GEO4210
Introd Intr oduc ucti tio on to Pet Petro role leum um Geol Ge olog ogy y an and d Geo Geoph phy ysi sics cs Geophys hysical Methods in Hydr Hydroc ocar arbo bon n Expl Explor orat atio ion n
About this part of the course • Purpose: to give an overview of the basic geop ge ophy hysi sica call me meth thod ods s used used in hydr hydroc ocar arbo bon n exploration • Working Plan: – Lecture: Prin – Lecture: Princi cipl ples es + Int Intro ro to E Ex xerci ercise se – Practical: – Practical: Seis Seismi mic c In Inte terp rpre reta tati tion on exce excerc rcis ise e
Lect ctur ure e Contents • Geophysical Methods • Theory / Principles • Extensional Sedimentary Basins and it its Seis Seismi mic c Signa ignatu turre • Introduction to the Exercise
Geophysical methods • Passive: Method using the natural fie fields of the Earth, e.g. gravi avity and magnetic
• Active: Met etho hod d tha hatt req equi uirres the inp npu ut of artific ficial ially ge gene nerrat ate ed energy energy,, e.g. e.g. seismic seismic reflec reflection tion
• 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 content
Geophysical methods Method
Measured parameter
“Operative” physical property
Gravity
Spatial variations in the strength of the gravitational field of the Earth
Density
Magnetic
Spatial variations in the strength of the geomagnetic field
Magnetic susceptibility and remanence
Electromagnetic Response to Electric (SeaBed electromagnetic radiation conductivity/resistivity Logging) and inductance Seismic
Travel times of reflected/refracted seismic waves
Seismic velocity (and density)
Further reading • Kear Keary y, P P.. & Broo Brooks ks,, M M.. (19 (1991 91)) An An Int Intro rod ducti uction on to Geophy Geophysi sical cal Explo Explorat ration ion.. Black Blackwe wellll Scient Scientifi ific c Publications. • Mussett, A. A.E. & Khan, M. M. (2 (2000) Lo Looking into the Earth – An Int Introd roducti uction on to Geologi Geological cal Geophys Geophysics ics.. Cambridg Cambridge e Univ Un iver ersi sity ty Pres Press. s. • McQu McQuilillilin, n, R., R., Bac Bacon on,, M. M. & Bar Barcl clay ay,, W. (198 (1984) 4) An An Intr In trod oduc ucti tion on to Seis Seismi mic c In Intterpr erpret eta ation tion – Re Refl flec ecti tion on Seismic Seismics s in Petrole Petroleum um Explora Exploration tion.. Graham Graham & Trotman Trotman.. • Badl Badley ey,, M.E M.E.. (198 (1985) 5) Prac Practi tica call Seis Seismi mic c In Intterpr erpret etat atio ion n. D D.. Reid Re idel el Publ Publis ishi hing ng Co Comp mpan any y. http://www.learninggeoscience.net/modules.php
Gravity • Gravity surveying measures spatial variations in the Earth’s gravitational field caused by diff differ eren ence ces s in th the e density of sub-surface rocks • In fact, it measures the variation in the accelaration due to gravity • It is expressed in so called gravity anomalies (in milligal, 10-5 ms-2), i.e. i.e. devia deviatio tions ns from from a pred predef efin ined ed refe refere renc nce e lev level, el, geoid (a sur surfa face ce over over which the gravitational field has eq equ ual value lue) • Gravity is a scalar
Gravity • Newton’s Universal Law of Grav Gravit itat atio ion n fo forr sma mallll masse masses s at th the e ea eart rth h surface:
F = – – – –
G × M × m R
2
• Spherical • Non-rotating • Homogeneous
= mg → g =
G = 6.67x10 -11 m3kg-1s-2 R iis s tth he Earth’s radius M is the mass of the Earth m is the mass of a small mass
G × M R
2
g is constant!
Gravity • Non-spherical Elli Ellips pse e of rota rotati tion on • Rotating Cent Ce ntrrifug ifugal al fo forrces ces • Non-homogeneous Subsurface heterogeneities
Distu isturb rba ances ces in th the acce ccelerati ratio on
N
Ellipse of rotation
Earth surface continent
Ellipse of rotation
Geoid ocean Geoid Geoid = main main seasea-lev level el Sphere Geoid gav = 9.81 m/s2 gmax = 9.83 m/s2 (pole) gmin = 9.78 m/s2 (equator)
Anomaly
NGU, 1992
Magnetics • Magn Magnet etic ic sur surve veyi ying ng aim aims s to inv inves esti tiga gate te the the sub subsu surf rfac ace e geology by measuring the strength or intensity of the Earth’s magnetic field. • Lateral variation in magnetic susceptibility and remanence give give rise rise to spat spatia iall var varia iati tion ons s in th the e ma magn gnet etic ic fie field • It is ex expressed in so called magnetic anomalies , i.e. deviations from the Earth’s magnetic field. • The unit of measurement is the tesla (T) which hich is volts volts·s· ·s·m m-2 In ma magn gnet etic ic surv survey eyin ing g th the e nanotesla is used (1nT = 10-9 T) • The magnetic field is a vector • Na Natu tura rall magne magneti tic c ele eleme ment nts: s: iron iron,, coba cobalt lt,, nic nicke kel, l, ga gado dolilini nium um • Ferr Ferrom omag agne neti tic c miner mineral als: s: magne magneti tite te,, ilmen ilmenit ite, e, he hema mati tite te,, pyrrhotite
