A MULTIDISCIPLINARY ASSESSMENT OF HYALOCLASTITE DEPOSITS IN PETROLEUM SYSTEMS USING FIELD STUDIES, DRILL CORE, BOREHOLE IMAGE AND WIRE-LINE LOG DATASETS

Abstract

This thesis aims to document hyaloclastite deposits in different depositional environments from field outcrops in Iceland to characterise lithofacies heterogeneity enabling comparison to subsurface datasets. Field hyaloclastite datasets from Stóri-Núpur (subaerial-marine transition) and Hjörleifshöfði (an emergent Surtseyan volcano) are used to support the interpretation of hyaloclastite and associated volcanic deposits in core, borehole image logs and wire-line log datasets from Hawaii (Hawaiian Scientific Drilling Project II – HSDP II borehole) and the Faroe-Shetland Basin (LOPRA1/1A well and the Rosebank field).
This study provides additional constraints on lava delta formation in predominantly basaltic systems where hyaloclastite depositional profiles reflect localised extrusion pathways and syn-sediment reworking which control 3D lithofacies architecture. Furthermore this thesis documents the evolution of Hjörleifshöfði using field mapping and major/trace element geochemistry. Hjörleifshöfði can split into five phases of deposition charting the submarine to emergent building of the volcano, unique as it also records a phase of silicic volcanism (Sólheimar Ignimbrite) which dates late stage volcanism to no earlier than 12,383 C14 years BP.
Petrophysical and petrographic observations suggest hyaloclastite deposits are unique in terms of their velocity/density and P and S wave relationships due to palagonite formation, basalt clast support, phenocryst and zeolite component amongst others which impacts on depth conversion and the calculation of reflection coefficients. Wire-line log response gamma-ray (GR), resistivity (RES), P-wave sonic velocity (Vp) is also closely linked to the dominant interstitial secondary minerals and phenocryst components of sideromelane glass.
Borehole image log analysis of mixed volcanic and volcaniclastic rocks allows the accurate characterisation of distinct internal lava flow features, contact relationships and joint networks enabling better characterisation of volcanic sequences in the subsurface via careful comparison with field examples.
Field, core and wire-line log data is combined to form a multidisciplinary assessment of hyaloclastite deposits in the subsurface suggesting that the complexity and scaling issues in hyaloclastite rocks is generally overlooked which may impact on future petroleum exploration in volcanic basins.