bluuebunny/crossref_metadata_2025 huggingface dataset:
Geology and hydrocarbon occurrences in the Ghadames Basin, Algeria, Tunisia, Libya
Authors: Echikh K.
Published: April 2008

The Ghadames Basin is a large intra-cratonic basin, covering portions of Algeria, Tunisia and Libya. The three countries are independently conducting petroleum exploration in their portions of the basin, using different play concepts and consequently obtaining different exploration results. This paper presents the first published basin-wide view of petroleum stratigraphy and play types. Some 700 exploratory wells have been drilled in the basin, resulting in the discovery of at least 150 oil pools with 9500 MMBO (million barrels of oil) in place. Most wells were located in the structurally higher parts of the basin, with deeper portions being less explored because of shifting dune conditions and an expectation of reservoir thinning into the basin centre. Silurian and Devonian source rocks occur across large parts of the basin and have generated volumes of hydrocarbon orders of magnitude above those discovered. The numerous structural phases (Taconic, Caledonian, Hercynian and Austrian) that have affected the basin have had important implications for depocentre migration, structural style, and for patterns of trap formation, alteration and destruction. The erosion pattern and topography developed on the Hercynian unconformity is a key control on petroleum systems within the basin, controlling the preservation of Palaeozoic hydrocarbons, communication between source and higher reservoirs and patterns of long-distance migration in the Triassic reservoir. For each of the producing plays, an analysis is presented of the geological factors controlling the habitat of hydrocarbons (e.g. the effect of various tectonic phases, the influence of basin-scale facies variations on reservoir quality, and the source rock-reservoir relationship), and is related to the variations that have been observed in exploration success rate and producibility. It is believed that a substantial volume of hydrocarbons still remains undiscovered in a range of trap and play types.
Regional depositional setting and pore network systems of the El Garia Formation (Metlaoui Group, Lower Eocene), offshore Tunisia
Authors: Loucks R. G. , Moody R. T. J. , Bellis J. K. , Brown A. A.
Published: April 2008

The Metlaoui Group was deposited on a broad ramp that deepened to the northeast into the Tethyan Sea. The ramp contains a series of broad facies belts. The inner-ramp facies belt is composed of three facies tracts: (1) Faid sabkha (most landward) characterised by skeletal-poor dolomudstones and wackestones and interbedded evaporites, (2) Ain Merhotta restricted shallow-lagoon facies composed of sparse skeletal wackestones, packstones, and cross-bedded gastropod grainstones; (3) updip El Garia high-energy shoal complex (most seaward) composed of red algal-discocyclinid grainstones and packstones which are locally cross-bedded. The El Garia carbonates in the mid-ramp setting are predominantly composed of large, flat nummulites. Dominant textures are burrowed, mud-poor packstones and poorly sorted or bimodal grainstones. The accumulation was located below fair-weather wave base and above storm wave base, at a water depth estimated to have ranged from 30 to 60 m (middle neritic). The deep-water (deep neritic), outer-ramp Ousselat Member consists of coarse silt- to medium sand-sized nummulithoclastic debris that grades from coarser to finer debris down the ramp. The Bou Dabbous Formation is composed of basinal foraminiferal mudstones, wackestone and packstones deposited by suspension and less commonly by gravity-flow deposition in upper bathyal water depths. Reservoir quality is variable, with most lime packstones and grainstones having moderate to high porosity (related to intraparticle and microporosity), but only poor to fair permeability (generally less than 10 mD). The higher-quality reservoirs have preserved interparticle porosity with permeabilities ranging from tens of millidarcys to several darcys. Permeable nummulitic packstones and grainstones are favoured by the following factors: (1) low abundance of lime mud, (2) low abundance of nummulithoclastic debris, (3) low abundance of echinoderm fragments, (4) moderate sorting, (5) minor precipitation of late burial cements, (6) dolomitization.
Basin evolution and deposition during the Early Paleogene in Tunisia
Authors: Zaïer A. , Beji-Sassi A. , Sassi S. , Moody R. T. J.
Published: April 2008

The marine Paleocene and Ypresian deposits of Tunisia, within the El Haria Formation and the Metlaoui Group, have been intensively studied because of the commercial interest in phosphates and hydrocarbons. This paper presents the latest updates of isochron, lithofacies and palaeogeographical maps, and interprets the patterns identified in light of synsedimentary structure. This reveals a close association between structure, basin geometry and subsidence. Facies distribution during the Early Paleogene is thought to be structurally controlled along basement lineaments. These major fault systems were reactivated several times during the Mesozoic and Tertiary, with the last movements occurring as Neogene and post-Villafranchian events. The structural control of facies is most evident during the Ypresian, particularly along the ‘North-South Axis’ (Nosa) a sub-meridian orogenic segment of Central Tunisia. In this area the Ypresian deposits exhibit a preferred alignment with the Nosa exerting a notable influence on basin geometries and rates of local subsidence. In addition to two large islands indicated in the onshore area of Tunisia, a number of ‘bald’ highs are identified in the offshore, representing original submarine highs. The general pattern across Tunisia throughout the period is of a number of small tectonically controlled basins. The distribution of phosphorites, organic-rich shales and evaporites can be particularly linked to the development of restricted basins during the period.
Kinematics of the Watchet-Cothelstone-Hatch Fault System: implications for the fault history of the Wessex Basin and adjacent areas
Authors: Miliorizos M. , Ruffell A.
Published: April 2008

