Seismic Reflection Data: what it is, how it can be used, & an application at Elk Hills, CA - Hudec and Martin, 2004

Seismic Reflection Data: Definition Records seismic waves at the surface that are reflected off of subsurface stratigraphic interfaces Seismic Reflection subject to or caused by an earth vibration return of a wave from a surface that it strikes into the medium through which it has traveled Images the subsurface using acoustic methods - Clay, 1990

- Kansas Geological Survey reflection patterns are described by Snell's Law Seismic Reflection Data: Acquisition Source Receivers cross lines inlines Spacing 12.5 m for Wytch Farm Oil Field 16.7 m for Elk Hills Oil Field grid of receivers for 3D survey:

- Common Midpoint Method Seismic Reflection Data: Processing

- Kansas Geological Survey Seismic x-section Seismic Reflection Data: Processing two-way travel time (ms) interface reflects energy proportional to impedance difference impedance = velocity x porosity subsurface geometries are reconstructed stratigraphic sections show up layered

Seismic Reflection Data: Resolution Vertical Horizontal Minimum separation between two features such that we can tell that there are two features rather than only one

Seismic Reflection Data: Interpretation Cross -section Time slice

Seismic Reflection Data: Interpretation 2D image of normal faults from offshore Lebanon: Stratigraphic variations Faulting on the order of 30+ meters due to faulting due to depositional changes

- Fort et al., 2004 Structural Applications (Scale for this x-section) Individual layers and faults Stratigraphic packages and fault zones Various scales of interpretation - Kattenhorn and Pollard, 2001

Seismic Reflection Data: Application 20 miles east of the San Andreas Fault - Elk Hills Oil Field

Part 1 of 2 step project Ultimate goal: fracture (stress) prediction across a specified stratigraphic layer a specified stratigraphic layer fracture pattern today is a composite of fracturing during each distinct slip event slip-induced stresses vary as faults interact evolutionary history important! Elastic models forward model for slip-induced stress perturbations Elk Hills: Motivation

Gain insight into fault geometry and timing at Elk Hills Industry: implications for hydrocarbon entrapment and charge General: implications for evolution of thrust systems of thrust systems

WestEast A A’ B B’ Elk Hills: Stratigraphy 31S 29R McDonald structure A A’ 2 mi N B B’ NWS Syn-depositional growth faulting: - Thinning onto highs - Thickening across faults Use sedimentary features to constrain fault movements with time of deposition of specific layers

Elk Hills: Stratigraphic Constraints Stratigraphic Constraints on Fault Timing: 2D analysis (cross sections) 3D analysis (isochores) Chronological fault evolution model

Elk Hills: Stratigraphic Constraints Pseudowell thickness plots Pseudowell analysis Calitroleum 2 mi N West A A’ MYA4-A Wilhelm Calitroleum BRR ms AA’ A SW NE

Elk Hills: Stratigraphic Constraints Cross section analysis Example 1: Onlap Syn-faulting strata Active faulting and uplift during deposition Fault Movement Indicators

Elk Hills: Stratigraphic Constraints Cross section analysis Example 2: Offset beds of equal thickness Pre-faulting strata Fault Movement Indicators Faulting post deposition

Elk Hills: Stratigraphic Constraints Cross section analysis Fault Movement Indicators NESW

Elk Hills: Stratigraphic Constraints Cross section analysis Fault Movement Indicators Offset onlap: 1. 3R>1R 2. 3R>BRR 0.5 mile 100 ms

Elk Hills: Stratigraphic Constraints Isochore: line drawn through points of equal vertical (apparent) thickness of a unit BRR Calitroleum Wilhelm MYA4-A McDonald x1x2x3x4x5 1 mile 500 ms

Elk Hills: Stratigraphic Constraints A A’ 2. Thin beds b) c) A A’ c) filled in paleo high (post tectonic) b) fault below interval (syn tectonic) 1. Close contours a) a)fault cut interval at time (syn tectonic) Two Signatures: Structural high A A’ A

Elk Hills: Stratigraphic Constraints Isochore: McDonald to Base Reef Ridge N km 012 miles ft Fault cut Structural high - active fault Structural high - active fault A A’ 2R 3R 1R 5R A A’ 2R 3R 1R 5R No expression - inactive fault 1R 5R 1R 2R 3R McDonald BRR 6R 7 7

Elk Hills: Stratigraphic Constraints Conclusions from all stratigraphic analyses - Reid, 1990 McDonald 7, 2R, 3R 2R, 3R, 1R 2R, 3R, 1R, 5R Active faults fault initiations bracketed between horizons: AA’ 2R 3R 1R 5R 6R west B B’ 6R 7 east

Elk Hills: Stratigraphic Constraints Conclusions from all Analyses Stage 1 Stage 2 Stage 3 Stage 4 WEST A A’ 3R A A’ 2R 3R pre-McDonald (mid Miocene) A A’ 2R 3R 1R pre-Base Reef Ridge (early Pliocene) A A’ 2R 3R 1R 5R pre-Wilhlem (mid Pliocene) pre-McDonald (mid Miocene) Stage 1 Stage 2 EAST B B’ 7 B 6R pre-McDonald (mid Miocene) Post MYA-4A (late Pliocene) 31S 29R A A’ 2 mi N B B’ 7

Elk Hills: Stratigraphic Constraints Fault Geometry & Timing input for elastic models determine faulting related stress perturbations