A fresh approach to ditch cutting analysis as an aid to exploration in areas affected by large igneous province (LIP) volcanism

Abstract

Where hydrocarbon exploration targets occur within basins affected by large igneous province (LIP) sequences, an understanding of the volcanic stratigraphy is essential in compiling accurate basin models at all scales. Ditch cutting samples are one of the most commonly available sources of data yielding information in the context of LIP stratigraphy such as phenocryst load, degree of secondary precipitation and lava composition, largely unattainable by remote sensing. Where core data is limited or absent, cuttings provide the only means of accessing such data along with valuable inference of volcanic facies development and down-hole conditions. Interpretations based on cuttings data are widely used in industry and, as such, a repeatable and well-defined methodology for the analysis and designation of volcanic facies from cuttings is an important requirement for regional and individual play modelling. Such an approach has not been common practice to date. We propose a system of basic percentage-based cuttings analysis and ternary classification specifically tailored to LIP sequences, and argue for the benefits of a coherent and transparent basin-wide approach. The classification system is further developed into a logstyle output for easy integration and comparison with other down-hole geophysical and biostratigraphic data. Exploration in areas affected by large igneous provinces (LIPs) such as much of the offshore West of Shetland North Atlantic region has many associated difficulties. In the West of Shetland Basin (WSB) region these may include deep water, difficulties in seismic imaging, regular bad weather, difficult formation properties and a relative lack of regional down-well data with which to predict the extent and volume of volcanic rocks (Archer et al. 2005;Close et al. 2005;Gallagher & Dromgoole 2007). With additional difficulties come additional costs and, as such, it is a matter of no small significance to gain the absolute maximum in information from completed wells. In this contribution we specifically approach the analysis of ditch cuttings generated through LIP sequences in an attempt to promote a standardized analytical procedure. At present a transparent and repeatable system for analysis is not apparent and limits the usability and correlative potential of cuttings-based data on LIP stratigraphy to date. We propose a simple nongenetic percentage-based cutting classification system from which an assessment of the degree of error inherent in cuttings data can be made. The classification system also provides a platform from which subsequent genetic inference can be made where appropriate. Further, as ditch cuttings data are continuous in good well conditions, a subsequent stratigraphic output is proposed whereby easy comparison between other down-hole data sources and, importantly, other wells can be made. This development has been largely undertaken using data from, and in the context of, offshore exploration for hydrocarbons. The results and methodology are however deemed equally applicable to o...

Frontier exploration and the North Atlantic Igneous Province: new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

“…These include the main facies building blocks of LIPs; simple tabular lavas, compound braided lavas, hyaloclastites, intrusions and interbeds (Jerram 2002;Nelson et al 2009a). (Planke 1994) and velocity histograms (Nelson et al 2009a) allowing confidence in the facies association whilst corroborating inference from cuttings (Millett et al 2014). Within this section around 20 lavas can be identified by their diagnostic asymmetric log profiles (Planke 1994) ranging in thickness from 6 to 40 m (average 16 m).…”

“…The analyses of cuttings from the Lagavulin well using the ternary classification scheme of Millett et al (2014) are summarised in Figures 2 and 3. Intervals where cuttings data are deemed to be affected significantly by drilling-related issues have been highlighted on the compiled cuttings log (column C in Fig.…”

“…Cuttings are routinely collected and described at the well-head in real time. Drill cuttings vary enormously in their quality, quantity and depth-accuracy depending on a range of factors (Millett et al 2014). Ditch cuttings for the Lagavulin Well were taken every 10 feet (10 feet = 3.048 m) giving approximately 900 individual ditch cuttings samples from the top of the volcanic interval to TD (terminal depth).…”

A combined Y/Ho, high field strength element (HFSE) and Nd isotope perspective on basalt weathering, Deccan Traps, India

“…By extension, the ability to fingerprint variations in the stratigraphy of mafic lava flows using other HFSE is of benefit in localities where flow contacts are not necessarily identifiable, such as in ancient, metamorphosed paleosols or drill-hole samples (e.g., Millet et al, 2014). It is important to stress that in such circumstances chemical stratigraphies can only be produced from those elements from the primary magmatic inventory that are demonstrably immobile and not added to the profile from an external source during or after weathering.…”

Hydrocarbon exploration and exploitation West of Shetlands

“…Watton et al (2014) demonstrate the value of formation microscanner images in unravelling different basaltic lithofacies, comparing the subsurface images with outcrop analogues in a classic calibration study. A fresh approach to ditch-cuttings analysis is presented by Millet et al (2014) who describe how a robust volcanic stratigraphy can aid the construction of accurate basin models, especially where core data is limited or absent. Morton et al (2014) also address basement history through the use of detrital zircon age dating in the northern Rockall Basin and conclude that the Hebridean Platform is a westward extension of the Lewisian Complex, with age dates that can be directly correlated with events identified in the Outer Hebrides and NW Scotland.…”