The Watchet-Cothelstone-Hatch Fault (WCHF) comprises a system of northwest-southeast-trending basement and cover faults that are traceable for at least 40km from the Bristol Channel, southeast into the western Wessex Basin. The west-dipping WCHF displays outcrop and seismic evidence of a complex movement history involving early (Variscan) and late (?Cretaceous or Tertiary) strike-slip, separated by phases of normal extension. In the Variscan basement of North Somerset, the WCHF shows a 14–16km dextral offset to Devonian markers and Variscan fold axes, and can be linked to similar trending faults in South Wales. Offsets to early Permian (and possibly late Carboniferous) rifts indicate a dextral transfer of north-south extension from the Crediton and Tiverton troughs, across the WCHF and into the Wessex Basin. In Mesozoic cover, the WCHF swings to become part of the east-west Central Bristol Channel Fault Zone in the north and Lopen Fault Belt (western Wessex Basin) in the south. This geometry reflects a present-day preservation of a later sinistral movement. The structure of juxtaposed Palaeozoic and Mesozoic successions along the WCHF provides an analogue for what might be expected at the basement-cover contact beneath the Wessex Basin. The existence of Mesozoic cover to the WCHF most likely explains why no Cenozoic pull-apart basins were formed, unlike the Sticklepath Fault (in granites and metamorphic rocks) to the southwest.
Transpression (or transtension) zones of triclinic symmetry: natural example and theoretical modelling
Authors: Lin Shoufa , Jiang Dazhi , Williams Paul F.
Published: April 2008

We describe a natural shear zone with triclinic symmetry, present a general model for triclinic shear zones based on natural examples, and investigate the kinematics and strain geometry within such zones.

In the Roper Lake shear zone in the Canadian Appalachians, the orientation of a stretching lineation is oriented approximately down-dip near the shear zone boundary and becomes gradually shallower towards the centre. The structures in the central portion of the shear zone exhibit approximately monoclinic symmetry where the poles to both the S- and C-surfaces, the stretching lineation on the S-surfaces and the striations on the C-surfaces all plot in a great circle girdle. However, the lineations from the marginal portion do not plot in the same girdle, and the bulk symmetry of the shear zone is triclinic. Theoretical modelling shows that the observed strain geometry can be interpreted by an oblique transpression with a larger ratio of simple shear to pure shear in the centre of the shear zone than in the margin. The latter suggests a higher degree of localization of the zone boundary-parallel movement component relative to the boundary-normal compression component.

We emphasize that, as the imposed boundary displacements for most natural shear zones lie between dip-slip and strike-slip, their movement pictures are generally triclinic; monoclinic shear zones are special end members. Structural data that exhibit monoclinic symmetry do not necessarily mean that they resulted from a monoclinic movement picture; the present modelling demonstrates that a triclinic movement picture with a high ratio of boundary-parallel movement to boundary-normal movement can result in apparent monoclinic structural geometry. The results of the modelling also show that the simple statement made for simple shear zones that stretching lineations will align with, and therefore indicate, the shear direction cannot be extrapolated to three-dimensional transpressional (or transtensional) shear zones.
Tectonic evolution of the Northern Salinian Block, California, USA: Paleogene to Recent shortening in a transform fault-bounded continental fragment
Authors: Tavarnelli Enrico
Published: April 2008

The complex structural setting of the western margin of North America is interpreted to result from oblique convergence of the North American and Pacific plates, accommodated by both right-lateral slip along the San Andreas fault and shortening east and west of it. Strike-slip movements along the San Andreas fault led to the detachment of a continental fragment (the Salinian Block) from the North American margin during early Miocene, and its translation northwestwards for over 300 km. Structural analysis in the Northern Salinian Block, west of the San Andreas fault, reveals a NE-SW-directed shortening accommodated by NW-SE-trending folds and thrusts with a dip-slip kinematic character. The stratigraphic record of progressively younger unconformities affected by both folds and thrusts, as well as the overprinting relationships among these contractional structures, enables us to clarify the tectonic evolution of the region since Paleocene time. The Paleocene to Recent history of the Salinian Block was dominated by strike-slip along the San Andreas fault, and by shortening perpendicular to it. The partitioning between strike-slip and dip-slip movements appears to be controlled by a pre-existing tectonic feature. The results from structural analysis along the Salinian Block are integrated into a deformation model for the western margin of North America, providing additional constraints on the timing of deformation and helping to clarify the role of strain-partitioning processes in the obliquely convergent California margin.
Ocean floor volcanism: constraints from the integration of core and downhole logging measurements
Authors: Brewer T. S. , Harvey P. K. , Lovell M. A. , Haggas S. , Williamson G. , Pezard P.
Published: April 2008

