Australian NW Shelf Inversion Study: Carnarvon Basin

Introduction

The 2700 km2 Willem 3D seismic survey that was acquired by Woodside Petroleum in 2006, is situated in a highly prospective area of the Carnarvon Basin with multiple play and trap configurations. Six wells have been drilled within the survey area, from Urania-1 in 2000 until the recent Pyxis-1 gas discovery drilled in 2015 by Woodside Petroleum. However, despite the availability of 3D seismic data and being an AVO-rich area, drilling success has had mixed results with some wells not encountering hydrocarbons when testing seismic amplitudes.

The main exploration risks within the survey are absence of hydrocarbon charge, top-seal leakage and the absence of economically sized structures, given that the water depths are mostly > 1000m.

This study aimed to add value by reprocessing and inverting the Willem 3D seismic data using Ikon Science approaches and workflows. The data was reprocessed through a broadband processing workflow and pre-stack depth migration, and targeted two exploration risks - the presence of hydrocarbon charge and top-seal leakage. After the data had been reprocessed, it was inverted using Ikon Science's state-of-the-art technology; Ji-Fi - Joint Impedance Facies Inversion - read more about Ji-Fi here.

The study sought to:
  • Predict fluids and lithology at existing wells
  • Produce facies models consistent with the wells & geology
  • Predict the recent discovery by Woodside at Pyxis
  • Investigate potential in open acreage



Benefits

The reprocessed data provided a highly improved model of the area, allowing petroleum explorers to reassess areas previously passed over.

Ji-Fi is a Bayesian simultaneous inversion that solves jointly for impedance and facies from seismic data. There is no need to build complex stratigraphic low frequency models, since Ji-Fi requires only the definition of facies based rock property trends - this entire study was completed in just 3 weeks!

This study highlights how applying Ji-Fi to historic data, has the potential to provide significant uplift.

Click here to read January 2016 Leading Edge article all about this study.



Deliverables

  • A perfect fluid contact found in Ji-Fi facies results at Pyxis and to the West of Pyxis-1 & gas column thickness map
  • Strong correlations to all the input & blind test wells
  • The Ji-Fi workflow highlighted the recent gas Pyxis discovery, at a stratigraphic level in which no other discoveries in the region had been identified previously
  • 100km of 2D facies traverse generated on Bellatrix-1 & Urania-1 well data
  • Despite the limited well control from two wells, the prediction of facies by comparison with three other wells at various levels provides significant confidence in predictions away from well control

Study coverage

Seismic data

  • The 2700 km2 Willem 3D seismic survey
  • 12 million traces of PSDM seismic data in time domain as four partial angle stacks (courtesy of Searcher Seismic)
    • Due to time constraints no conditioning was applied to the seismic data

Well data

  • Open source
    • No petrophysical analysis due to time constraints
  • Two wells with elastic logs: Bellatrix & Urania
  • Three wells with no elastic logs: Ixion, Pluto & Guilford
  • One well with no logs: Pyxis
  • The elastic logs were QC’ed and facies were interpreted consistently across the wells