Magnetics • Magnetic susceptibility, k a dimensionless property which in ess essence nce is a measure of how susceptible a material is to becoming magnetized
• Sedimentary Rocks – Lime Limest ston one: e: 10-2 10-25. 5.00 000 0 – Sand Sands sto tone ne:: 0-21 0-21.0 .000 00 – Shal Shale: e: 60-1 60-18 8.6 .600 00
• Igneous Rocks – Grani ranite te:: 10 10-6 -65 5 – Perido Peridoti tite: te: 95 95.50 .500-1 0-196. 96.000 000
• Minerals – Qua uarrtz: -15 – Ma Magn gneti etite: te: 70 70.0 .00000-2x 2x10 107
Magnetics • Magnetic Force, H • Intensity of induced magnetization, Ji • Ji = k · H • Induced and remanent magnetization H
• Magnetic anomaly = regi region onal al - resi residu dual al
J i
J res
J r
NGU, 1992
Electromagnetics Elect Electrom romag agne neti tic c me meth thod ods s use the response of the groun round d to th the e prop propag agat atio ion n of inci incide den nt alt alterna ernati ting ng elect electrom romag agne neti tic c waves, waves, made up of two orth orthog ogon onal al vecto ectorr components, an electrical inte intens nsit ity y (E) (E) and and a mag ma gne neti tiz zing ing fo forc rce e (H) (H) in a plane plane perp perpend endic icula ularr to the direction of travel
Electromagnetics Transmitter
Prima Primary ry field field
Primary field
Receiver
Secondary field
Conductor
Elec El ecttrom omag agne nettic ano nom mal aly y = Pr Priima marry Fie ielld – Sec econ onda darry Fiel eld d
Electromagnetics – Sea Bed Logging SBL is a marine electromagnetic method that has the ability to map the subsurface subsurface resistiv resistivity ity remotely remotely from the seafloor. seafloor. The basis of SBL is the use of a mobile horizontal electric dipole (HED) source transmitting a low frequency electromagnetic electromagnetic signal and and an array array of seafloor electric field receivers. A hydrocarbon filled reservoir will typically have high resistivity resistivity compared with with shale and a water filled reservoirs. SBL therefore therefore has the unique unique potential potential of distinguishing distinguishing between between a hydrocarbon filled and a water filled reservoir
Refl Re fle ect ctio ion n Se Seis ismo molo logy gy Marine multichannel seismic reflection data
Refl Re fle ect ctio ion n Se Seis ismo molo logy gy
Refl Re fle ect ctio ion n Se Seis ismo molo logy gy
Refl Re fle ect ctio ion n Se Seis ismo molo logy gy Inci Incide dent nt ray ray Amplitude: A0
Reflect Refle cted ed ray Amplitude: A1
Layer 1
ρ1,
v1
Layer 2
ρ2,
v2
ρ2,
v2
≠ ρ1,
Acou Ac oust stic ic Imp Imped edan ance ce:: Z = ρ·v Reflect Refl ection ion Coef Coeffici ficient ent:: R = A1 /A0 R =
v1
2
v2 − ρ 1v1
ρ 2 v2 + ρ 1v1
=
Z 2 − Z 1 Z 2 + Z 1
Transmis Tran smissio sion n Coe Coeffic fficien ientt: T = A2 /A0
Trans Transmit mitte ted d ray ray Amplitude: A2
T =
2
v
1 1
ρ 2 v2 + ρ 1v1
-1 ≤ R ≤ 1 R=0 Alll incid Al inciden entt en ener ergy gy tr tra ans nsmi mitt tted ed (Z1=Z2) no re refl flec ecti tion on R = -1 or +1 Alll incid Al incident ent en ener ergy gy re refle flecte cted d strong str ong re refle flecti ction on R<0 Phase Pha se cha change nge (18 (180° 0°) in ref reflec lected ted wa wave ve
Refl Re fle ect ctio ion n Se Seis ismo molo logy gy • Shotpoint interval 60 seconds • 25-120 re receivers • Sampling rate 4 milliseconds • Normal seismic line ca. 8 sTWT
Refl Re fle ect ctio ion n Se Seis ismo molo logy gy
Sed ediimen enttar ary y Bas asiins • Hydrocarbon provinces are found in sedimentary basins • Important to kn know ho how basins are formed • Basin Analysis – Hydrocarbon traps – Strat ratigrap raphy of • Source rock • Reservoir rock • Cap rock
– Maturation of source rocks – Migration path-way -ways s
Exten ensi sio onal Sediment nta ary Basi sin ns • • •
Offshore Norway – Viking Graben, Central Graben Late Jurassic – Early Cretaceous Mature Hydrocarbon Province
Basi sin n Analysi sis s PRE-RIFT
SYN-RIFT
POST-RIFT
Syn-Rift Rotated Fault Blocks
Increasing Fault Displacement
Seismic Signature of Extensional Sedi Se dime ment ntar ary y Ba Basi sins ns
INTRODUCTION TO EXERCISE
Seismic Signature of Extensional Sedimentary Ba Bas sins – Offshore Norw rwa ay
Strat St ratig igrap raphy hy – Of Offs fshor hore e Norw Norway ay
Summ Su mmar ary y Of Offs fsho hore re Norw Norwa ay • Main Rifting Event: Late-Jurassic – Early Cretaceous • Structural Traps – Fault bounded • Main Re Reservoir: Upper Triassic – Middle Jurassic Jurassic,, containi containing ng Tarber Tarbert, t, Ness, Ness, Rannoch, Cook, Statfjord and Lunde Fms. • Source Rock: Up Upper Jurassic, Heather Fm • Cap Rock: Early Cretaceous
Exercise • Interprete seismic line NVGTI92-105 • Interprete pre-, syn- and post-rift sequences • Interprete possible hydrocarbon traps • Point out source-, reservoir, and cap-rock