The volcanic architecture of oceanic crust records the diversity in volcanic activity during its development in the neovolcanic zone of individual ridge systems. Potentially there exists a spectrum of lithological architectures which may primarily be related to the spreading rate and the dynamics of individual magma chambers along different ridges. Recent studies have emphasized the observable spatial variations within different neovolcanic zones, although direct extrapolation into the third dimension can only be achieved by the use of drilling results. To study the structure of the volcanic layer it is essential that individual lithologies (sheet flows, pillow lavas and/or breccias) can be discriminated from the core and/or logging results and mapped within the borehole. Unfortunately a problem with the drilling of the volcanic basement during the Ocean Drilling Program has been the generally low (typically c. 25%) and biased core recoveries, which produce an erroneous picture of the lithological diversity of the volcanics. This problem is further compounded by the difficulty in determining the volcanic stratigraphy, particularly when the key information is lost during coring (i.e. boundaries/contacts). Downhole logging provides near continuous records of the physical/chemical properties of the borehole which when integrated with core measurements, yield a detailed picture of the architecture of the volcanic layer. Logging results from ODP Hole 896A are of sufficient quality that sheet flows, pillow lavas and brecciated units can be discriminated and mapped effecively within the borehole. From their distribution it is evident that sheet flows become more abundant in the lower part of the hole, which probably correlates with ridge axis volcanism whereas, the predominance of pillow lava flows (<340 mbsf (metres below sea floor)) in the upper part of the hole, is probably related to off-axis volcanism within the neovolcanic zone.
A novel approach for constraining heat flow histories in sedimentary basins
Authors: Gallagher K. , Morrow D. W.
Published: April 2008

We examine the application of a novel inversion procedure to determine a heat flow history from down-hole thermal indicator data from 2 wells in northern Canada. The approach is based around Occam’s razor in that we attempt to constrain the simplest heat flow history consistent with the observed data. Simple models are defined in terms of the variation about the present day heat flow and the inverse problem is to minimise a combined function of the data misfit, weighted by the individual error on each observation, and the model complexity.

The results obtained for the 2 wells, Pan Am Beaver River YT G-01 and Imperial Island River No. 1 in the Liard Basin, agree with heat flow histories determined from forward modelling. However, the inversion approach discussed provides useful estimates of model resolution and sensitivity. The effect of variations in the data errors and the nature of the forward model are also considered. As we expect intuitively, the quality of the solution depends directly on the quality of the input data and forward model.
Geology of the Sierra de Fiambalá, northwestern Argentina: implications for Early Palaeozoic Andean tectonics
Authors: Grissom Grady C. , Debari Susan M. , Snee Lawrence W.
Published: April 2008

Field mapping in conjunction with structural, metamorphic, and geochronological data document the tectono-thermal history of exhumed deep crustal rocks in the Sierra de Fiambalá, NW Argentina. The range consists of two structural blocks distinguished by different metasedimentary sequences and different grades of metamorphism. Orthogneiss and paragneiss in the northern structural block may have a Precambrian history. Greenschist- to amphibolite-facies metamorphism, intrusion, and injection migmatization affected all rocks at 540–550 Ma. A subsequent event in the Late Cambrian to Ordovician (
c.
515 to 470 Ma) involved amphibolite- to granulite-facies metamorphism, mafic intrusion, and deformation, followed by cooling through mid-Palaeozoic time. The emplacement of Carboniferous (325–350 Ma) post-tectonic granites caused reheating and retrogression that was strongest toward the northeast part of the range.

The Cambrian, Ordovician, and Carboniferous events in the Sierra de Fiambalá were of regional extent as indicated by temporal correlations with events reported for other deep crustal rocks of the northern Sierras Pampeanas. Correlations between periods of intrusion and high-grade metamorphism in the northern Sierras Pampeanas and volcanic-sedimentary events in the adjacent supracrustal exposures confirm that rocks in the northern Sierras Pampeanas formed at deep (10–25 km) structural levels in the early Palaeozoic continental margin of Gondwana.
The granitoids of the Sierra de San Luis
Authors: Llambías E. J. , Sato A. M. , Suárez A. Ortiz , Prozzi C.
Published: April 2008

The Sierra de San Luis consists of Lower Palaeozoic igneous and metamorphic rocks. The granitoids were emplaced between Late Cambrian and Early Carboniferous times, and the most important metamorphic and deformational events occurred during this period. Older metamorphic events have also been recognized but they are poorly constrained. The most important group of granitoids are as follows: (1) Deformed tonalites and granites. The tonalites have a calc-alkaline signature with low La/Yb ratio. They are the Early Famatinian, pre-orogenic, granitoids of Late Cambrian to Early Ordovician age and form part of a magmatic arc. (2) The La Escalerilla granite is a deformed biotite monzogranite and consists of a single elongated pluton; its relation with the other granitoids is not clear. (3) Garnet-biotite-muscovite granodiorites and granites and pegmatites. The granitoids have S-type signatures, with high
87
Sr/
86
Sr and La/Yb ratios. They are possible crustal magmas related to a thickened crust and are interpreted as early- to syn-orogenic. (4) Biotite- and biotite-hornblende granites and minor tonalites of Devonian to Early Carboniferous age. The granites form large, post-orogenic, plutons and batholiths related to crustal uplift, and represent the last igneous activity of the Famatinian cycle (and also of the Palaeozoic era) in the Sierra de San Luis.
Fluid inclusion constraints on conditions and timing of hydrocarbon migration and quartz cementation in Brent Group reservoir sandstones, Columba Terrace, northern North Sea
Authors: Wilkinson J. J. , Lonergan L. , Fairs T. , Herrington R. J.
Published: April 2008

