Volcaniclastic rocks and their implications for hydrocarbon, CCUS and geothermal exploration: Examples from the Ethiopian Flood Basalt Province

Introduction

Volcaniclastic rocks are often considered problematic and are overlooked or avoided during the development of exploration plays due to their perceived poor reservoir potential and the problems swelling clays can cause during drilling. This is despite the existence of commercial intravolcanic hydrocarbon reservoirs, e.g. Rosebank, Faroe-Shetland Basin, as well as the discovery of volcaniclastic seals in working hydrocarbon systems, e.g. the Turkana Basin, North Kenya. In addition, assessing the petrophysical characteristics of volcaniclastic rocks can also help develop the growing potential to store CO₂ in volcanic(lastic) rocks through mineral trapping and better inform geothermal resources in volcanically-affected basins.

Despite this demand, there is still a distinct lack of quantitative studies assessing the character of volcaniclastic rocks, particularly the complex secondary mineralisation that can occur during burial and diagenesis and its effects on the petrophysical properties of these sequences. In the East African Rift System, the dynamic interplay between different volcanic systems, sedimentary systems and tectonism can often result in thick volcaniclastic successions being present within potential hydrocarbon and geothermal basins. It is, therefore, imperative to quantify the nature, thickness, and architecture of volcaniclastic rocks to better understand their role in the prospectivity of volcanic-bearing basins.

Methods

This ongoing project uses well-exposed onshore analogues of flood basalt provinces from the Ethiopian Flood Basalt Province, as well as the NE Atlantic Margin (East Greenland, Faroe Islands), to quantify the thickness and architecture of intravolcanic sedimentary sequences. Analysis of 280 intravolcanic samples (volcaniclastic and siliciclastic) collected from these analogues at varying depths of burial (0-5 km) and geothermal gradients has investigated their reservoir properties and mineral trapping potential. Quantitative X-Ray Diffraction (QXRD) analysis on 33 samples has quantified their mineral assemblage, so that the primary composition and burial temperature control on the presence and abundance of certain secondary minerals, including various zeolite and clay mineral species, can be assessed. Mercury Injection Capillary Pressure (MICP) and conventional Poro-Perm data from these samples quantify their petrophysical attributes, including air permeability, microporosity, pore throat distribution, and entry capillary pressures. These tools have enabled a thorough assessment of reservoir potential and sealing capacity using the Sneider classification scheme.

Results

Principal component analysis of the QXRD and MICP data indicates that the growth of secondary minerals (clays and zeolites) extensively impacted the petrophysical characteristics of volcaniclastic rocks. For the first time the relationship between secondary mineralisation and sealing potential in volcaniclastic rocks is quantified. 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 MICP analyses indicate that certain mineral species have a greater influence over the sealing capacity of volcaniclastic rocks, and where present can support column heights of up to 1181 m, 671 m and 574 m for oil, gas and CO₂, 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-derived, felsic-derived or mixed-derived volcanic material), burial depth, environmental conditions, and meteoric alteration.

Conclusions

These insights are being incorporated into basin-scale studies using the Blue Nile Basin, Ethiopia, as an example, to understand the distribution of various volcaniclastic facies. The aim is to ultimately predict the distribution of favourable volcaniclastic reservoir and seal characteristics needed for viable hydrocarbon systems, CO₂ storage sites or geothermal energy projects in volcanic settings. This study has major implications for volcanic-bearing basins, where sub- and intra-lava volcaniclastic sequences are prolific and laterally extensive.