Hot Rocks in Cold Basins—A Guide for Petroleum Explorers in Regions Containing Intrusive and Extrusive Igneous Rocks

Abstract

Intrusive and extrusive igneous rocks pose important challenges to petroleum explorers working in many passive margin basins in Australia and beyond. This presentation draws on a host of examples, from seismic and well data, of igneous rocks that have influenced basin structure and prospectivity along the southern Australian margin (e.g. the Bass and Bight basins), the North West Shelf (Carnarvon basin) and the NW European Atlantic margin (the Faroe-Shetland Basin) in order to illuminate the main impacts of magmatism on petroleum systems. Our main focus is on intrusive systems, which often form interconnected networks of sills, laccoliths and dykes that typically exploit pre-existing structural or stratigraphic planes of weakness. Experience from the Faroe-Shetland Basin suggests that as much as 90% of intrusive igneous material within the shallow sedimentary fill of a basin may be seismically unresolvable, and hence understanding and predicting the distribution and effects of these rocks is of vital importance. Determining the timing of magmatism in relation to hydrocarbon charge is a critical issue. Given that magma often preferentially intrudes normal fault zones, post-charge magmatism poses a major risk to fault-dependent traps. In instances where charge is post-magmatism, interconnected, low-permeability sheet intrusions can compartmentalise significant volumes of source and reservoir rock and thereby reduce migration efficiencies. However, there is evidence from some basins that fractured intrusions may facilitate the migration, or even store significant volumes of hydrocarbons. Fractured intrusions that act as conduits for long-term fluid flow can also undergo significant alteration, which may inhibit the ability to detect them on seismic data, with implications for pore pressure prediction during drill planning. Intrusive systems are commonly associated with phases of highly focussed, sub-vertical, transient fluid flow resulting in the formation of hydrothermal vent complexes on the palaeo-landsurface or seabed. Such highly focussed fluid flow may lead to the breaching of previously effective sealing horizons. This presentation will provide a number of workflows designed to aid petroleum explorers in evaluating risks associated with igneous rocks that are likely to have influenced elements of the petroleum system.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015