Fluid inclusion data have been obtained from Brent Group sandstones from the Columba Field on the margin of the East Shetland Basin, northern North Sea. Cogenetic primary aqueous and hydrocarbon inclusios trapped during the initial stages of late diagenetic quartz overgrowth are common, indicating that the pore fluid present during the onset of a major phase of quartz-kaolinite(-illite) diagenesis consisted of immiscible saline aqueous and petroleum phases. Similar inclusions are also observed in abundant planar arrays cross-cutting detrital and authigenic quartz. Homogenization temperature and salinity data from the two types of aqueous inclusions are statistically indistinguishable, as are the properties of the two types of hydrocarbon inclusions, suggesting that the same two-phase fluid was present probably throughout the main phase of quartz cementation. Fluid inclusion thermobarometry shows that the majority of the fluids were trapped in the range 104 ± 6°C and 300 ± 33 bars. These data are consistent with either (1) trapping of a warm, over-pressured hydrocarbon + aqueous fluid, probably in thermal and chemical disequilibrium with the host reservoir rocks, derived from a Kimmeridgian source in the deep Viking Graben basin to the east of the reservoir within the period 85–55 Ma, or (2) trapping of a hydrostatically pressured fluid, in thermal equilibrium with the host reservoir rocks, with hydrocarbon ± aqueous fluids derived by lateral up-dip migration from a local Kimmeridgian hydrocarbon source in the East Shetland Basin within the period 65–40 Ma. A maximum duration of 10 Ma for the fluid flow event can be estimated based on a typical burial history model for the area.
Small seismic-scale fault identification and mapping
Authors: Townsend C. , Firth I. R. , Westerman R. , Kirkevollen L. , Hårde M. , Andersen T.
Published: April 2008

The primary focus of this paper is to emphasise the large volumes of information related to faults and fault systems that are present in, and extractable from, 3D seismic data. During most interpretations, this information is seldom included and transferred to the reservoir model so that their effects can be accounted for during reservoir simulation. Reasons why they are not included are generally related to the time constraints imposed on studies when commercial considerations are often given precedence above any scientific justification. The inadequacies of what are presently considered acceptable models are highlighted and methodologies that could lead to improved reservoir models are proposed. These methodologies are derived from an investigation of how fault systems will manifest themselves in seismic data, based on both theoretical concepts and the use of synthetic models. From these information sources, principles are derived for the identification of faults and fault systems in 3D seismic volumes. These principles are then tested in two case studies selected to emphasize the limitations imposed on seismic resolution by both target depth and seismic frequency content. After highlighting the diversity of fault related information, which is accessible and currently under-utilized by current reservoir modelling techniques, potential methods for automatic mapping and digital extraction of fault information are proposed. When these automated methods are implemented into seismic interpretation software, commercial reasons for ignoring small seismic-scale faults will be significantly reduced and as a result, reservoir performance predictions will become more realistic.
Space and time propagation processes of normal faults
Authors: Marchal D. , Guiraud M. , Rives T. , van den Driessche J.
Published: April 2008

Studies of normal fault development in space and time by X-ray tomography imaged analogue modelling suggest that different processes occur during the propagation of these faults. The conceptual model proposed distinguishes two types of propagation: horizontal and vertical. Examination of horizontal propagation reveals two separate levels of organization. Firstly, an isolated fault segment develops, alternately over time, by radial propagation and by linkage with secondary fault segments (tip faults) arising at the end of the main fault segment (parent fault). These are arranged either in an en echelon or stepwise pattern. Secondly, isolated faults evolve by linkage. Two isolated faults connect up via a relay fault area which arises in the relay zone of the two isolated faults. The relay faults develop by radial propagation and link with the two major faults. Undulating fault traces result from propagation by tip-to-parent fault segment linkage or isolated-to-isolated fault linkage. Upward and downward vertical propagation are identified and exhibit equivalent propagation processes to those involved in horizontal propagation. Space-time sequences combining horizontal and vertical propagations over time suggest that the evolution of normal faults observed in space also reflects their evolution over time.
Quaternary neotectonism and intra-plate volcanism: the Coorong to Mount Gambier Coastal Plain, southeastern Australia: a review
Authors: Murray-Wallace C. V. , Belperio A. P. , Cann J. H.
Published: April 2008

