Flood basalt facies from borehole data: implications for prospectivity and volcanology in volcanic rifted margins

Abstract

Flood basalt successions cover many potentially prospective sedimentary basins world-wide, and a few instances exist of intra-basalt petroleum discoveries. However, little is known about the architecture and rock properties of the lava flows, intrusions and other lithologies that make up these successions. We present a simple, effective method of obtaining information from borehole data on the different volcanic facies within a flood basalt succession. Our aims are: (1) to provide a means of determining proportions of different volcanic facies without detailed examination of borehole data or where borehole data are limited; (2) to explore the relationship between onshore and offshore observations. The facies classification scheme providing the framework for this research includes tabularclassic lava flows, compound-braided lava flows, hyaloclastites and intrusions. We show how this scheme can increase our knowledge of the offshore succession and can be useful in hydrocarbon exploration.In the Faroe Islands, three different basalt formations display a range of facies onshore. Boreholes have been drilled through these, and several kilometres' depth of log data collected. The proximity of these boreholes to onshore observations allows the identification of different facies within the wireline log data. This work demonstrates that histograms of P-wave velocities provide an efficient method of identifying the different facies, and we also explore why these distributions are so different. When applied to borehole data from published ODP wells and one commercial well, it is possible to estimate proportions of the different volcanic facies using the velocity distributions alone.

Characterising extrusive and intrusive magmatism at the Kipper Field, Gippsland Basin, using 3D seismic data

“…Typical signatures in logs consist of low gamma ray signatures, bulk density and neutron porosity. This is consistent with log signatures for extrusive lava flows, in contrast to intrusives which would show high bulk density (Nelson et al 2009). Due to weathering they are quite prone to washout, visible on caliper logs and generally have a high photoelectric (PEF) log response, although this is quite sensitive to washout and mudcakes (Figure 3).…”

Eocene volcanism in offshore southern Baffin Bay

“…Several studies have shown that the physical volcanology of a flood basalt sequence (including the lava flow facies) can be derived from wireline log data using characteristic velocity and density profiles (e.g. Planke 1994;Nelson et al, 2009). A full sequence of wireline data was available from the Delta-1 well, and this provided an assessment of the type and style of volcanism present in the well.…”

“…A typical flood basalt lava flow is divided into three parts (e.g. Self et al, 1997;Nelson et al, 2009): a vesicular/fractured rubbly crust, a massive mostly non-vesicular core and a thin vesicular base. These three parts have very different physical properties: the core has a high density and high V p , whereas the crust has a relatively low density and low V p .…”

Igneous seismic geomorphology of buried lava fields and coastal escarpments on the Vøring volcanic rifted margin

“…Jerram et al, 2009;Planke et al, 2015;Schofield et al, 2016), as well as an understanding of the petrophysics of the volcanic rocks from borehole core and wireline data (e.g. Planke et al, 1999;Japsen et al, 2004;Nelson et al, 2009;Watton et al, 2014;Millett et al, 2015). With these advances in our understanding of volcanic margins, targeted 3D seismic data over key volcanic areas will provide valuable insights into how these margins develop through time and are preserved in the offshore rock record.…”