Simon Stewart

Structure and emplacement of mud volcano systems in the South Caspian Basin

The term “mud volcano system” is coined to describe the set of structures associated with a constructional edifice (mud volcano) and feeder complex that connects the volcano to its source stratigraphic unit. Three-dimensional (3-D) seismic data from the South Caspian Basin are used to investigate the structural elements and evolution of these systems. Mud volcano systems initiate via early, kilometer-scale, biconic edifices termed “pioneer” cones. These are fed by fluidization pipes tens of meters in width. Subsequent kilometer-scale mud volcanoes grew via persistent extrusion, fed by numerous additional fluidization pipes injected in the country rock. This subvolcanic intrusion complex creates a densely intruded, cylindrical zone, similar in cross section to gryphon swarms observed at an outcrop onshore. Wall rock erosion and compaction of the intruded zone leads to the collapse of a downward-tapering cone enveloping the cylindrical zone, capped by ring faults that define a kilometer-scale caldera that downthrows the overlying mud volcano. Mud volcanoes get buried during basin subsidence and can look like intrusive laccoliths at first glance on seismic data. Reactivation of mud flow through a conduit system generates a stack of superimposed mud volcanoes through time. Large volcanoes continue to dewater during burial and may locally remobilize. This model of mud volcano evolution has similarities with igneous and salt tectonic systems. To reduce drilling and geologic uncertainty, mud volcano system extent and impacts on a reservoir can be assessed on 3-D seismic data.nnSimon Stewart received his Ph.D. from the Imperial College London in 1992. In 1992–2000, he was seismic interpreter at Amerada Hess, in support of exploration and appraisal drilling in the North Sea. In 2000, he moved to BP and has since worked on structural geology and well planning in the Americas, Middle East, and the former Soviet Union. His special interest is in the integration of structural geology and seismic interpretation. Richard Davies received his Ph.D. from the University of Edinburgh in 1995. In 1995–2003, he was with Mobil and ExxonMobil, working on field development and exploration, the North Sea, the west of Shetlands, and the west Niger Delta deep-water slope. In 2003–2005, he was a senior lecturer in earth sciences at Cardiff University, United Kingdom. Currently, he is professor and director of CeREES (Center for Research into Earth Energy Systems), Durham University, United Kingdom. His special interest is in seismic-scale expression of fluid migration and diagenesis.

Emplacement of giant mud volcanoes in the South Caspian Basin: 3D seismic reflection imaging of their root zones

Exceptional quality 3D data for the largest mud volcano yet described provide the first detailed imaging of the plumbing architecture that connects a major volcanic edifice to its source layer at depth. The volcano is in the South Caspian Basin and consists of an extruded submarine mud bicone, 10 km wide and 1.4 km thick, overlying an oval caldera 1.2–1.6 km in width and 0.5 km in depth. The caldera narrows downwards into a zone of collapsed country rock forming a downward tapering cone, 1 km in height, the vertex of which is located close to the top of the mud source layer. The imaged structural elements lead to an evolutionary model. A narrow, steep fluidization pipe fed the oldest, ‘pioneer’ cone. We propose that numerous additional fluidization pipes injected the country rock, forming a densely intruded, cylindrical zone, similar to ‘gryphon’ swarms observed at outcrop onshore. Wall-rock erosion and compaction of the intruded zone led to collapse of the downward tapering cone that linked upwards into ring faults that define the caldera margins. Later mud flowage focused on the conical sheared margins. Volumetric contraction of the extruded volcanic cone led to an unusual concentric system of minor, outward-facing normal faults. This model has many similarities to syntheses of igneous maar–diatreme–caldera systems, for which it may be analogous.

The structure and formation of mud volcano summit calderas

Circular depressions bounded by inward-dipping faults found at the upper terminations of large mud volcano systems (>500 m diameter) are termed ‘mud volcano summit calderas’. From new mapping and comparison with previously identified examples we describe a series of common structural and morphological features found at a number of calderas and propose a mechanism for caldera formation. A typical example consists of concentric deformational and volcanic zones including an outermost topographic rim, inward-dipping circular fault system, ‘moat’ and raised central ‘pedestal’ of freshly extruded mud volcanic sediment. This distinctive ‘moat and pedestal’ morphology can be explained in terms of the quantity and rheology of extruded mud, and appears characteristic of calderas from the South Caspian Basin and elsewhere. The association of fresh mud volcanic deposits with the calderas in combination with extensive literature on caldera modelling leads us to conclude that the most likely causal mechanism is subsidence as a result of evacuation of fluids and sediment from shallow structural levels during eruptions. Summit calderas mapped in this study characterize dormant periods of mud volcanism in subaerial and submarine settings, and appear to be a common structural element in the extrusive domain of mud volcano systems.