The Coorong to Mount Gambier Coastal Plain in southeastern South Australia preserves a lengthy record of Quaternary temperate carbonate sedimentation in the form of high wave energy barrier shoreline deposits and associated back-barrier lagoon facies. The barriers occur sub-parallel to the modern coastline and to each other and increase in age landwards. The age of the barriers is now generally well established within a framework of radiocarbon, thermoluminescence, uranium-series disequilibrium, amino acid racemization dating and the position of the Brunhes-Matuyama boundary between the East and West Naracoorte Ranges. This framework is in accord with the deep-sea δ
18
O record of global ice volume change and the inferred timing of sea-level highstands for this interval. Individual barriers such as the last interglacial Woakwine Range (oxygen-isotope substage 5e) may be traced laterally for distances up to 300km and record a history of neotectonism in the form of regionally varying epeirogenic uplift associated with Quaternary volcanism.
The Namib Rift: a rift system of possible Karoo age, offshore Namibia
Authors: Clemson J. , Cartwright J. , Swart R.
Published: April 2008

This paper describes a hitherto poorly documented, deep-seated rift system offshore Namibia, here named the Namib Rift. The structural history and rift geometry of this system are determined from reflection seismic data. No wells penetrate the rift-fill succession but from its stratigraphic context, regional thickness relationships and by analogy with onshore Karoo stratigraphy, the age of the rift is interpreted as broadly equivalent to the Karoo Supergroup. The rift is divisible into two major arms, with rift segmentation evident in the north. An angular unconformity separates a rift and post-rift sequence. This is interpreted as a Mid-Triassic hiatus related to a regional compressional events. The basement comprises the Pan-African Fold Belt along the coast, with the Damaran arm extending inland. In the Cape Cross-Walvis Bay area, the Autseib Lineament separates the Kaoko and Damaran arms. This lineament, and the adjacent basement fabrics, are shown to have strongly influenced rift geometry in this area. South of the Autseib Lineament, Damaran basement fabrics largely determined fault trends within the Namib Rift. To the north, Kaoko basement fabrics strongly influenced basement extensional faulting. This area was also subjected to Mid-Triassic compression. South of the Autseib Lineament, large-scale uplift was distributed across large folds whereas to the north, uplift occurred close to pre-existing faults. North-northeast directed compression was common to both areas, but the Autseib Lineament partitioned the structural response. McKenzie-type pure shear extension is favoured as the rift mechanism, with Mid-Triassic uplift modifying thermal subsidence. The final compressive event of the Cape Orogeny, coeval with the regional Mid-Triassic hiatus, terminated rifting in offshore Namibia.
Falkland Plateau evolution and a mobile southernmost South America
Authors: Barker P. F.
Published: April 2008

Assessment of southwest Gondwana break-up and its implications for regional hydrocarbon prospectivity must now take into account the origin of the southeast margin of the Falkland Islands as a volcanic rifted continental margin, and of the floor of the major part of the Falkland Plateau Basin as elevated oceanic crust. A reconstruction of the Falkland Plateau against southern Africa shows a southward extensional widening of the Outeniqua Basin across the line of the Falkland-Agulhas Fracture Zone, changing from stretched continental to oceanic crustal structure. Such a model for Outeniqua Basin opening, and the independent westward and clockwise rotation of the Falkland Islands block, suggests that southernmost South America was also a collection of microplates moving independently within a generally extensional environment in the Late Triassic-Early Jurassic. This is incompatible with assumptions of a rigid southernmost South America over this time, and a dominant role for a continuous dextral strike-slip Gastre Fault.
Evidence for multi-phase rifting in the North Falklands Basin
Authors: Bransden P. J. E. , Burges P. , Durham M. J. , Hall J. G.
Published: April 2008

The post-Gondwanide basin development of the North Falklands Shelf has been evaluated using seismic sequence stratigraphy to determine the structural and stratigraphic evolution of the region.

Two areally distinct post-Gondwanide basin systems (megasequences) are identified on the North Falklands Shelf, each with an independent structural and stratigraphic history. This contrasts with published models which interpret all basin areas to have been initiated in the early-mid Jurassic.

Megasequence I is developed in a series of northwest and subsidiary north and east trending half-graben, and typically shows sedimentary growth into faults indicating an active rifting environment. Pervasive small-scale synthetic faulting is largely non-rotational, suggesting that it represents later deformation. No post-rift sediments associated with this phase of extension are identified. Comparisons with seismic geometries of other basins in the region, based on a restored plate model, indicate that Megasequence I represents part of a widespread mid-late Jurassic rifting event, ultimately leading to the opening of the Rocas Verdes (oceanic) Basin.

Megasequence I is cross-cut by a north trending half-graben system which represents a failed arm of the South Atlantic Rift. The sedimentary fill of this basin, Megasequence II, can be subdivided into two units. The lower unit locally demonstrates sedimentary growth into the main eastern bounding fault of the half-graben and is thought to represent the synrift phase of Atlantic extension. Within the upper unit of Megasequence II there is no evidence of fault-controlled sediment growth into the main easterly bounding fault and the overall geometry is indicative of passive fill of pre-existing topography, supporting a post-rift origin for this section.

