High-Throughput Analysis of Offshore Well Drill-Cuttings via SEM-Automated Mineralogy Using Single-Step Trans-Vertical Moulds

David Grant,Dylan J Goudie,Derek Wilton

doi:10.3390/min12030337

Abstract

Single-step trans-vertical moulds used to prepare grain mounts for SEM-automated-mineralogy based instruments have been shown to lead to more representative sample statistics. For samples that contain a variety of mineral phases, denser phases can settle to the bottom of an epoxy mould during the curing phase. Slicing and polishing a cross section through density-stratified moulds of the sample material can compensate for this. Aliquots from offshore drill well cuttings were prepared using three different methods: (1) traditional epoxy mounting, (2) two-step trans-vertical moulds, and (3) single-step trans-vertical moulds. All were analysed with a FEI Quanta 650 FEG instrument using MLA software with similar acquisition parameters. The results indicate that the single-step trans-vertical moulds are reproducible, lead to more accurate statistics, and yield MLA calculated elemental assays that closely match with data from standard analytical methods. In addition, these moulds can be made in half the time of the two-step trans-vertical moulds, and because of the unique geometry of the custom-made SEM holders, the analytical throughput is doubled. Depending on the size fraction of the well cuttings, the throughput can be further increased by slicing off the remaining epoxy to allow for more samples in the 26x TV holder. A unique cell holder was created to accommodate these trans-vertical samples, allowing them to fit into a LA-ICP-MS instrument such that detailed follow-up microanalyses can be conducted on specific minerals. For instance geochronology can be conducted on grains of interest (e.g., zircons), which had been mapped by the SEM-MLA.

Highlights

Scanning electron microscopy (SEM) based automated mineralogy has become a vital tool for mineralogical research over the last 30 years [1,2]
It is demonstrated that the single-step trans-vertical mould can be used to analyse drill well cuttings from offshore Newfoundland, an area containing four fields that account for 5% of Canadian oil production
If one were to polish the bottom of this sample and analyse it by the scanning electron microscopy with mineral liberation analysis (SEM-MLA), the resulting 2D surface would be overrepresented in minerals with high density or Z phases, while underrepresenting those with low Z values

Summary

Introduction

Scanning electron microscopy (SEM) based automated mineralogy has become a vital tool for mineralogical research over the last 30 years [1,2]. If one were to polish the bottom of this sample and analyse it by the scanning electron microscopy with mineral liberation analysis (SEM-MLA), the resulting 2D surface would be overrepresented in minerals with high density or Z (mean atomic number) phases, while underrepresenting those with low Z values. SEM-MLA is limited to analysing a 2D surface, so any sampling bias on this surface due to this density settling in 30 mm round mounts would be reflected and carried forward in the final results. To account for this bias, trans-vertical sections can be produced and analysed to obtain a more accurate analysis by analysing along the vertical direction in the epoxy, parallel to the settling direction [4–6]. The only answer to this problem, besides transvertical sections, is to mount a smaller amount of sample material (e.g., 0.3 gm) to create a thin monolayer (cf. [9]); the trade-off of this method is fewer sample grains to analyse

Methods

Results

Conclusion