Log Interpretation in Non-Hydrocarbon Environments - Methods and Applications -.pdf

ICDP

International Continental Drilling Program

Log Interpretation in Non-Hydrocarbon Environments - Methods and Applications -

Dr. Renate Pechnig Aachen University of Technology

Log data for lithology prediction Enhanced interpetration for lithology reconstruction is required if: information on lithology is available only from cuttings e.g. KTB main hole core recovery is very low and cuttings are not available e.g. ODP hole in oceanic crust (504B) core recovery is high, but information on petrophysical characteristics of the drilled rocks are also required e.g HSDP2, Hawaii

KTB

Examples from the KTB boreholes

Overview KTB boreholes

Motivation for KTB

The KTB main hole has reached a depth of 9101 m. Drilling strategy was targeted to avoid expensive coring. The total core available from the main hole is only about 85 m. In contrast, the KTB pilot hole was completely cored down to 4000 m.

Target

Transfer of log data into lithological information

Strategy

Calibration of log responses in the fully cored 4 km deep KTB pilot hole Transfer of knowledge to the more than 9 km deep main hole and predict lithology from logging data.

Data Compilation and Calibration Selection of calibration intervals Compilation of all available core, cuttings and log data

Comparing of core and log data and classification of electrofacies

Electrofacies definition

Serra (1986) „An electrofacies is a set of log responses which characterizes a rock type and permits it to be distinguished from the others“

Identification of Electrofacies

1) Manual identification by examining the shape of the various log curves and by relating log boundaires to core stratigraphy. 2) Cross-plot techniques to identify and separate the different rock types by their log responses.

Grouping of electrofacies in the pilot hole

Training and transfer to uncored sections Learn stage: Storing the specific information of each electrofacies into a multidimensional data base by using e.g. neural networks, discriminance analysis. Transfer of the electrofacies data base to uncored sections –> level by level lithology prediction. Result: a synthetic lithological profile, the EFA LOG

Example KTB – Paragneisses pilot hole

EFA-Log versus core profile of a paragneiss section in a calibration section in the pilot hole. Core recovery in this depth section is almost 100%.

Example KTB – Metabasites pilot hole

EFA-Log versus core profile of a metabasites section in the pilot hole. Core recovery in this depth section is almost 100%.

Example KTB – Metabasites main hole

EFA - Log constructed from logs in the main hole compared to the cuttings profile. Resolution of the log derived profile is much higher!

ODP

Examples from ODP Hole 504B

Drilling Location of Holes 504B and 896A

American Plate MidAtlanic Ridge

Cocos Plate Costa Rica Rift

Pacific Plate

504B 896A Nazca Plate

Motivation in ODP Hole 504B Need for lithology reconstruction in ODP Hole 504B

504B is the deepest hole drilled in oceanic crust core recovery is extremely low < 20 % lithostratigraphic information from core is not complete

Simplified log responses of pillows and lavaflows

Cross plots: resistivity versus gamma ray

896A

10

10

5

5

0

0 10

100

electrical resistivity (Ωm)

10

total gamma ray (API)

total gamma ray (API)

504B

100

electrical resistivity (Ωm)

massive units thin flows pillow basalts

Cross plots: resistivity versus velocity

896A 7

6

6

5

5

4

4

3

3

2

2 10

100

electrical resistivity (Ωm)

10

VP (km/s)

VP (km/s)

504B 7

100

electrical resistivity (Ωm)

massive units thin flows pillow basalts

Results of cross plot analysis massive units high electrical resistivity high velocity low gamma ray

slightly altered slightly fractured

thin flows intermediate resistivities intermediate velocity intermediate gamma ray

intermediate alteration intermediate fracturing

pillow basalts low electrical resistivities low velocity high gamma ray

highly altered strongly fractured

y e o c

Lithology Reconstruction

) 5 8 9 1 n m d (A y p g ra -s o ith L 2 0

/s k V m h o D )L I(% H P N 6 4 0 2

rto lib )3a m /c (g B O H R

0 1

3 .0 2

0 3

E tly a rm c is d n

g o F -L A

m 1 <

2 0 1 3

) s b

yn l a b p d re tc o

0 3

t( p e d

d e tc n ly a b ro p

5 0 4 3

: d n g e L

LLD (Ωm) 1 500

Core 500

300

550

350

600

400

Depth (mbsf)