Similar seismic geometries are observed in parts of the North Atlantic Rift System and the northern North Sea. In particular, the North Shetland Basin has a Permo-Triassic to late Jurassic rift and post-rift section. Renewed extension in the early Cretaceous resulted in break-up and domino faulting, with the generation of high relief fault blocks. The intervening lows were sites of deposition for onlapping deepwater sediments.
Structural controls on the hydrocarbon and mineral deposits within the Kutai Basin, East Kalimantan
Authors: Colke I. R. , Craig J. , Blundell D. J.
Published: April 2008

The Kutai Basin, Kalimantan, is the deepest sedimentary basin in Indonesia. It contains both substantial reserves of hydrocarbons and minerals. The basin formed in the middle Eocene as a result of extension linked to the coeval opening of the Philippines Sea, the Celebes Sea and the Makassar Straits. Seismic profiles across the northern margin of the Kutai Basin reveal inverted half-graben of middle Eocene age oriented north-northeast-south-southwest and north-south. Further extension during the late Oligocene occurred on northwest-southeast trending extensional faults, reactivating basement structures. Restoration of deformed half-graben suggests that the syn-rift sequence, which contains important coal horizons, was sufficiently deeply buried to generate hydrocarbons prior to inversion. Shortening from the early Miocene to the present day resulted in breaching of some structural and stratigraphic traps, and also in the generation of new traps, in particular folds in the hanging walls of inverted half-graben and as a result of short-cut faults. Middle Eocene, late Oligocene-early Miocene and Plio-Pleistocene volcanic activity was focused at the intersection of basement weaknesses and set up hydrothermal systems which formed several mineral deposits. Reactivation of northwest-southeast and northeast-southwest trending basement structures as a result of a variation in the stress regime has controlled the location of hydrocarbon and mineral deposits in the Kutai Basin. This study highlights the importance of a regional understanding of varying stress regimes in controlling the orientation and location of structures active in a basin at a particular time, and in predicting the location of mineral deposits.
Extensional faults that localize Irish syndiagenetic Zn-Pb Deposits and their reactivation during Variscan compression
Authors: Hitzman Murray W.
Published: April 2008

The syndiagenetic Zn-Pb deposits of south-central Ireland, which are hosted by Lower Carboniferous carbonate sediments, occur along generally northeast trending, segmented normal fault arrays which were active during sedimentation. Individual sulphide bodies are located at points of maximum throw on east-west trending faults within the northeast trending fault arrays. All of the known major Irish Zn-Pb deposits in the south-central Midlands occur on north-side-down normal faults. The Irish Midlands underwent compressive shortening during the Variscan phase of the Hercynian orogeny. North-side-down normal faults were largely unaffected by this event while favourably oriented, south-side-down, Early Carboniferous normal faults were reactivated and transformed into reverse faults. The absence of significant sulphide deposits along south-side-down Chadian-Arundian extensional faults is probably due mainly to dismemberment, and/or compressional uplift and erosion, of such zones during Variscan deformation.
Fracture-controlled fluid flow in the Lower Palaeozoic basement rocks of Ireland: implications for the genesis of Irish-type Zn-Pb deposits
Authors: Everett C. E. , Wilkinson J. J. , Rye D. M.
Published: April 2008

A fluid inclusion study has been carried out to test whether circulation of fluids within fractured very low grade metasedimentary basement rocks was an important process in the genesis of the carbonate-hosted base metal deposits of Ireland. In the Silvermines district, Silurian greywackes and mudrocks are host to several different cross-cutting vein types which are locally abundant in east-east-northeast and northwest-north-northwest trending, high angle, brittle fault zones. Structural analysis indicates that the vein orientations are consistent with formation under a northeast-southwest dextral trans-tensional regime, controlled locally by reactivation of pre-existing faults. Three vein types have been identified: (1) early hematitic calcite-quartz ± pyrite; (2) quartz-calcite ± sphalerite, galena, chalcopyrite, pyrite, barite; (3) ankerite-ferroan dolomite-quartz ± sphalerite, pyrite. Vein types 2 and 3 are associated with weak to locally intense sericite-chlorite-carbonate alteration.

Primary fluid inclusion data from vein Types 2 and 3 show homogenization temperature-salinity characteristics (123–238°C, 9.7–20.6 wt% NaCl equivalent) overlapping significantly with the deposit data. In addition, primary inclusions hosted by quartz and sphalerite commonly contain CO
2
as indicated by the formation of gas clathrate and, rarely, solid CO
2
on cooling. Bulk fluid inclusion analyses on quartz vein samples show that the fluid composition is comparable with experimental data on fluids equilibrated with greywackes at temperatures of 200–350°C. Alkali geothermometry gives temperatures (158–219°C) not significantly different to homogenization temperatures suggesting that the veins formed under low fluid pressures.

New inclusion data from Navan brown sphalerite confirm the importance of a relatively high temperature, moderate salinity (187–220°C, 15–18 wt% NaCl equivalent) mineralizing fluid as observed at Tynagh, Silvermines, Lisheen and in the Lower Palaeozoic basement. In these inclusions, gas clathrate has been identified. This is the first reported occurrence of this phase in fluid inclusions from the Irish base metal deposits.