Structural controls on mud volcano vent distributions: examples from Azerbaijan and Lusi, east Java

Abstract: Structural mapping, nearest neighbour and two-point azimuth statistical analysis of mud volcano vent distributions from nine examples in Azerbaijan and the Lusi mud volcano in east Java are described. Distributions are non-random, forming alignments subparallel to faults within anticlines, ring faults, conjugate faults and detachment faults; this finding confirms a spatial relationship and supports a model for subsurface flow along these features as well as showing fractionation at depth. As fracture and fault orientations are related to structures such as anticlines and the in situ stress state they are therefore predictable. We use vent distributions in Azerbaijan, where the structural geology is well constrained, to propose what controls the distribution of 169 vents at the Lusi mud volcano. This mud volcano system shows evidence for initial eruptions along a NE–SW trend, parallel to the Watukosek fault, changing to eruptions that follow east–west trends, subparallel to regional fold axes. Our analysis indicates that regions east and west of the Lusi mud volcano are more likely to be affected by new vents than those to the north and south, owing to probable onset of elongate caldera collapse within a 10 km diameter of the central vent.

Heterogeneous exhumation in the Inner Moray Firth, UK North Sea: constraints from new AFTA® and seismic data

Integration of regional seismic interpretation, sonic velocity, vitrinite reflectance and apatite fission-track analysis (AFTA®) studies has demonstrated that the western region of the Moray Firth rift arm (UK North Sea) experienced pronounced exhumation during the Cenozoic. Although this basin is usually considered to have experienced regionally uniform exhumation, interpretation of new seismic data has revealed the presence of a major system of post-Jurassic normal faults, with throws commonly in the range of 10–300 m and locally exceeding 1 km. New, high-quality seismic data are used in combination with AFTA and vitrinite reflectance data to investigate the role of extensional faulting during exhumation of this basin. Results of this interpretation not only confirm the offsets across major faults, but also show that greater exhumation and erosion occurred on their footwalls than on their hanging walls. We conclude that the localized, differential exhumation is the result of superposition of local or short-spatial-wavelength extensional tectonics upon regional, long-spatial-wavelength exhumation. These results suggest that differential exhumation might be characteristic of unroofed rift basins where normal faults subcrop the exhumation-related unconformity and that, in such cases, thermal histories from footwall locations may yield inaccurate predictions of the burial history of hanging-wall depocentres. Inaccurate burial histories will lead to a misrepresentation of the thermal history, with an impact on the estimation of hydrocarbon source rock maturity for petroleum basins.

Estimates of yet-to-find impact crater population on Earth

Abstract: One hundred and seventy-six craters on Earth are currently accepted as resulting from bolide impact; this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations. Here we take the subset of craters most relevant to sedimentary basin exploration and utilization (Phanerozoic kilometre-scale craters) and tackle the question of how many remain to be found in Phanerozoic strata on Earth. A surprising range in predictions of crater numbers is obtained from published crater population functions, reaching two orders of magnitude variation for craters in the range 1–5 km diameter. This range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere. Also required is quantification of the total area and timespan of Phanerozoic sediment currently preserved on Earth; an estimate of 8.87 × 1015 km2 a is derived here. Together with a relatively conservative crater population function this yields a prediction of 845 craters; subtracting the 131 known Phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714, of which 228 are larger than 2.5 km. Uncertainties, subsets of this population, and spatial distribution are also considered.

Displacement distributions on extensional faults: Implications for fault stretch, linkage, and seal

