A Methodology for Exploration in Mature Rift Basins - why play mapping integrated with well failure analysis matters - an example from the Rift Systems of the North Carnarvon Basin, North West Shelf, Australia.

Author(s):	Ian M. Longley	, Australia
	John Bradshaw	FIXME

Many sedimentary rift basins have been intensively explored, typically over many decades in a competitive environment where the full application of the technological armory available to explorers today (e.g. 3D seismic imaging and depth conversion, AVO, etc) has been extensively refined and deployed. Typically in these basins all of the potential source kitchen areas have been tested and in the proven charge areas all of the significant simple structural traps such as anticlines and highside fault blocks/horsts have been drilled. In these areas exploration is consequently now be focussed on the remaining smaller sub-economic simple traps and on larger more complicated trap types. Many of these more complex trap types involve base seals, fault seals and/or pinch outs making them inherently more risky and harder to define than the simpler highside traps which are generally well defined by modern seismic methods.

The challenge for explorers in these basins is how to collate what is typically a voluminous dataset of wells and seismic data in such a way that the most prospective areas for subtle and or complex traps can be efficiently determined together with a calibrated understanding of the relative historical success rates and discovery sizes for these different trap types. An effective methodology is the application of well-established play fairway mapping such that areas of proven reservoir, seal and charge are mapped out spatially on a play by play basis. There are many approaches for doing this but the best method, particularly in areas of complex and laterally variable geology, is the construction of “split risk” common risk segment (CRS) maps where the shared and independent risk elements are separated for each risk element. The key additional element in the proven fairway areas is the proven areas is systematic classification and analysis of well failures, discoveries and prospect inventories by specified trap types Exploration failures associated with any complex traps that are beyond the proven play fairway are consequently excluded in this analysis, such that the explorer can then determine both a spatial focus and extract the real exploration record for these subtle/complex tests and thus ultimately quickly determine and high grade the best remaining focus areas for detailed mapping and prospect generation efforts. An example is presented of low side fault block traps in the Upper Jurassic plays of the Barrow- Dampier-Exmouth portion of the North Carnarvon Basin, along with a review of the exploration record for this trap type, the methodology used, and the remaining potential identified.

The methodology presented is repeatable and applicable to any mature basin type but is particularly applicable in mature rift basins where the trap target and failure analysis data is publically available for collation. Large complex trap discoveries are not common and are frequently non-amplitude supported and solitary (e.g. the Buzzard discovery in the Inner Moray Firth in the North Sea), but when they occur they do have a transformational impact on both exploration companies and government treasuries and always re-invigorate the exploration efforts in what was previously considered a relatively inactive or fallow basin.