These data provide convincing evidence for the regional flow of high temperature fluids, similar to the Lower Carboniferous ore-forming fluids, within the Lower Palaeozoic metasedimentary basement. Systematic regional variations in homogenization temperature-salinity characteristics imply variable infiltration of local, low temperature, surface-derived fluids, consistent with a density driven convection model. In combination with existing isotopic data and thermal constraints, the new data strongly suggest that deep circulation of fluids was a fundamental process in the genesis of the Irish deposits. Therefore, purely topographically driven fluid flow models solely utilizing the Old Red Sandstone as a regional aquifer are inappropriate for the Irish orefield.
Two-dimensional modelling of stratigraphy and compaction-driven fluid flow in the Pannonian Basin
Authors: Van Balen R. T. , Lenkey L. , Horváth F. , Cloetingh S. A. P. L.
Published: April 2008

During the Pliocene-Quaternary time interval, the peripheral parts of the Pannonian Basin system have been uplifted and subsidence in the basin centre accelerated, causing a distinctive truncation pattern in the basin stratigraphy. Stress analyses indicate that the Pannonian Basin system, originally formed in an extensional regime, is subjected to a compressive stress since the early Pliocene. Results of forward modelling of basin subsidence and sedimentary filling along a cross-section through the southern part of the Pannonian Basin demonstrate that a change of the basin shape due to the compressive stress can successfully explain the observed pattern of differential uplift and subsidence occurring since the early Pliocene. In addition, the forward modelling of subsidence and fill provides constraints for the depth of lithospheric necking during extension, the palaeowater-depth history and lake-level changes in the southern part of the Pannonian Basin.

Compaction-driven fluid flow modelling shows that the first significant overpressures in the southern part of the Pannonian Basin developed during progradation of a large deltaic system, at a time when sedimentation rates increased rapidly. Due to the stress-induced acceleration of subsidence during Pliocene to Quaternary times, sedimentation rates increased again, causing a further increase of overpressure. The Pliocene stress induced uplift of the basin flanks combined with a preceding lake-level fall created a larger gravity potential of the groundwater table, enhancing the influx of meteoric water into the basin. This can explain observed diagenetic patterns in the southern part of the Pannonian Basin.
Ductile extension and the formation of the Aegean Sea
Authors: Jolivet L. , Patriat M.
Published: April 2008

The Aegean Sea and Tyrrhenian sea areas offer almost unique examples of actively collapsing orogens where the internal velocity field, kinematic boundary conditions and seismic activity can be compared and where older structures formed during the early stages of the same process are exhumed. We study these processes along two major transects. The first runs from Mt Olympos to the centre of the Cyclades, it is perpendicular to the direction of extension and offers the opportunity to study the brittle-ductile transition by direct observation of rocks and deformation. A gradient of finite exhumation along the transect has brought various crustal levels to the surface, and the localization of deformation during exhumation along major detachments has preserved older penetrative structures formed below. As these outcrops are distributed along the strike of a single crustal-scale tilted block the observed structures probably corresponds to a single tectonic event, and we can sample the associated deformation at increasing depth along the transect. We propose a stratification of deformation regimes associated to extension: localized normal faults in the brittle crust, distributed extensional shear bands with a partition between domains of noncoaxial and coaxial flow below 8–10 km, and distributed coaxial flow below 20 km. Flat-lying extensional shear bands extend brittle normal faults within the brittle-ductile transition. The second transect runs from Crete to Naxos and Mykonos and is parallel to the main direction of extension. It shows the transition from tectonic accretion and syn-orogenic extension near the subduction front to post-orogenic extension in the backarc region. The exhumation of metamorphic rocks proceeds in two stages: synorogenic ‘cold’ exhumation along detachments in the upper part of the accretionary complex, with a good preservation of HP-LT parageneses, and post-orogenic exhumation in a warmer environment in the backarc region with a complete retrogression of HP-LT parageneses. The 3D finite strain field is then compared to recent space geodesy data and the mechanism of crustal collapse discussed.
Tethyan sutures of northern Turkey
Authors: Okay Aral I. , Tüysüz Okan
Published: April 2008

The two main Tethyan sutures of Turkey, the İzmir-Ankara-Erzincan and the Intra-Pontide sutures, are reviewed through several well-studied transects crossing the suture regions. Both sutures have formed during the Early Tertiary continental collisions following northward subduction of Tethyan oceanic lithosphere. The İzmir-Ankara-Erzincan suture is represented along most of its
c.
2000 km length by Paleocene and younger thrust, which emplace the upper crustal rocks of the northern continent over that of the southern continent with an intervening tectonic layer of Cretaceous subduction-accretion complexes. These thrusts constitutes a profound stratigraphic, structural, magmatic and metamorphic break, of at least Carboniferous to Palaeocene age and form the main boundary between Laurasia and Gondwana in the Turkish transect. Voluminous subduction-accretion complexes of Triassic and Cretaceous ages occur respectively to the north and south of the suture giving the antithetic subduction polarities during these two periods. This, and evidence for a major accretionary orogeny of Late Triassic age north of the İzmir-Ankara-Erzincan suture suggest that two separate oceanic lithospheres, of Carboniferous to Triassic (Palaeo-Tethys) and of Triassic to Cretaceous ages (Neo-Tethys) respectively have been consumed along the suture. The final continental collision along the İzmir-Ankara-Erzincan suture was slightly diachronous and occurred in the earliest Palaeocene to the west and in the Late Palaeocene to the east. The
c.
800 km long Intra-Pontide suture is younger in age and have formed during the Early Eocene and younger continental collisions linked to the opening of the Western Black Sea Basin as an oceanic back-arc basin. At present the North Anatolian Fault, which came into existence in the Late Miocene, follows the course of the older Intra-Pontide suture.
Thermal conductivity of sedimentary rocks: uncertainties in measurement and modelling
Authors: Midttømme Kirsti , Roaldset Elen
Published: April 2008

