Structural Evolution and Petroleum Systems of the Murchison Basin

J. Mac BeggsGeoSphere Ltd, Lower Hutt

Francesca C. GhisettiTerraGeologica, Christchurch

GSNZ Annual Meeting

Wellington, Nov. 23-26, 2008

• Inner margin ofCompressionalInversion Orogen

• Footwall of the Alpine Fault in the bend region

• Overlies major basementdiscontinuities (TerraneBoundaries)

• Bounded by sets of largecrustal faults

• Heat flow anomaly (up to 108 mW/m2

(Townend, 1999)

Upwelling of slab-derived fluids inferred from MT (Wannamaker et al., 2007)

BULLERTAKAKA

M. BATHOLITH

MURCHISON BASIN

• Flexural foredeep during L. Oligocene through M. Miocene (30-10 Ma)

• Persistent subsidence from the

extensional through the early

compressional stages

Extensional basin (hosting terrestrialcoal measures) during mid-late Eocenerifting (45-35 Ma) (tip of N-migratingEmerald Basin)

*

*

~ 20 Ma

MB

Large sediment accommodation space with > 7.5 km of infilling Tertiary sediments

Beggs, Ghisetti

and Tulloch, 2008

Emerald B.

E. Miocene Ranges

• Strong vertical mobility

• Rapid uplift (post 10 Ma) with removal of > 3.5 km of Eocene and younger section (Gibson et al., 1996)

Ghisetti and Sibson (2006)

BASE TO THE OLIGOCENE SEQUENCE

> 40% post L. Miocene shortening

• Crustal seismicity

• Large historical earthquakes

• Reverse NNE-SSW faults

HYDROCARBON SYSTEM

Evidence for generation andmovement of hydrocarbon fluids

• Problem of location and quality of seismic lines

• Unsuitable location of wells

SEEPS

Murchison-1 1927

Blackwater-1 1968

Bounty-1 1970

Matiri-1 1985

Location, extent, and maturation of source rocks

Location, geometry, size and depth of stratigraphic/structural traps

Survival of hydrocarbon fluids in the trap-seal assemblage

CONTRIBUTION OF STRUCTURAL ANALYSIS FOR DEFINITION OF SUBSURFACE STRUCTURE

KEY ISSUES FOR EXPLORATION

ManglesGeological Sheet1984

Lihou 1993

East

East

West

West Longford Syn.

Longford Syn.

SEISMIC LINE MB87-01 (REPROCESSED)

Longitudinal Transverse

LONGFORD SYNCLINE

• Inclined non-cylindrical folds with opposite vergence

• Open synclinestight, disrupted anticlines 37-34.3 Ma

34.3-21.7 Ma

21.7-15.9 Ma

15.9-15 Ma

• Disharmonic style controlled by competence contrasts in the sedimentary sequence and byinherited structures

• Forced folds

• High-angle inherited and reactivated normal faults

• Low-angle cross-cutting thrust faults accomodatingsignificant amounts of shortening

1. Buckling and detachment (latest Oligocene-Early Miocene)

2. Fold growth and amplification associated with compressionalinversion (Middle Miocene)

3. Crosscutting by low-angle thrusts (footwall shortcuts of AF) since 10 Ma

Modified from Ghisetti and Sibson (2006)

• Difficult downdip projection of surface and near-surface data

• Correct definition of structural style is a key for exploration and well location

• Low-angle crosscutting thrust faults seal stratigraphic traps and early structural traps in the footwall

• 3D migration pathways of fluids are likely to be structurally controlled

IMPLICATIONS FOR HYDROCARBON EXPLORATION