Protagonist or antagonist: Secondary mineralisation in volcaniclastic rocks and its implications for hydrocarbon exploration

Introduction

The impact of volcanism on hydrocarbon systems in prospective basins has come to the fore with the recognition of viable commercial volcaniclastic reservoirs, most prominently, the 2004 Rosebank discovery in the Faroe-Shetland Basin. In addition, the discovery of volcaniclastic seals in working hydrocarbon systems, e.g. the Turkana Basin, North Kenya, has given rise to new hydrocarbon play concepts involving volcaniclastic sequences. Despite this, there is still a distinct lack of quantitative studies assessing the character of volcaniclastic rocks, particularly the complex secondary mineralisation which can occur during burial and diagenesis. As a result, volcaniclastic rocks are often considered problematic and are overlooked or avoided during the development of exploration plays. It is, therefore, imperative to quantify the nature, thickness, and architecture of volcaniclastic rocks to better understand their role in hydrocarbon prospectivity in volcanic-bearing basins.

Methods

This ongoing project uses well-exposed onshore analogues of flood basalt provinces from the NE Atlantic Margin (East Greenland, the Faroe Islands), and Ethiopia, to quantify the thickness and architecture of intralava sedimentary sequences. More than 280 intervolcanic samples (volcaniclastic and siliciclastic) have been collected from these analogues at varying depths of burial (0-5 km) to analyse. Thirty-three selected samples have undergone Quantitative X-Ray Diffraction (QXRD) analysis to quantify their mineral assemblages, and to assess how primary composition and burial temperature control the presence and abundance of certain secondary minerals, including various zeolite and clay mineral species. The samples also have Mercury Injection Capillary Pressure (MICP) and conventional Poro-Perm data to quantify their petrophysical attributes, including air permeability, microporosity, pore throat distribution, and entry capillary pressures. This has enabled a thorough assessment of their reservoir potential, as well as an assessment of their sealing capacity using the Sneider classification scheme.

Results

Principal component analysis of the QXRD and MICP data enables a comprehensive examination of how secondary minerals (clays and zeolites) can extensively impact the petrophysical characteristics of volcaniclastic rocks. This has (for the first time publically) led to the relationship between secondary mineralisation and sealing potential to be established and quantified in volcaniclastic rocks. The results of this study show, that as well as providing potential reservoirs with good effective porosity, volcaniclastic rocks can also form effective top and side seals at shallow depths (<2 km), which can form relatively early in play development. The results from the MICP analysis indicate that certain mineral species have a greater influence over the sealing capacity of volcaniclastic rocks, and where present can support hydrocarbon column heights of up to 1181 m (3873 ft) and 671 m (2203 ft) for oil and gas, respectively. The combination of QXRD and MICP data also allows for an assessment of the factors which control mineralisation, and as a consequence, the sealing or reservoir potential e.g. primary composition (mafic versus felsic), burial depth, geothermal gradients and environmental conditions.

Conclusions

This study has major implications for volcanic-bearing basins, where sub- and intra-lava volcaniclastic sequences are prolific and laterally extensive. It provides a comprehensive assessment of the role volcaniclastic rocks play in hydrocarbon exploration, and reconsiders the antagonistic perception of volcaniclastic sequences in hydrocarbon-bearing basins.