In spite of the fact that thermal conductivity is a key factor in basin modelling, knowledge regarding the thermal conductivity of sedimentary rocks is scarce. In particular, hardly any information exists about claystones and mudstones which can make up 70–80% of a sedimentary basin. For the Upper Jurassic sediments of a section across the northern North Sea, basin modelling programs, using the geometric mean model, gave a deviation of 50°C for the present-day temperature, simply because the matrix conductivity was estimated by two different models. These uncertainties in the determination of thermal conductivities are partly due to problems in measuring, and partly to difficulties in modelling the thermal conductivity. Future work should concentrate on comparative studies and tests on all types of sedimentary rocks, directed towards improved measurement methods. This is of particular importance for clays, mudstones and shales, as measurements on these are associated with the greatest uncertainties. Standardized procedures that include detailed descriptions of sampling, sample preparation and measurement techniques are important to ensure that the thermal conductivity measured is unaffected by factors related to the measurement method. In thermal conductivity modelling it is our experience that the influence of sediment texture is underestimated by most models currently applied. Estimates based on the geometric mean model seem to be most reliable when the determination of matrix conductivity is restricted to the sediment type, mineralogy and texture of the sediments.
Principal aspects of compaction and fluid flow in mudstones
Authors: Bjørlykke K.
Published: April 2008

Compaction and fluid flow in sedimentary basins are usually modelled as mechanical compaction assuming that the main driving force is the effective stress. However, in the deeper part of sedimentary basins (> 2–3 km, 70–100°C), compaction also involves dissolution and precipitation of minerals, and these processes are strongly controlled by temperature and to a lesser extent by variations in effective stress. The total pore-water flux is relatively independent of the permeability and pressure gradients because it is controlled by the rate of compaction. The main effect of permeability variations is the focusing of the compaction-driven flow. Fluids are also released by petroleum generation and dehydration of clay minerals; both these processes are also essentially temperature dependent. If the minimum permeability in one sedimentary layer (seal) falls below a critical value and there is little lateral drainage, the pressure will build up to fracture pressure. Pore pressures and fluid flow cannot be calculated from the matrix permeability of the rock when fracture pressures are reached, because much of the flow occurs along the generated fractures. The permeability produced by fracturing is a dynamic variable adjusting itself to the flux; it is not a rock property like the intergranular permeability. The intergranular permeability of shale samples before fracturing is also difficult to measure reliably in the laboratory as a result of unloading. This makes pressure prediction using one-dimensional pressure modelling uncertain. The prediction of permeability distributions in three dimensions is much more difficult.
In sight of the suture: the early Palaeozoic geological history of the Isle of Man
Authors: Woodcock N. H. , Quirk D. G. , Fitches W. R. , Barnes R. P.
Published: April 2008

The pre- and syn-Caledonian rocks of the Isle of Man are now known to comprise three distinct units: the early Ordovician Manx Group, the mid-Silurian Dalby Group and the ?late Silurian-early Devonian Peel Sandstones.

The Manx Group is dominated by Arenig deep-marine turbidites and debrites deposited in oxygenated basins on the northwest-facing margin of Avalonia. Its organization into a sand-rich lower part and a mud-rich upper part invites comparison with the Skiddaw Group (Lake District) and Ribband Group (Leinster) and points to control by margin-wide events, in part eustatic sea-level changes. Episodes of mass-wasting and Fe-Mn fluid exhalation also correlate along the margin. A mid-late Ordovician volcanic arc is missing above the Manx Group, although parts of its intrusive substructure may be preserved.

The Dalby Group comprises northwest-derived turbidites, sedimented into an anoxic basin during Wenlock (mid-Silurian) time. These turbidites were deposited in a successor basin above the Iapetus suture zone. The Dalby Group sits with a tectonic contact on the Manx Group.

No evidence has been found of a pre-Silurian cleavage. The main Caledonian D1 and D2 shortening phases are post-Wenlock, comparable in age with those further along the margin in the Lake District and Leinster. The Peel Sandstones preserve a Lower ‘Old Red Sandstone’ sequence, mostly removed by post-Caledonian erosion elsewhere along this outboard part of the Avalonian margin. The unit does not host a definite Caledonian cleavage, and it must have been deposited late in the deformation history. The granitic intrusions into the Manx Group range from early in D1 to late in D2. The intrusions generate only local aureoles, and the high metamorphic grade in parts of the Manx Group may be enhanced by favourable protolith compositions.