EARS 5131 STUCTURE AND HYDROCARBON PROSPECTIVITY OF BASIN

NORMAL FAULTS, ASSOCIATED STRUCTURES AND HYDROCARBON TRAPS. GROUP 3 RACHEL FRANCIS CHU’KA NICK

OUTLINE OF PRESENTATION DEFINITION OF NORMAL FAULTS TYPES OF NORMAL FAULTS ASSOCIATED NOMENCLETURE CHARACTERISTICS OF NORMAL FAULTS GEOMETRY OF NORMAL FAULTS FORMATION OF NORMAL FAULTS STRESS REGIMES BASINS & ASSOCIATED STRUCTURES HYDROCARBON STRUCTURES ASSOCIATED WITH NORMAL FAULTS CASE EXAMPLES

WHAT ARE FAULTS? FRACTURES ACROSS WHICH THERE HAS BEEN DISPLACEMENT IN THE PLANE OF THE FRACTURE. FAULTS DIVIDE ROCK INTO FAULT BLOCKS (HANGING WALL AND FOOT WALL). CLASSIFIED BASED ON ORIENTATION OF RELATIVE DISPLACEMENT. DIP-SLIP FAULTS. STRIKE-SLIP FAULTS. OBLIQUE-SLIP FAULTS.

FAULT TYPES

DEFINITION OF NORMAL FAULTS INCLINED DIP-SLIP FAULTS IN WHICH BLOCKS ABOVE FAULT PLANE (HANGING WALL) MOVES DOWN RELATIVE TO BLOCK BELOW THE FAULT (FOOT WALL) PLANE. MODE II OR III FRACTURES MOVEMENT OF THE BLOCKS ARE RECORDS OF THE KIND OF FORCE EXPERIENCED IN THE REGION IN THE PAST AND PROBABLY AT PRESENT.

NORMAL FAULTS CAUSED BY EXTENSIONAL FORCES INCLINED DIP-SLIP FAULTS IN WHICH BLOCKS ABOVE FAULT PLANE (HANGING WALL) MOVES DOWN RELATIVE TO BLOCK BELOW THE FAULT (FOOT WALL)PLANE (Sv>SH>Sh)

ASSOCIATED NOMENCLETURE FOOTWALL IS THE BLOCK BELOW THE FAULT PLANE HANGING WALL IS THE BLOCK ABOVE THE FAULT PLANE HEAVE IS THE MAXIMUM HORIIZONTAL DISPLACEMENT THROW IS THE MAXIMUM VERTICAL DISPLACEMENT DIP IS THE ANGLE BETWEEN THE FAULT PLANE AND HORISONTAL

Example: Thick Limestone Beds, Northern Mexico. If you stood on the fault plane, the block on the right would be under your feet. This is thus the footwall. The red line marks equivalent layers on opposite side of the fault. Since the hanging wall dropped relative to the foot wall, this is clearly a normal fault.

CHARACTERISTICS OF NORMAL FAULTS Normal faults extend the crust in a direction perpendicular to the fault trace. Because the hanging wall moves downward, normal faults place younger rocks over older rocks. Missing stratigraphic section applies only to a vertical section through the fault. Horizontal plane shows repetition of stratigraphy. Vertical plane shows repetition of stratigraphy, if apparent dip on beds is in same direction as, but steeper than the fault.

GEOMETRY AND DISPLACEMENT OF NORMAL FAULTS In cross section: Planar or curved (Listric). In Plan view: Curved and sinous traces. Fault Zones: In 3D: Large Scale Faults are Composed of Fault Arrays. Displacement varies over fault surface in a systematic manner. Maximum displacement in center with concentric contours of displacement (zero displacement on tip line). Displacement can be transferred between overlapping segments.

Normal fault geometry in 3D

FAULT ZONE The actual 3D architecture of a normal fault zone (From Silje Støre Berg, 2004 Ph.D. thesis)From Silje Støre Berg, 2004 Ph.D. thesis

Fault Shape & Length: Displacement Properties

TYPES OF NORMAL FAULTS LISTRIC NORMAL FAULTS GROWTH FAULTS. LOW ANGLE NORMAL FAULTS DOMINO AND IMBRICATE NORMAL FAULTS. CONJUGATE NORMAL FAULTS.

LISTRIC NORMAL FAULTS GROWTH FAULTS Form at same time as sedimentation (syn-sedimentary). Sediment thickness decreases away from normal faults. Fault dip shallows with increasing depth. Associated with roll-over anticlines in syn-depositional settings. Also associated with synthetic and antithetic faults. Forms collapsed crest structures when detached faults can’t accommodate sediment load. Growth index (Ratio of sediments on both sides of major growth faults).

LISTRIC NORMAL FAULTS

Listric normal fault on seismic profile

LOW ANGLE NORMAL FAULTS. Merge into detachments at depth; ‘Domino’ and Imbricate Listric faults. Detachment confined to crust: Extension Balanced by compression. Gulf Coast and Perido Fold Belt. Detachment Fault cuts whole lithosphere: Ductile shear zone at 10-15km. Basin and Range Pronvince.

CONJUGATE NORMAL FAULTS Fault planes dip towards each other.

STRUCTURAL ASSOCIATIONS OF NORMAL FAULTS. FOLDS. Rollover Anticlines Drag Folds COMMONLY PRESENT AS SYSTEMS OF MANY ASSOCIATED FAULTS. Synthetic faults –Usually smaller and parallel to the major fault and have same direction of dip. Antithetic faults –In conjugate orientation to major faults and have opposite dip. Ring faults –Concentric normal faults developed as surficial rock collapse into subsurface cavity: Calderas. Strike-Slip faults HORSTS AND GRABENS

ASSOCIATED BASINS AND REGIONAL STRUCTURES RIFT BASINS MID-OCEANIC RIDGES GRAVITY COLLAPSE FEATURES SALT DOMES BASINS ASSOCIATED WITH EXTENSIONAL DYNAMICS

FORMATION OF HYDROCARBON TRAPS BY NORMAL FAULTS

A PICTURE OF ME STANDING CLOSE TO A FAULTED CHALK DEPOSITE AT WEST YORKSHEIR COAST

CLASSIFICATION OF FAULTS FAULTS COULD BE CLASSIFIED ACCORDING TO THEIR STRESS REGIME. (ANDERSON’S SYSTEM) SH Sh Sv

TYPES OF FAULTS NORMAL FAULTS (Sv>SH>Sh) THRUST FAULTS (SH>Sh>Sv) STRIKE-SLIP FAULT (SH>Sv>Sh)