DOCSLIB.ORG

Fault Characterization by Seismic Attributes and Geomechanics in a Thamama Oil Field, United Arab Emirates

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Fault Characterization by Seismic Attributes and Geomechanics in a Thamama Oil Field, United Arab Emirates GeoArabia, Vol. 9, No. 2, 2004 Gulf PetroLink, Bahrain Fault characterization, Thamama oil field, UAE Fault characterization by seismic attributes and geomechanics in a Thamama oil field, United Arab Emirates Yoshihiko Tamura, Futoshi Tsuneyama, Hitoshi Okamura and Keiichi Furuya ABSTRACT Faults and fractures were interpreted using attributes that were extracted from a 3-D seismic data set recorded over a Lower Cretaceous Thamama oil field in offshore Abu Dhabi, United Arab Emirates. The Thamama reservoir has good matrix porosity (frequently exceeding 20%), but poor permeability (averaging 15 mD). Because of the low permeability, faults and fractures play an important role in fluid movement in the reservoir. The combination of the similarity and dip attributes gave clear images of small-displacement fault geometry, and the orientation of subseismic faults and fractures. The study better defined faults and fractures and improved geomechanical interpretations, thus reducing the uncertainty in the preferred fluid-flow direction. Two fault systems were recognized: (1) the main NW-trending fault system with mapped fault-length often exceeding 5 km; and (2) a secondary NNE-trending system with shorter faults. The secondary system is parallel to the long axis of the elliptical domal structure of the field. Some of the main faults appear to be composed of en- echelon segments with displacement transfer between the overlapping normal faults (relay faults with relay ramps). The fault systems recognized from the seismic attributes were correlated with well data and core observations. About 13 percent of the fractures seen in cores are non-mineralized. The development of the fault systems was studied by means of clay modeling, computer simulation, and a regional tectonics review. The existing fluid-flow characteristics of individual faults and fractures in the field can be modeled using the present-day stress regime, with the maximum horizontal stress oriented north-northeast. Slip-tendency and dilation-tendency analyses simulating present-day regional stress conditions are indicators of fault and fracture transmissibility. The NNE-striking secondary fault system is parallel to the present- day maximum horizontal stress and could act as a flow conduit in the reservoir. INTRODUCTION Present-day stress regimes acting on fault and fracture systems can control where fluid flows within hydrocarbon reservoirs (e.g. Heffer and Dowokpor, 1990). The objective of this study is to accurately map the fault and fracture systems that affect fluid flow in the Thamama reservoir of an offshore United Arab Emirates oil field (Figure 1). Fault and fracture systems occur at many spatial scales, and are grouped here into: (1) seismic scale, (2) subseismic scale, and (3) micro scale. Seismic-scale faults are routinely imaged with 2-D and 3-D seismic data. Recent improvements, particularly in 3-D seismic acquisition and advanced processing techniques, provide clear images of subtle structural features and facies distribution in hydrocarbon reservoirs. By utilizing a variety of attributes extracted from 3-D seismic data (e.g. dip, similarity, curvature), the subsurface can be further analyzed to estimate the geometry of small-displacement faults (Figure 2). Subseismic-scale fault and fracture systems, however, are not possible to image using compressional waves alone. These more subtle systems may be indirectly modeled by using outcrop analogs, and physical experiments of both clay and sand materials. These analog models are then calibrated to the actual reservoir by using sensitivity analysis of the experimental settings through numerical simulation. This study used an analog-modeling apparatus consisting of two arms to apply regional stress, and a balloon for dome growth in the central part. These experiments were done at Southwest Research Institute in San Antonio, Texas, USA (D.A. Ferrill, D.W. Sims, A.P. Morris and D.J. Waiting, unpublished JNOC Report 2001). 63 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/2/63/4564557/tamura.pdf by guest on 25 September 2021 Tamura et al. Maydan Fateh 52°E54Salman ° Hamidiyah 56° Mahzam Bul Falah Fateh SW Hanine (Sassan) North Dome Sajaa Idd Al-Shargi Nasr Rashid Moveyeid Studied Field Juwaiza South Dome Kahaif Abu Al Bu Danah Dubai Mandous A-Structure North Bukhoosh Khubai Umm 25° Al-Karkara El Shaif Margham Bunduq Belbazem Arabian Gulf QATAR A-Structure South Bu Haseer Umm Umm Al Dholou 25°N Satah Al Salsal Bin Nashef Zakum Jarnain Umm Arzanah Saath Khusub Al Razbooth Mubarraz Al Dalkh Bu Jufair Umm C Abu Dalma Dhabi Al Anbar UNITED ARAB A EMIRATES Ghasha Zal Hair B Umm Al Lulu Dalmah Neewat Al Ghalan Hail Jarn Yaphour Shanayel Shuweihat Al Dabb'iya Rumaitha N Arjan 050 Ruwais Bida Al Qemzan Murban-Bab OMAN km Sahil Shams TURKEY Caspian SAUDI Sea Bu Hasa ARABIA SYRIA Asab Riqeah NDhafra Safah Med Sea IRAN 0 300 Marzuk Huwaila Haliba IRAQ km JORDAN Mushash KUWAIT 23° Arabian L-II-B Bu Qalla L1 G Lekhwair Madiq Gulf Al Barakah BAHRAIN Shah ° QATAR 2 23 Lekhwair E2 Daleel EGYPT Arabian Zarrarah Qusahwira Mender UAE Shield 1 OMAN 3 Mezoon Dhulaima 52°SAUDI ARABIA 54° Shaybah Red SUDAN Sea YEMEN Figure 1: Location map of the studied field in offshore United Arab ERITREA Arabian Sea Emirates. ETHIOPIA Gulf of Aden Micro-scale fractures are usually analyzed using WORK FLOW cores and Borehole Image Logs. In our study they were analyzed mainly from core samples (vertical Fault Geometry Interpretation and deviated wells), since the image quality of 1) Seismic attribute interpretation acquired Borehole Image logs did not allow for - For seismic-scale faults; conventional approach utilizing seismic attributes (dip, similarity, curvature) their detailed analysis. 2) Analog study - For subseismic-scale faults; modeling experiments, GEOLOGICAL SETTING numerical simulation and outcrop observations The oil producing Thamama Group (Figure 3) in the studied oil field consists of multiple layers of Integrated Geological Model chalky carbonate reservoirs with uniformly poor core permeability, averaging 15 mD, associated with relatively high porosity, frequently above Fault Flow Characterization 20 percent. The net-to-gross thickness is estimated 3) Geomechanics interpretation to be approximately 300/600 ft. The matrix - Slip tendency and dilation tendency properties of these reservoirs can be enhanced by 4) Well data interpretation - Fracture observation on core and borehole images a fluid conductive fault/fracture network, and by - Multi well interference tests fracture corridors. The domal structure of the field is likely to be related to a deep-seated salt diapir Figure 2: Work flow summary illustrating the of infra-Cambrian Hormuz Salt (Figure 4). integration of seismic mapped faults with the Previous studies provided evidence that wrench clay/sandbox analog modeling experiments and tectonics played an important role in the structural geomechanical interpretations. evolution in the region (Marzouk and El Sattar, 64 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/9/2/63/4564557/tamura.pdf by guest on 25 September 2021 Fault characterization, Thamama oil field, UAE UNITED ARAB EMIRATES HYDROCARBON HABITAT AND STRATIGRAPHY Seq. Central and Strati- Western Northeast Southeast Shows graphy (Sharland System Group et al., 2001) Limestone Simsima Fiqa Oolitic grainstone Upper Aruma Halul Cretaceous Laffan Dolomite Ruwaidha Tuwayil Mishrif Gypsum, anhydrite Shilaif Mishrif AP8 Sandstone Wasia Middle Mauddud Cretaceous Nahr Umr Shale Shu'aiba Bab Member Source rock Kharaib Aquifer Lekhwair Lower Lower Gas Thamama Cretaceous Habshan MFS J110 Manifa Oil Asab Hith Oolite 149 Ma Minor oil Arab ABC Arab-D (Lower) Arab-D Upper Hanifa MFS J70 Diyab Upper Jurassic Tuwaiq Mountain Lst. Hadriya Upper Araej Sila AP7 Lower Araej Uwainat Figure 3: Jurassic Izhara and Cretaceous stratigraphy of Jurassic Hamlah the United Arab Lower/Middle Marrat Emirates. 1995, Figure 4). At least two distinct tectonic events with different compressive stress directions are interpreted from the features of the Arabian Plate. One is the EW-directed ‘Oman stress’ associated with the Oman Mountain nappes and ophiolite obduction during the Mesozoic. The other is the NNE-directed Cenozoic ‘Zagros stress’. These different stress events reactivated pre-existing basement fault networks, and triggered salt swelling of the infra-Cambrian salt. The salt activation resulted in the structural growth of anticlines above the salt pillows and salt domes in the region. Rapid structural growth at the end of the Middle Cretaceous under the ‘Oman stress’ regime is one of the key factors for the structural doming and faulting of the studied field. FAULT GEOMETRY INTERPRETATION In order to investigate seismic-scale fault