 6 Implications.......................................................................................................
75 6.1 Hydrocarbon exploration .....................
76 6.2 Geohazards……............................................
77 6.3 Methane emission to the atmosphere ………
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79 7 A leading case-study: the Caspian mud volcanism
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81 8 Emerging issues and future research
82 8.1 Mud volcanism on other planets.............................................................
83 8.2 Seepage and microbial activity…………………………
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85 9 Sediment-hosted geothermal systems
86 10 Conclusions .........................................................................................................
87 References .....................................................................................................................
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89 Supplementary Material: Methods and data tables
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91 1 Introduction
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93 Mud volcanoes (hereafter reported as MVs) are surface expressions of focused fluid flow
94 inside hydrocarbon-bearing sedimentary basins. They are a specific category of hydrocarbon
95 seeps, connected hydraulically to petroleum (natural gas and oil) rich sediments and
96 accumulations, which may or may not have commercial importance. Mud volcanism, or
97 sedimentary volcanism, represents one of the most intriguing phenomena of the Earth’s crust,
98 not least for its implications in energy resource exploration, seismicity, hazard and
99 atmospheric budget of greenhouse gases. MVs can, in fact, (a) indicate subsurface petroleum
100 accumulations, (b) may react to or reveal precursor signals of earthquakes, (c) induce hazards
101 for people and industrial facilities, and (d) release large amounts of methane into the
102 atmosphere. For these reasons MVs, occurring both onshore and offshore, have been the
103 object of wide research since the early 1900s (
e.g. Goubkin and Fedorov, 1938). Books and
104 review papers, published since the end of 1990s (e.g. Guliyev and Feizullayev, 1997; Milkov,
105 2000; Dimitrov, 2002; Kopf, 2002), summarised the basic and important concepts of MVs,
106 describing their distribution, the tectonic settings, activity and products, as well as the
107 mechanisms of formation. However, after those reviews, in the last 15 years, a great deal of
108 new geological, geophysical and geochemical data has been acquired, which clarified
109 ambiguous concepts and significantly improved our knowledge of MVism. The scope of the
110 present review is to provide updated information on the meaning and implications of MVs,
111 some of which have been neglected in previous reviews.
Today, the list of peer-reviewed
112 articles dealing with MVs occurring in Europe, Asia, America, Oceania and almost all
113 marginal seas, is immense: it is not the aim of this paper to provide an inventory of all the
114 available works. Rather, our main objectives are to summarise, discuss and provide new
115 concepts regarding:
116 (a) the terminology used to describe different processes and structures, which appear to
117 be confused in some articles (Section 2);
118 (b) the physical, chemical and morphologic characteristics of the several fluid emission
119 structures (Section 3);
120 (c) the chemical properties of the released fluids, in particular the molecular and isotopic
121 composition of the gas (Section 4);
122 (d) the MV formation dynamics (Section 5);
123 (e) the implications of MV for petroleum exploration, geo-hazards and global
124 atmospheric methane budget (Section 6).
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126 As an illustrative case study, we provide an overview of the MVism in the Caspian Basin
127 (Section 7) that a) represents all the main characteristics of a typical geological setting prone
128 to the formation of MVs; b) displays the largest density and variety of MV types on Earth;
129 and c) has been extensively studied for both scientific and petroleum exploration purposes. In
130 this respect, we provide 22 new, unpublished, compositional and isotopic data from four MVs
131 in Azerbaijan. Gas and water samples were collected and analysed in 2005 and 2006, as
132 described in the Supplementary Material. These data confirm and complete some general
133 concepts addressed in Section 4.2.
134 We then discuss emerging issues and opportunities for future research, including MVism on
135 other planets (Mars and Titan), and microbial activity associated with MV seepage (Section
136 8). Finally, a short discussion is dedicated to Sediment-Hosted Geothermal Systems (SHGS,
137 Section 9), which are peculiar fluid flow systems incorporating some similarities with MVs,
138 and thus may be confused with them, but that substantially are driven and controlled by
139 different factors, i.e. they do not represent sedimentary volcanism.
140
141 2 Fundamentals: terminology, distribution and morphologies of mud
142 volcanoes
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144 2.1 Definitions and terminology
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146 MVs (Fig. 1) are the surface expression of subsurface processes characterised by movements
147 of large masses of sediments and fluids, collectively indicated as “sedimentary volcanism”.
148 The subsurface processes, which may or may not give rise to MVs, are generically referred to
149 as “piercement structures”, which include diapirs, diatremes, domes, dewatering pipes, mud
150 intrusions, mud mounds, chimneys, pipes
(see definition, for example, in Kopf, 2002; and in
151 Skinner and Mazzini, 2009). “Mud volcano” has often been considered as a descriptive term,
152 indicating substantially and generically a surface discharge of mud, water and gas,
153 independent of the geological processes and settings that drive and control the fluid
154 manifestation. As a result, the term was often incorrectly applied to volcanic (magmatic) or
155 geothermal and non-sedimentary settings, resulting in an unintended divergence of consistent
156 scientific discussion. For example, some hydrothermal manifestations at the Yellowstone