The facies architecture of large igneous provinces: an integrated geological and geophysical approach to the characterisation of volcanic successions in 3-D

Abstract

Quantifying the facies architecture of flood volcanic provinces is important as it can be used to understand the physical volcanology and rock property variations throughout the igneous succession. This is very important to the petroleum industry exploration efforts in volcanic rifted margins as volcanic successions commonly mask geophysical images of sub-volcanic petroleum plays. This problem is known as the 'sub-basalt imaging problem' and is caused by factors including the geometrical heterogeneities and elastic velocity and density contrasts through the volcanic pile. The study of facies architecture is broken down into a series of orders of scale. These scales reflect a systematic approach to the characterisation of the facies architecture, from a centimetre through to kilometre-scale, and incorporates 3D modelling of a range of data types for constructing the 3D structure of the flood volcanic successions. A system for the characterisation of lava flow scale facies is presented termed the 'intrafacies scheme'. This may be used to assess and interpret the geological facies heterogeneities present on a 'micro-scale' and link the interpretations to geophysical rock properties. The scheme is applied to outcrop-scale case studies in the Talisker Bay area of the Skye Lava Field on the Isle of Skye, Scotland. On a lava field scale of study ('meso-scale'), the geometrical relationships of several flood basalt provinces are studied, focusing on the Skye Lava Field. This is studied in ID through to 3D, revealing that the lava field may be divided into architectural sequences based on lava flow facies interpretations. The facies evolve upwards through the volcanic succession from geometrically complex thin, olivine-basaltic compound-braided lava flow facies towards the base, to simple, thick basaltic-andesite tabular lava flows. The lower lavas are interpreted to have formed on the gently dipping flanks of a shield volcano. The observations and understanding of flood volcanics on a lava field scale of observation and the facies forming the building blocks of lava fields are used to interpret the GFA-99 2D seismic data from the Faeroe-Shetland Basin. The interpretation is developed into 3D and thicknesses of the Faeroes Lava Group are calculated. The complete study of facies from intrafacies through to basin-scale interpretations reveal that flood volcanic successions contain substantial geometrical and rock property heterogeneities, and that these can be characterised in the 3D modelling environment into geologically realistic geophysical flood basalt facies architectural models.