geometry, the following seismic attributes were extracted from single 3-D seismic data. (1) Time Dip; (2) Azimuth; (3) Edge Enhancement; (4) Instantaneous Phase; and (5) Curvature (Figure 5, Curvature not illustrated). In this particular oil field, the Time Dip-processed image gives the best visual fault enhancement. The faults were mapped using time slices in each attribute cube every 4 or 8 milliseconds. Lineations observed on time slices were interpreted as seismic-scale faults after being confirmed by other attributes and by vertical seismic cross-sections. Figure 6 shows an example of fault interpretation on a Time Dip attribute image. Two fault systems are present in this field. The main NW-trending normal faults are commonly
Load more

Recommended publications
  • THE GROWTH of SHEEP MOUNTAIN ANTICLINE: COMPARISON of FIELD DATA and NUMERICAL MODELS Nicolas Bellahsen and Patricia E
    THE GROWTH OF SHEEP MOUNTAIN ANTICLINE: COMPARISON OF FIELD DATA AND NUMERICAL MODELS Nicolas Bellahsen and Patricia E. Fiore Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 e-mail: [email protected] be explained by this deformed basement cover interface Abstract and does not require that the underlying fault to be listric. In his kinematic model of a basement involved We study the vertical, compression parallel joint compressive structure, Narr (1994) assumes that the set that formed at Sheep Mountain Anticline during the basement can undergo significant deformations. Casas early Laramide orogeny, prior to the associated folding et al. (2003), in their analysis of field data, show that a event. Field data indicate that this joint set has a basement thrust sheet can undergo a significant heterogeneous distribution over the fold. It is much less penetrative deformation, as it passes over a flat-ramp numerous in the forelimb than in the hinge and geometry (fault-bend fold). Bump (2003) also discussed backlimb, and in fact is absent in many of the forelimb how, in several cases, the basement rocks must be field measurement sites. Using 3D elastic numerical deformed by the fault-propagation fold process. models, we show that early slip along an underlying It is noteworthy that basement deformation often is thrust fault would have locally perturbed the neglected in kinematic (Erslev, 1991; McConnell, surrounding stress field, inducing a compression that 1994), analogue (Sanford, 1959; Friedman et al., 1980), would inhibit joint formation above the fault tip. and numerical models. This can be attributed partially Relating the absence of joints in the forelimb to this to the fact that an understanding of how internal stress perturbation, we are able to constrain the deformation is delocalized in the basement is lacking.
    [Show full text]
  • Stress Fields Around Dislocations the Crystal Lattice in the Vicinity of a Dislocation Is Distorted (Or Strained)
    Stress Fields Around Dislocations The crystal lattice in the vicinity of a dislocation is distorted (or strained). The stresses that accompanied the strains can be calculated by elasticity theory beginning from a radial distance about 5b, or ~ 15 Å from the axis of the dislocation. The dislocation core is universally ignored in calculating the consequences of the stresses around dislocations. The stress field around a dislocation is responsible for several important interactions with the environment. These include: 1. An applied shear stress on the slip plane exerts a force on the dislocation line, which responds by moving or changing shape. 2. Interaction of the stress fields of dislocations in close proximity to one another results in forces on both which are either repulsive or attractive. 3. Edge dislocations attract and collect interstitial impurity atoms dispersed in the lattice. This phenomenon is especially important for carbon in iron alloys. Screw Dislocation Assume that the material is an elastic continuous and a perfect crystal of cylindrical shape of length L and radius r. Now, introduce a screw dislocation along AB. The Burger’s vector is parallel to the dislocation line ζ . Now let us, unwrap the surface of the cylinder into the plane of the paper b A 2πr GL b γ = = tanθ 2πr G bG τ = Gγ = B 2πr 2 Then, the strain energy per unit volume is: τ× γ b G Strain energy = = 2π 82r 2 We have identified the strain at any point with cylindrical coordinates (r,θ,z) τ τZθ θZ B r B θ r θ Slip plane z Slip plane A z G A b G τ=G γ = The elastic energy associated with an element is its θZ 2πr energy per unit volume times its volume.
    [Show full text]
  • Evidence for Controlled Deformation During Laramide Orogeny
    Geologic structure of the northern margin of the Chihuahua trough 43 BOLETÍN DE LA SOCIEDAD GEOLÓGICA MEXICANA D GEOL DA Ó VOLUMEN 60, NÚM. 1, 2008, P. 43-69 E G I I C C O A S 1904 M 2004 . C EX . ICANA A C i e n A ñ o s Geologic structure of the northern margin of the Chihuahua trough: Evidence for controlled deformation during Laramide Orogeny Dana Carciumaru1,*, Roberto Ortega2 1 Orbis Consultores en Geología y Geofísica, Mexico, D.F, Mexico. 2 Centro de Investigación Científi ca y de Educación Superior de Ensenada (CICESE) Unidad La Paz, Mirafl ores 334, Fracc.Bella Vista, La Paz, BCS, 23050, Mexico. *[email protected] Abstract In this article we studied the northern part of the Laramide foreland of the Chihuahua Trough. The purpose of this work is twofold; fi rst we studied whether the deformation involves or not the basement along crustal faults (thin- or thick- skinned deformation), and second, we studied the nature of the principal shortening directions in the Chihuahua Trough. In this region, style of deformation changes from motion on moderate to low angle thrust and reverse faults within the interior of the basin to basement involved reverse faulting on the adjacent platform. Shortening directions estimated from the geometry of folds and faults and inversion of fault slip data indicate that both basement involved structures and faults within the basin record a similar Laramide deformation style. Map scale relationships indicate that motion on high angle basement involved thrusts post dates low angle thrusting. This is consistent with the two sets of faults forming during a single progressive deformation with in - sequence - thrusting migrating out of the basin onto the platform.
    [Show full text]
  • 24. Structure and Tectonic Stresses in Metamorphic Basement, Site 976, Alboran Sea1
    Zahn, R., Comas, M.C., and Klaus, A. (Eds.), 1999 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 161 24. STRUCTURE AND TECTONIC STRESSES IN METAMORPHIC BASEMENT, SITE 976, ALBORAN SEA1 François Dominique de Larouzière,2,3 Philippe A. Pezard,2,4 Maria C. Comas,5 Bernard Célérier,6 and Christophe Vergniault2 ABSTRACT A complete set of downhole measurements, including Formation MicroScanner (FMS) high-resolution electrical images and BoreHole TeleViewer (BHTV) acoustic images of the borehole wall were recorded for the metamorphic basement section penetrated in Hole 976B during Ocean Drilling Program Leg 161. Because of the poor core recovery in basement (under 20%), the data and images obtained in Hole 976B are essential to understand the structural and tectonic context wherein this basement hole was drilled. The downhole measurements and high-resolution images are analyzed here in terms of structure and dynamics of the penetrated section. Electrical resistivity and neutron porosity measurements show a generally fractured and consequently porous basement. The basement nature can be determined on the basis of recovered sections from the natural radioactivity and photoelectric fac- tor. Individual fractures are identified and mapped from FMS electrical images, providing both the geometry and distribution of plane features cut by the hole. The fracture density increases in sections interpreted as faulted intervals from standard logs and hole-size measurements. Such intensively fractured sections are more common in the upper 120 m of basement. While shallow gneissic foliations tend to dip to the west, steep fractures are mostly east dipping throughout the penetrated section. Hole ellipticity is rare and appears to be mostly drilling-related and associated with changes in hole trajectory in the upper basement schists.
    [Show full text]
  • Scale Brittle Rock Structures and the Estimation of “Paleostress” Axes – a Case Study from the Koralm Region (Styria/Carinthia)______
    Austrian Journal of Earth Sciences Volume 107/2 Vienna 2014 Small- to meso - scale brittle rock structures and the estimation of “paleostress” axes – A case study from the Koralm region (Styria/Carinthia)______________________ Franz-Josef BROSCH1) & Gerald PISCHINGER1)2)*) KEYWORDS paleostress analysis brittle tectonics Koralm Tunnel 1) Institute of Applied Geosciences, TU Graz, Rechbauerstraße 12, A-8010 Graz, Austria; Florianer beds 2) Geoconsult ZT GmbH, Hölzlstraße 5, A-5071 Wals bei Salzburg, Austria; Eastern Alps Badenian *) Corresponding author, [email protected] Koralpe Abstract Mapping of fault slip data and the consecutive estimation of paleostress orientations are routine procedures during structural geo- logical investigation. In addition, there are various other small- to mesoscale features of brittle deformation which are frequently nei- ther mapped nor routinely used for paleostress analysis. In this paper we present such data, i.e. conjugate shear fractures, faulting related secondary fractures, tensile/extensional fractures, veins, low angle shears and listric faults, from the Koralpe, Eastern Alps. Paleostress orientations are determined with the help of stereographic projection techniques and compared to the results of published fault slip data and fault slip data gathered during tunnelling of the Koralm Tunnel in the Badenian Florianer beds of the western Sty- rian Basin. The analysed data yield paleostress axes which may be attributed to the three Andersonian fault types with similar ori- entations as the ones achieved by fault slip analysis. Yet, a reliable (relative) timing of the different tectonic events is not possible on the basis of the structures analysed and time constraints from the surrounding basins are limited to Ottnangium till Sarmatium.
    [Show full text]
  • Advanced Stress Analysis
    City University of New York (CUNY) CUNY Academic Works Open Educational Resources City College of New York 2018 Advanced Stress Analysis Benjamin Liaw CUNY City College How does access to this work benefit ou?y Let us know! More information about this work at: https://academicworks.cuny.edu/cc_oers/83 Discover additional works at: https://academicworks.cuny.edu This work is made publicly available by the City University of New York (CUNY). Contact: [email protected] FALL 2018 SYLLABUS Page 1/1 ME 54100: ADVANCED STRESS ANALYSIS Courses: ME I4200: APPLIED STRESS ANALYSIS Time & Tuesday & Thursday, 11:00 a.m. – 12:15 p.m. Place: Steinman Hall, Basement ST-B64 (Materials Science Lab) Course Stress and strain. Principal stresses & directions. Generalized Hooke's Law (constitutive relations) Description: for elastic materials. Plane-stress/plane strain formulations in Cartesian/polar coordinates. Failure criteria. Bending of straight & curved beams. Torsion of shafts. Thick tubes, rotating disks, shrink fits. Thermal stresses in rings, tubes, and disks. Energy methods in structural mechanics. Applications of finite element methods in stress analysis. Prerequisites: ME 24700: Engineering Mechanics II (Kinematics and Dynamics of Rigid Bodies) ME 33000: Mechanics of Materials ME 37100: Computer-Aided Design Instructor: Prof. Benjamin Liaw E-mail: [email protected] Office: Steinman Hall, Room ST-247 Tel: (212) 650-5204 Hours: Monday: 4:00 p.m. – 5:00 p.m. Fax: (212) 650-8013 Wednesday: 1:00 p.m. – 2:00 p.m. Textbook: B.M. Liaw, Advanced Stress Analysis, CUNY City College of New York, Open Educational Resources. References: 1. F.P.
    [Show full text]
  • Stress Rotation — the Impact and Interaction of Rock Stiffness and Faults
    https://doi.org/10.5194/se-2020-129 Preprint. Discussion started: 3 August 2020 c Author(s) 2020. CC BY 4.0 License. Stress rotation — The impact and interaction of rock stiffness and faults Karsten Reiter TU Darmstadt, 64287 Darmstadt, Schnittspahnstraße 9 Correspondence: Karsten Reiter ([email protected]) Abstract. It has been assumed, that the maximum compressive horizontal stress (SHmax) orientation in the upper crust is governed on a regional scale by the same forces that drive plate motion. However, several regions are identified, where stress orientation deviates from the expected orientation due to plate boundary forces (first order stress sources), or the plate wide pattern. In some of this regions a gradual rotation of the SHmax orientation has been observed. 5 Several second and third order stress sources have been identified, which may explain stress rotation in the upper crust. For example lateral heterogeneities in the crust, such as density, petrophysical or petrothermal properties and discontinuities, like faults are identified as potential candidates to cause lateral stress rotations. To investigate several of the candidates, generic geomechanical numerical models are utilized. These models consist of up to five different units, oriented by an angle of 60◦ to the direction of contraction. These units have variable elastic material properties, such as Young’s modulus, Poisson ratio and 10 density. Furthermore, the units can be separated by contact surfaces that allow them so slide along these faults, depending on a selected coefficient of friction. The model results indicate, that a density contrast or the variation of the Poisson’s ratio alone sparsely rotates the horizontal stress orientation (517◦).
    [Show full text]
  • Evaluation of Computational Methods of Paleostress Analysis Using Fault
    EVALUATION OF COMPUTATIONAL METHODS OF PALEOSTRESS ANALYSIS USING FAULT-STRIATION DATA Abstract of a thesis presented to the Faculty of the State University of New York at Albany in partial fulfillment of the requirements for the degree of Master of Science College of Science and Mathematics Department of Geological Sciences Steven Henry Schimmrich 1991 ABSTRACT Over the past 12 years, many different computational methods or variations of existing methods have been proposed for determining paleostress tensors from fault populations and their slip directions. These methods are all based upon well-known relationships between stress and shear and use iterative, non-linear mathematical algorithms which seek to minimize the angles between the calculated maximum shear stress direction and the observed movement directions on each fault plane in a population. The solution returned is the best-fit paleostress tensor for the population. By taking the Coulomb failure criterion into account, several paleostress analysis programs have been able to use linear, rather than non-linear, methods to solve for a paleostress tensor. The advantages of using linear equations is that they are less computationally-intensive and are far easier to solve. A major problem with computational methods of paleostress analysis is that very little work has been done on evaluating their effectiveness and/or possible limitations. If the techniques return results consistent with other methods of estimating paleostress directions, or with various kinematic analysis methods, they are often used by geologists. If not, an attempt may be made to explain why, but geological explanations are usually sought rather than criticizing the paleostress analysis methods.
    [Show full text]
  • Focal Mechanisms and Stress Field in the Southern Aegean
    Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Solid Earth Discuss., 5, 1721–1770, 2013 Open Access www.solid-earth-discuss.net/5/1721/2013/ Solid Earth SED doi:10.5194/sed-5-1721-2013 Discussions © Author(s) 2013. CC Attribution 3.0 License. 5, 1721–1770, 2013 This discussion paper is/has been under review for the journal Solid Earth (SE). Focal mechanisms Please refer to the corresponding final paper in SE if available. and stress field in the Southern Aegean Focal mechanisms in the Southern W. Friederich et al. Aegean from temporary seismic networks – implications for the regional stress field Title Page and ongoing deformation processes Abstract Introduction Conclusions References 1 1 3 2 W. Friederich , A. Brüstle , L. Küperkoch , T. Meier , and Egelados Working Tables Figures Group1 1 Ruhr-University Bochum, Institute of Geology, Mineralogy and Geophysics, Bochum, J I Germany 2Christian-Albrechts-University Kiel, Institute of Geosciences, Kiel, Germany J I 3 BESTEC GmbH, Landau, Germany Back Close Received: 12 September 2013 – Accepted: 15 September 2013 – Published: 24 October 2013 Full Screen / Esc Correspondence to: W. Friederich ([email protected]) Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion 1721 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract SED The lateral variation of the stress field in the southern Aegean plate and the subducting Hellenic slab is determined from recordings of seismicity obtained with the CYCNET 5, 1721–1770, 2013 and EGELADOS networks in the years from 2002 to 2007. First motions from 7000 5 well-located earthquakes were analysed to produce 540 well-constrained focal mech- Focal mechanisms anisms.
    [Show full text]
  • Local and Far Field Stress-Analysis of Brittle Deformation in the Western Part of the Northern Calcareous Alps, Austria
    Geol. Paläont. Mitt. Innsbruck, ISSN 0378–6870, Band 26, S. 109–136, 2003 LOCAL AND FAR FIELD STRESS-ANALYSIS OF BRITTLE DEFORMATION IN THE WESTERN PART OF THE NORTHERN CALCAREOUS ALPS, AUSTRIA Hugo Ortner With 25 figures and 1 table Abstract Brittle deformation was analysed in 86 stations with in the western Northern Calcareous Alps (NCA). Six paleostress ten- sor groups were defined, which constrain the deformational history since onset of Alpine deformation and can be related to distinct stages of deformation in other parts of the orogen. The superposition of deformational events with stress ten- sors of the same geometry, but different ages was estblished. The evolution of the far field stresses (1st order stress), which are thought to be related to plate movements, was simple: Cretaceous NW-directed compression was followed by Paleocene/Eocene NNE-directed compression, which in turn was replaced by Oligocene/Miocene NNW-directed compres- sion. Local stress fields (2nd order stress) complicate the picture: Cretaceous NW-compression was partitioned into NNW- directed compression in the NCA and W-directed compression in the Central Alps. Oligocene/Miocene NNW-directed compression was superposed by secondary stress fields related to lateral escape of crustal blocks during formation of the metamorphic core complex of the Tauern Window: N- to NNE-directed compression near large faults delimiting eastward moving blocks and E-directed extension. Third order stress fields are related to folding. The succession of these 3rd order stress fields is not systematic. Additionally, stress tensors related to Late Miocene E-W compression were recorded, which are related to collision in the Carpathian orogen.