Depth (mbsf)

EFA-Log

650

massive units dikes (core only)

450

700

500

750

thin flows pillow basalt

Core recovery

Core 250

Core recovery

EFA-LOG of Hole 504B EFA-Log LLD (Ωm) 1 500

ICDP

Examples from HSDP2, Hawaii

Location Map ‘Big Island’

HSDP - Drilling Location

Drill Hole

http://www.gps.caltech.edu/faculty/stolper/deep_drilling.html

Depth Core (ftbsl) Lithology 0

Core Recovery [%]

Lithology of HSDP2 Depth (mbsl) 0

1000

Final depth: 3110 mbsf

500 2000

3000 1000

4000

5000

1500

Core recovery: 95%

Legend Aa Pahoehoe

6000 2000 7000

8000 2500

9000

10000

L O G G I N G

3000

I N T E R V A L

Transitional Massive Pillow Hyaloclastite

Logging Program HSDP 2 Logging Sections

Bitsize

412 ft/ 126 m

Sonic

6007 ft/ 1831 m

Magnetometer

Resistivity γ Spectrum

BHTV

DTS

GR TEMP Caliper Inclination

1981 ft/ 604 m

8930 ft/ 2723 m USGS Uni Hawaii

performed by GFZ Uni GötPotsdam tingen

1st Run: July 1999 2nd Run: December 1999

Motivation for log analysis Objective: Reveal the internal structure of Mauna Kea and constrain the understanding of volcano hydrogeology. Understanding of volcano hydrogeology requires information on porosity and permeability Only few petrophysical measurements were made on cores Log data provides the only continuous information for porosity prediction Porosity prediction form logs needs a prior understanding of in-situ petrophysics and rock characteristics

Lithology reconstruction in the subaerial stage In fo rm at io n C or e

Calibration Core Lithology Depth Resistivity medium Total Gamma Ray (API) (mbsl) (Ohmm), log 1

10

100 5

10

Result

15

F r a c s

V e s i c l e s

Lava Flow Succession

Core Lithology

A l t e r a t i o n

Core Recovery (%) 700

U119c

U119

705

705 U120a U120b

710

U120

U120c U120e

715 720

815 725

825

710

U120d U120f U121a U121b

U121

820

700

U119d

Discrimance Analysis

U123 U124

??

715 720

U124a U124b

725

U125a

730

U125

U125b

730

U126a

735

735 U126 U126b

740

740

U127a

U127b

745

745

U127c

830

835

750

750 U127

U127d

755

U127e

755

760

U127f

760

U128a

765

840

U128

770

765 770

U129

775

845

U130

780

775 780

U131

785

785

850 790

U132

795

855

790 795

U133

800

800

Log variability in the submarine stage Depth [ftbsl]

Resistivity(a) deep [Ohmm]

Total (a) Gamma Ray [API]

Total Field(b) [nT]

Depth [mbsl]

3600

LU 2

3800 4000 4200

Rocks described from core as hyaloclastites show significant differences with depth

4400

Low resistivity‚ high GR

4600

LU 3

4800 5000

Low resistivity‚ low GR, strong magnetic anomalies

5200 5400 5600

LU 4

5800 6000

(a) Borehole data, measured by GFZ-Potsdam, Operation Support Group (July 1999) (b) Borehole data, measured by University of Goettingen, Institute of Geophysics (July 1999)

Log Unit Boundary

Changes in Total Field = Magnetic Anomaly

High resistivity‚ high GR

Log lithology and internal structure of Mauna Kea Depth [mbsl]

Resistivity Total deep Gamma Ray [Ohmm] [API] 1

Aa-, Pahoehoe Lava widely brecciated partly low potassium Aa-, Pahoehoe Lava predominantly massive Hyaloclastite, polymict/monolithologic high matrix content, weak consolidation Hyaloclastite, polymict/monolithologic, high matrix content, strong consolidation Hyaloclastite, monolithologic few matrix content, weak consolidation

15

600

LU1

meteoric alteration

subaerial flows

1000

LU2

volcanoclastic apron low consolidation

Massive units weakly fractured, Pillow units, massive to strongly fractured

10,000 4

1500

LU3 landslide - debris flow?

LU4 2000

LU5 LU6 LU7

2500

volcanoclastic apron high consolidation

LU8 LU9

transition from pillow core complex to volcanoclastic apron