AAPG/GSTT HEDBERG CONFERENCE
“Mobile Shale Basins - Genesis, Evolution and Hydrocarbon Systems”
June 4-7, 2006 - Port-of-Spain, Trinidad and Tobago
Mud Volcano Evolution from 3D Seismic Interpretation and Field Mapping in Azerbaijan
Robert Evans*1, Richard Davies2, Simon Stewart3
(1) 3D Lab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3YE, UK
(2) CeREES, (Centre for Research into Earth Energy Systems) Department of Earth Sciences,
University of Durham, Science Labs, Durham DH1 3LE
(3) BP Azerbaijan, c/o Chertsey Road, Sunbury on Thames, Middlesex, TW16 7LN, UK
* Presenting Authors email: evansrj8@cf.ac.uk
Introduction
Mud volcanoes are an important structural and sedimentary expression of the dewatering and degassing of sediments as they undergo burial and compaction in mobile shale basins. The strong genetic link between mud volcano development and the host basin’s fluid flow and tectonic regime means that interpreting the evolutionary sequence of individual mud volcanoes may offer insight into the basins structural and hydrodynamic development. Of particular significance to this approach is the mud volcanic edifice, the extrusive component of the mud volcano system. The stratigraphy of this feature, in theory, records the history of mud volcanic eruption. However, accurately describing the internal architecture of mud volcanic edifices has been problematic in the past due to a lack of good three-dimensional exposure on land and the typically poor quality of seismic data that images mud volcanoes. Consequently the evolutionary histories of most large mud volcanoes can only be speculated on.
We now present the results of detailed 3D seismic reflection mapping within a giant mud volcanic edifice from the South Caspian Sea. Seismic data used is of sufficient quality to enable us to subdivide the edifice into its subcomponent units and in doing so reconstruct the eruptive history of the mud volcano. The study is complemented through the integration of seismic and sub-seismic scale structural details observed at an active onshore mud volcano. Some structural elements mapped at this volcano may be analogous to those observed in the seismic data. The study offers new insights into mud volcano system evolution which may help to gain a better understanding of the drainage histories and hydrodynamic characteristics of mobile shale basins.
The main subject of this study is the Chirag mud volcano, located approximately 100 km southeast of Baku, offshore Azerbaijan. At around 11 km in diameter and up to 1.4 km thickness the edifice of this volcano is believed to be the largest yet described using seismic data. Conventional seismic stratigraphic principles are adapted to subdivide the edifice into five subcomponent units (Fig 1A). Four out of the five units are interpreted as buried and extinct mud cones with one unit likely to represent a layer of background sediment deposited during a phase of eruptive quiescence. Calibration of unit bounding reflections to a nearby borehole enables us to estimate the eruption rate for each unit.
Figure 1: (A) Uninterpreted and interpreted seismic traverse through part of the Chirag edifice showing internal reflection detail that is the basis for stratigraphic subdivision. Note internal reflection terminations and cross sectional shape of mud cone units 1, 2, 4 and 5. (B) Interpreted seismic traverse through the Chirag edifice showing relationship of sub-component units with the asymmetric caldera. TMV= top mud volcano reflection, BMV= base mud volcano reflection.
The edifice is located above a near-circular fault-bounded caldera that represents the upward limit of a subvolcanic ring complex. The caldera measures just over 2 km in diameter and is made up of a series of steeply inward dipping arcuate extensional faults. It has a “trapdoor” geometry in cross section indicating that caldera collapse has been asymmetric (Fig 1B). Evacuation and deflation of the mud source region, surface loading and large amounts of compaction within the heavily intruded feeder system are all possible mechanisms to explain the occurrence of this caldera.
On the basis of reflection geometries and thickness characteristics we suggest that fault-controlled asymmetric subsidence of the caldera floor initiated following the first extrusion of mud. It continued until the later stages of edifice evolution when eventually fault activity abated and ceased (Fig 1B). If we assume that caldera collapse is linked to source deflation, then this observation may indicate that early pressure release from the mobile shale substrate was catastrophic with deflation and caldera collapse occurring as a result. Following this the newly established hydraulic gradient between the mobile shale and the surface would have the effect of drawing new mud laterally towards the volcano from within the mobile shale. This may explain the lack of significant caldera collapse during the later stages of edifice evolution as the source region is continuously being recharged at this time. However, a direct hydrodynamic link between the extrusive edifice and the source region is not proven and there are other ways of explaining the mechanism behind the calderas formation as noted above.
The Garadag Pilpila Mud Volcano
The Garadag Pilpila mud volcano is located approximately 27 km southwest of Baku in eastern Azerbaijan and has recently been the focus of detailed structural and geomorphological mapping. The volcano is around 80 m in height and measures just over 4 km in diameter. It is currently active and has a number of small mud cones (gryphons) near its crest. The principal structural feature of the volcano is a long curvilinear fault that is located near the crest of the volcano (Fig 2).
Figure 2: Non-perspective 3D view of the Garadag Pilpila mud volcano. Ikonos satellite image is draped over a Digital Elevation Model. Note position of large gryphons aligned along northern section of fault trace.
A variety of fracture styles make up the trace including sections of extensional displacement, rubbly vegetated depressions and open fracture zones. Most significant are a series of well-developed en-echelon extension cracks that bridge the fault trace along much of its length and indicate a component of strike-slip fault displacement. The greatest amount of extension is observed along the arcuate section of the fault trace with around 1-2 m of normal displacement indicated by offset of the volcano ground surface. A number of presently and recently active mud volcanic vents (gryphons) and mud pools (salses) are present near the summit of the volcano. These are mostly clustered in an area near to the highest point of the volcano outside of the fault-bound block. However, some of the largest gryphons are located along the northern fault segment and are actually cut by the fault trace (Fig 2).
Implications for Internal Volcano Structure
It is possible that the fault system mapped at Garadag Pilipila represents the upward extent of a subvolcanic ring complex that has propagated upwards through the edifice to the surface. If so it would be directly analogous to the Chirag collapse caldera. The synsedimentary development of the Chirag caldera fault early in the volcanoes evolution means that its upper tip must have been exposed at the surface as a circular fault trace at this time. The alignment of large gryphons along the exposed fault trace may indicate that the fault acts as some kind of conduit system to the surface. There are however other possible scenarios. For instance collapse over a more shallowly buried “mud chamber” located within the edifice or even as a feature relating to an incipient mass movement.
In addition to highlighting the power of 3D seismic data as a tool for the investigation of mud volcanoes this study serves to demonstrate that:
1. Conventional seismic stratigraphic methodology can be applied to the subdivision of large mud volcanic edifices.
2. Stratigraphic subdivision of the edifice and subsequent analysis of the geometries and lateral extents of subcomponent units may be used to reconstruct the eruptive history of the volcano.
3. Caldera-like collapse of country-rock beneath the edifice may be an important mechanism in volcano evolution, possibly as a result of source region evacuation and deflation.
4. Analysis of the erupto-stratigraphy of a mud volcanic edifice may offer insights into the hydrodynamic regime of the mud volcano system and possibly the underlying mobile shale substrate.
5. Seismic scale structural elements such as possible caldera faults can be observed at outcrop and may be analogous to seismically observed examples.
AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago