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Introduction to Petroleum Geology and Geophysics

Introduction to Petroleum Geology and Geophysics
GEO4210 Introduction to Petroleum Geology and Geophysics Geophysical Methods in Hydrocarbon ExplorationAbout this part of the course • Purpose: to give an overview of the basic geophysical methods used in hydrocarbon exploration • Working Plan: – Lecture: Principles + Intro to Exercise – Practical: Seismic Interpretation excerciseLecture Contents • Geophysical Methods • Theory / Principles • Extensional Sedimentary Basins and its Seismic Signature • Introduction to the ExerciseGeophysical methods • Passive: Method using the natural fields of the Earth, e.g. gravity and magnetic • Active: Method that requires the input of artificially generated energy, e.g. seismic reflection • The objective of geophysics is to locate or detect the presence of subsurface structures or bodies and determine their size, shape, depth, and physical properties (density, velocity, porosity…) + fluid contentGeophysical methods Method Measured parameter “Operative” physical property Gravity Spatial variations in the Density strength of the gravitational field of the Earth Magnetic Spatial variations in the Magnetic susceptibility strength of the and remanence geomagnetic field Electromagnetic Response to Electric (SeaBed electromagnetic radiation conductivity/resistivity Logging) and inductance Seismic Travel times of Seismic velocity (and reflected/refracted density) seismic wavesFurther reading • Keary, P. Brooks, M. (1991) An Introduction to Geophysical Exploration. Blackwell Scientific Publications. • Mussett, A.E. Khan, M. (2000) Looking into the Earth – An Introduction to Geological Geophysics. Cambridge University Press. • McQuillin, R., Bacon, M. Barclay, W. (1984) An Introduction to Seismic Interpretation – Reflection Seismics in Petroleum Exploration. Graham Trotman. • Badley, M.E. (1985) Practical Seismic Interpretation. D. Reidel Publishing Company. http://www.learninggeoscience.net/modules.phpGravity • Gravity surveying measures spatial variations in the Earth’s gravitational field caused by differences in the density of subsurface rocks • In fact, it measures the variation in the accelaration due to gravity • It is expressed in so called gravity anomalies (in 5 2 milligal, 10 ms ), i.e. deviations from a predefined reference level, geoid (a surface over which the gravitational field has equal value) • Gravity is a scalarGravity • Newton’s Universal Law • Spherical of Gravitation for small • Nonrotating masses at the earth • Homogeneous surface: G × M × m G × M F = = mg → g = 2 2 R R 11 3 1 2 – G = 6.67x10 m kg s – R is the Earth’s radius à g is constant – M is the mass of the Earth – m is the mass of a small massGravity • Nonsphericalà Ellipse of rotation • Rotatingà Centrifugal forces • Nonhomogeneousà Subsurface heterogeneities à Disturbances in the accelerationN Ellipse of Earth surface rotation continent Ellipse of rotation Geoid ocean Geoid = main sealevel Sphere Geoid 2 g = 9.81 m/s av Anomaly 2 g = 9.83 m/s (pole) max 2 g = 9.78 m/s (equator) minNGU, 1992Magnetics • Magnetic surveying aims to investigate the subsurface geology by measuring the strength or intensity of the Earth’s magnetic field. • Lateral variation in magnetic susceptibility and remanence give rise to spatial variations in the magnetic field • It is expressed in so called magnetic anomalies, i.e. deviations from the Earth’s magnetic field. 2 • The unit of measurement is the tesla (T) which is volts·s·m 9 In magnetic surveying the nanotesla is used (1nT = 10 T) • The magnetic field is a vector • Natural magnetic elements: iron, cobalt, nickel, gadolinium • Ferromagnetic minerals: magnetite, ilmenite, hematite, pyrrhotiteMagnetics • Magnetic • Sedimentary Rocks susceptibility, k – Limestone: 1025.000 – Sandstone: 021.000 a dimensionless – Shale: 6018.600 property which in • Igneous Rocks essence is a – Granite: 1065 measure of how – Peridotite: 95.500196.000 susceptible a material is to • Minerals becoming – Quartz: 15 magnetized 7 – Magnetite: 70.0002x10Magnetics • Magnetic Force, H • Intensity of induced magnetization, J i • J = k · H i • Induced and remanent magnetization J i H • Magnetic anomaly = J J r res regional residualNGU, 1992Electromagnetics Electromagnetic methods use the response of the ground to the propagation of incident alternating electromagnetic waves, made up of two orthogonal vector components, an electrical intensity (E) and a magnetizing force (H) in a plane perpendicular to the direction of travelElectromagnetics Primary field Transmitter Receiver Primary field Secondary field Conductor Electromagnetic anomaly = Primary Field – Secondary FieldElectromagnetics – Sea Bed Logging SBL is a marine electromagnetic method that has the ability to map the subsurface resistivity remotely from the seafloor. The basis of SBL is the use of a mobile horizontal electric dipole (HED) source transmitting a low frequency electromagnetic signal and an array of seafloor electric field receivers. A hydrocarbon filled reservoir will typically have high resistivity compared with shale and a water filled reservoirs. SBL therefore has the unique potential of distinguishing between a hydrocarbon filled and a water filled reservoirReflection Seismology Marine multichannel seismic reflection dataReflection SeismologyReflection Seismology Reflection Seismology Acoustic Impedance: Z = ρ·v Incident ray Reflected ray Amplitude: A Amplitude: A 0 1 Reflection Coefficient: R = A /A 1 0 ρ v ρ v Z Z 2 2 1 1 2 1 R = = ρ , v Layer 1 1 1 ρ v + ρ v Z + Z 2 2 1 1 2 1 ρ , v 2 2 Layer 2 Transmission Coefficient: T = A /A 2 0 ρ , v ≠ ρ , v 2 2 1 1 2ρ v 1 1 Transmitted ray T = ρ v + ρ v Amplitude: A 2 2 1 1 2 1 ≤ R ≤ 1 R = 0 à All incident energy transmitted (Z =Z ) à no reflection 1 2 R = 1 or +1 à All incident energy reflectedà strong reflection R 0 à Phase change (180°) in reflected waveReflection Seismology • Shotpoint interval 60 seconds • 25120 receivers • Sampling rate 4 milliseconds • Normal seismic line ca. 8 sTWTReflection SeismologySedimentary Basins • Hydrocarbon provinces are found in sedimentary basins • Important to know how basins are formed • Basin Analysis – Hydrocarbon traps – Stratigraphy of • Source rock • Reservoir rock • Cap rock – Maturation of source rocks – Migration pathwaysExtensional Sedimentary Basins • Offshore Norway – Viking Graben, Central Graben • Late Jurassic – Early Cretaceous • Mature Hydrocarbon ProvinceBasin Analysis PRERIFT SYNRIFT POSTRIFTSynRift Rotated Fault Blocks Increasing Fault DisplacementSeismic Signature of Extensional Sedimentary BasinsINTRODUCTION TO EXERCISESeismic Signature of Extensional Sedimentary Basins – Offshore NorwayStratigraphy – Offshore NorwaySummary Offshore Norway • Main Rifting Event: LateJurassic – Early Cretaceous • Structural Traps – Fault bounded • Main Reservoir: Upper Triassic – Middle Jurassic, containing Tarbert, Ness, Rannoch, Cook, Statfjord and Lunde Fms. • Source Rock: Upper Jurassic, Heather Fm • Cap Rock: Early CretaceousExercise • Interprete seismic line NVGTI92105 • Interprete pre, syn and postrift sequences • Interprete possible hydrocarbon traps • Point out source, reservoir, and caprock
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