Controls on primary porosity and permeability development in igneous rocks
Published:January 01, 2003
Some of the more important processes leading to the development of primary igneous porosity due to the cooling and crystallization of magma are reviewed. A distinction is made between volcanic and plutonic rocks, and crystalline and granular volcanic material. Porosity in each rock type is classified according to a proposed effective length scale and geometry into diffusive (Class D) and macroscopic flow (Class F) features. Estimated ranges in values of porosity and permeability are given for a wide selection of igneous rock types, and comparison is made with permeability variations (Δk) derived for both the continental and oceanic crust. While fracture porosity is dominant in most crystalline materials, primary porosity development may play an important role in the final (total) porosity in igneous basement. Some types of primary porosity and permeability in igneous rocks will be strongly time- and scale-dependent due to thermal effects associated with the emplacement and cooling of magmas and volcanic material. Tectonic reworking of the primary petrophysical properties of basement-forming igneous rocks may be significant in the development of regions of anisotropy and enhanced porosity.
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Hydrocarbons in Crystalline Rocks
‘Commercial oil deposits in basement rocks are not geological “accidents” but are oil accumulations which obey all the rules of oil sourcing, migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement rocks should be explored with the same professional skill and zeal as ccumulations in the overlying sediments’, Landes et al. (1960), AAPG Bulletin.
Given that most OPEC countries are currently at or within 5% Production capacity, there is a growing need to look for ‘new oil’ and other hydrocarbons in non-traditional sources.
While oil and gas fields in crystalline basement are still discovered mostly by accident, as shown in this book, such reservoirs can be very prolific, especially if the basement rock is highly faulted or fractured. The chapters in this volume cover a diverse range of topics related broadly to the theme of hydrocarbons in crystalline rocks, and challenge explorationists’ definition of basement rock, which needs to be less narrow and more responsive to new geological ideas.