Volcaniclastic rocks – antagonists or protagonists?

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.
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, architecture and composition of interlava sedimentary sequences. More than 280 interlava samples (volcaniclastic and siliciclastic) have been collected from these analogues at varying depths of burial (0-5 km) to analyse. This study assesses their reservoir potential and sealing capacity, and evaluates how compositional variances within the interlava sediments significantly control the petrophysical characteristics of these rocks.

Methods

Sixty-eight (predominantly volcaniclastic) samples have undergone petrographic examination and conventional Poro-Perm analysis. This has enabled a thorough assessment of their reservoir potential. Thirty-three samples from this selection, with permeabilities of <20 mD, have also been subjected to Mercury Injection Capillary Pressure (MICP) analysis to quantify their petrophysical attributes, including microporosities, pore throat distributions, and entry capillary pressures. This allows for their sealing capacity to be assessed using the Sneider classification scheme, which converts the air/mercury capillary pressures to equivalent hydrocarbon/water pressures. These samples have subsequently undergone Quantitative X-Ray Diffraction (QXRD) analysis to quantify their mineral assemblages, and to assess how secondary minerals control the petrophysical properties, and thus, the reservoir and sealing potential, of interlava sedimentary sequences.

Results

The porosities and permeabilities vary greatly for the 68 interlava samples. Generally, optical porosities are negligible, whilst helium porosities are very favourable. This indicates substantial microporosity, attributed to secondary alteration products of susceptible volcanic material, mainly clays and zeolites. This supports the petrographic observations which show varying degrees of volcanic alteration depending on the depth of burial, volcanic composition and grain size. Despite these high helium porosities, the volcaniclastic samples typically have negligible reservoir potential, although there are some exceptions that display excellent reservoir properties.
The petrophysical characteristics of the samples with the poorest reservoir potential were evaluated using MICP analysis to better understand the sealing capacity of these volcaniclastic units. The MICP results shows that, due to very small mean hydraulic radiuses (<10 nm) and high entry capillary pressures, volcaniclastic rocks can produce effective seals. At near-surface conditions (and 7.5 % leakage saturations), these seals have the potential capacity to support oil columns (35° API) of up to 1181 m and gas columns up to 671 m.

By comparing the MICP results with newly obtained QXRD data, the relationship between the sealing potential and the volume and type of secondary minerals in volcaniclastic rocks, has (for the first time publically) been established and quantified. Principal component analysis indicates that the presence and abundance of certain mineral species controls the petrophysical characteristics of volcaniclastic rocks, and subsequently, the sealing capacity of these units.

Conclusions

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). This is particularly prevalent in volcanic-bearing basins where sub- and inter-lava volcaniclastic sequences are prolific and laterally extensive. This study not only has major implications for hydrocarbon exploration, but it can also be applied to carbon dioxide sequestration, where volcaniclastic units are good candidate seals for geologic CO₂ storage. In conclusion, this study provides a comprehensive assessment of the role volcaniclastic rocks play in hydrocarbon exploration and CO₂ sequestration, and reconsiders the antagonistic perception of volcaniclastic sequences.