    [Show full text]
  • Bach Ho Field, a Fractured Granitic Basement Reservoir, Cuu Long Basin, Offshore Se Vietnam: a “Buried-Hill” Play
    Journal of Petroleum Geology, Vol. 32(2), April 2009, pp 129-156 129 BACH HO FIELD, A FRACTURED GRANITIC BASEMENT RESERVOIR, CUU LONG BASIN, OFFSHORE SE VIETNAM: A “BURIED-HILL” PLAY Trinh Xuan Cuong* and J. K. Warren** A combination of seismic, wireline, FMI and core data shows that Bach Ho field in the Cuu Long Basin, offshore SE Vietnam, is an unusual “buried hill” reservoir. There is little or no production from associated siliciclastic “grus” or granite wash, and the fractured reservoir matrix is largely made up of unaltered acid igneous lithologies (mostly granites and granodiorites). A major NE-SW late Oligocene reverse fault system cross-cuts the field, with about 2000 m of lateral displacement in the highly productive Central Block. The associated fracture meshwork greatly enhances reservoir quality. Transpressional wrench faulting in the late Oligocene in this part of the field emplaced a block of brittle granitic rock on top of organic-rich Eocene – Oligocene mudstones, and facilitated the early migration of hydrocarbons into the fracture network. Structure, not erosion, set up the 1000 m column of liquids in the fractured granodiorites which form the reservoir at Bach Ho. Faulted intervals with associated damage zones create an enhanced secondary porosity system in the granodiorite; effective porosities range from 3-5% and occasionally up to 20%. Some associated fractures are partially blocked by authigenic calcite and kaolinite. Features that degrade reservoir quality at Bach Ho include: (i) a thin, low-permeability clay- plugged “rind” created by surface-related (meteoric) Eocene – Oligocene weathering — this rind variably overprints the uppermost 10-40 m of exposed basement throughout the Cuu Long Basin; and (ii) widespread hydrothermal cements which largely predate late Oligocene wrench faulting; cementation mostly took place during post-magmatic cooling and precipitated zeolites, carbonates and silica in fractures which cut across both the igneous and the country rocks.
    [Show full text]
  • Principal Stresses
    Chapter 4 Principal Stresses Anisotropic stress causes tectonic deformations. Tectonic stress is defined in an early part of this chapter, and natural examples of tectonic stresses are also presented. 4.1 Principal stresses and principal stress axes Because of symmetry (Eq. (3.32)), the stress tensor r has real eigenvalues and mutually perpendicu- lar eigenvectors. The eigenvalues are called the principal stresses of the stress. The principal stresses are numbered conventionally in descending order of magnitude, σ1 ≥ σ2 ≥ σ3, so that they are the maximum, intermediate and minimum principal stresses. Note that these principal stresses indicate the magnitudes of compressional stress. On the other hand, the three quantities S1 ≥ S2 ≥ S3 are the principal stresses of S, so that the quantities indicate the magnitudes of tensile stress. The orientations defined by the eigenvectors are called the principal axes of stress or simply stress axes, and the orientation corresponding to the principal stress, e.g., σ1 is called the σ1-axis (Fig. 4.1). Principal planes of stress are the planes parallel to two of the stress axes, or perpendicular to one of the stress axes. Deformation is driven by the anisotropic state of stress with a large difference of the principal stresses. Accordingly, the differential stress ΔS = S1 − S3, Δσ = σ1 − σ3 is defined to indicate the difference1. Taking the axes of the rectangular Cartesian coordinates O-123 parallel to the stress axes, a stress tensor is expressed by a diagonal matrix. Therefore, Cauchy’s stress (Eq. (3.10)) formula is ⎛ ⎞ ⎛ ⎞ ⎛ ⎞ t1(n) σ1 00 n1 ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ t(n) = t2(n) = 0 σ2 0 n2 .
    [Show full text]