Most extensional faults are characterized by displacement distri butions that range from zero at the fault tips to a maximum value at some point along the length of the fault. Regardless of the pattern of displacement distribution, a geometrical requirement resulting from the displacement variation along the fault is that beds must stretch parallel with the strike of the fault. It has been suggested that minor faults and fractures evolve perpendicular to the main fault to accommodate this stretch. The amount of stretch that this subpopulation of faults must accommodate is quantified here using several alternative displacement profile models. The choice of pro file model is much less significant than the ratio of maximum displacement to fault length (Dmax/L). The amount of stretch rapidly increases from about 0.7% at Dmax/L of 0.05 to about 3% at the typical upper limit for Dmax/L of 0.1. These relationships predict tens to hundreds of meters of fault-parallel stretch associated with kilometer-scale extensional faults. The size and spatial distribution of stretch accommodation faults should be incorporated in three-dimensional (3-D) fault zone models, and there are several implications for fault linkage and seal. One possible strain accommodation scenario is that a small number of faults that have displacements significant enough to generate reservoir-juxtaposition leak points could exist. Alternatively, a subseismic fault and fracture population could either significantly degrade horizontal permeability parallel with the major fault or, if open, constitute a target for high-angle exploitation drilling. The intersections between the stretch accommodation faults and the main fault could provide conduits for fault valving or low-entry pressure leak points. The most commonly observed candidate stretch accommodation faults occur in fault overlap zones in segmented fault systems, coincident with maximum displacement gradients on the fault planes. In these cases, cutoff stretch accommodation could be an integral factor in fault segment linkage. (Begin page 588)

3D seismic technology: are we realising its full potential?

Abstract Three-dimensional (3D) seismic data have had a substantial impact on the successful exploration and production of hydrocarbons. Although most commonly acquired by the oil and gas exploration industry, these data are starting to be used as a research tool in other Earth sciences disciplines. However despite some innovative new directions of academic investigation, most of the examples of how 3D seismic data have increased our understanding of the structure and stratigraphy of sedimentary basins come from the industry that acquired these data. The 3D seismic tool is also making significant inroads into other areas of Earth sciences, such as igneous and structural geology. However, there are pitfalls that parallel these advances: geoscientists need to be multidisciplined and true integrators, and at the same time have an ever-increasing knowledge of geophysical acquisition and processing. Notably the utility of the 3D seismic tool seems to have been overlooked by most of the academic community, and we would submit that academia has yet to take full advantage of this technology as a research tool. We propose that the remaining scientific potential far exceeds the advances made thus far and major opportunities, as well as challenges, lie ahead.

Sector collapse of mud volcanoes, Azerbaijan

Abstract: Field data collected from mud volcanoes in Azerbaijan are used to describe a process in mud volcano development that involves portions of the constructional edifices collapsing outwards in ‘thin-skinned slides. These events create kilometre-scale scarps that are tens of metres in height, arcuate in plan view, elongate and facing downdip. Similar morphological features occur on igneous volcanoes and have been described as ‘sector collapse structures. The largest sector collapses in igneous volcanoes involve some 1012 tons of mobilized material; equivalent structures in mud volcanoes are several orders of magnitude smaller. We employ a shape parameter that can be utilized in field and satellite-based mapping, to distinguish between slope failure and eruptive deposits. Three mud volcanoes with kilometre-scale sector collapses are described and controlling mechanisms are reviewed. The updip domains of these collapses are characterized by fluid escape, showing that there is also linkage to deeper mud volcano structure. The observations are reconciled in a model consisting of a deflating mud chamber that triggers thin-skinned sector collapse. The updip domain of the sector collapse is localized above a deep-seated zone of volume loss and the downdip domain of the collapse runs down the edifice flank onto the surrounding plain.

Improved Drilling Performance through Integration of Seismic, Geological and Drilling Data

Abstract Unexpected incidents leading to lost time when the rig is on location cause unplanned cost to the hydrocarbon industry of over one billion dollars annually. Processing and interpretation of 3D seismic data usually focuses on reservoir levels. But from a drillers perspective, geological features of the overburden are often more significant than those at reservoir level, since over 90% of the well is typically spent drilling the overburden, coping with a wider variety of challenges than those associated with the reservoir itself. 3D seismic data defines overburden tectonostratigraphy, the framework of a geological model that can be used in well planning to reduce geological uncertainty, surprises and expense along the whole well track. Many technologies applied in reservoir modelling are equally valid in defining overburden features relevant to well planning. The overburden 3D volume can be populated with key parameters for well design, such as pore pressure and geomechanical attributes, though the complexity of the model will often be restricted by well cost and perception of drilling risk. The role of 3D seismic data in forming the tectonostratigraphic framework of multi-attribute, kilometre-scale Earth models, is illustrated here by a number of examples where model sophistication has been scaled to match project requirements. Overburden Earth models also provide a framework where several academic research themes, for instance 3D fault geometry, can be put into a commercial context. Construction of overburden models for well planning has also highlighted a number of future geological research areas that could have a significant impact on drilling performance. Some of these, such as hydraulic properties of fault systems, are highlighted here.