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 Magmatism in the Northern Carnarvon Basin, and some considerations for petroleum exploration

Presented by Michael Curtis

Abstract

Magmatism related to Jurassic rifting and Early Cretaceous breakup of Gondwana has played an important role in the tectonic evolution of the Northern Carnarvon Basin (NCB).  Crustal thinning and possible plume impingement beneath the Cuvier margin led to an elevated thermal regime through the late Jurassic which caused the emplacement of huge volumes of magma into the upper crust of the Exmouth Plateau and Exmouth Sub-Basin.  The presence of the igneous rocks resulting from this magmatic activity has implications for petroleum exploration and development in the region.

This project has used 3D seismic datasets to map intrusions across the NCB in unprecedented detail.  Intrusions are present in a range of morphologies: From individual, saucer shaped sills such as those in the Indian 3D survey, to huge interconnected, stacked complexes spanning 60+ km laterally in the Scarborough and Bonaventure 3D Surveys.  Occasionally, intrusions display magma pipes which terminate in lobate features which have been used to determine the direction of emplacement.  There are also several instances of thick intrusions ‘jacking up’ overlying sediments, creating 4-way dip closed fold structures.

The igneous system present in the NCB has been previously defined as a “Large Intrusive Igneous Province”.  No inboard extrusive activity had been identified, and it was thought that the mafic intrusions in the Exmouth region had not breached the surface due to relative buoyancy effects.  However, Tithonian rift-related volcanism has recently been recognised on 3D seismic reflection data in the Exmouth Sub-Basin.  Mt Aneto, adjacent to the Pyrenees oil and gas field, is a very well preserved volcano.  It is bisected and downthrown by extensional faults, and was covered by thick sediments of the Early Cretaceous Barrow Delta, protecting it from erosion by the regional Valanginian Unconformity (associated with final breakup of India from the Australian continent).  A chain of volcanoes at the western edge of the Exmouth Sub-Basin has also been revealed by intersection at Toro-1 and seismic interpretation as part of this study.  In contrast to Mt Aneto, the Toro volcanoes are eroded by the Valanginian Unconformity.

Typically, the ratio of intrusive to extrusive igneous rock in igneous provinces is between 5:1 and 10:1.  The volume of volcanism identified in the Exmouth region does not adequately account for the extent of the intrusive system, which possibly suggests that the ‘remaining’ volcanic complexes may have been completely removed by the breakup unconformity.

A post-mortem analysis of wells intersecting igneous rocks in the NCB has also been performed.  Intrusions in Yardie East-1 and Edel-1 have acted to mature source rocks and compartmentalise reservoirs.  In Palta-1, intrusions were missed by wellsite geologists entirely!  Low seismic velocities through the volcano at Toro-1 suggested overpressure, but were later found to be the result of alteration of the volcanics to clay.  Finally, ashfall deposits encountered by ODP 763, Stybarrow-2, Enfield-3 and Enfield-4 have been altered to smectite, a swelling clay, which can cause reservoir damage and potentially lead to difficulty in POOH if it becomes water saturated.