Interpretation of alluvial deposits in deep directional resistivity data: the Triassic Skagerrak Formation, Ivar Aasen, Utsira High

Abstract

A significant amount of hydrocarbons are contained within fluvial deposits, which constitute potential CO2 storage sites. The Skagerrak 2 reservoir zone of the Triassic Skagerrak Formation in the Ivar Aasen field (North Sea) is composed of fluvial channel-fills and floodplain- and lacustrine deposits. Fluvial deposits typically form heterogeneous reservoirs as the sandbodies are generally discontinuous, with variable lateral and vertical connectivity, displaying internal heterogeneities that are important for fluid flow. Deep Directional Resistivity (DDR) data is an important data type used in mature hydrocarbon fields. The data identify the tops and bottoms of the sandbodies and can be used to place wells optimally. However, DDR data contain an abundance of information about the sedimentary deposits that are commonly not used to their full potential. The data can be utilised to a greater extent to characterise the reservoir and improve reservoir modelling and well planning. Multiple cored intervals and wells with DDR data of the Skagerrak 2 reservoir zone at the Ivar Aasen field provide an excellent opportunity to examine DDR data from a sedimentological perspective. Specifically, this study aims to improve the understanding of DDR data in alluvial reservoirs and the sedimentary environment of the Skagerrak 2 interval at the Ivar Aasen field. DDR data from 18 wells and core- and wireline logs within the Skagerrak 2 interval have been used in this study to investigate the alluvial system. The interpreted DDR data are compared to the virtual outcrop, Cinca Canal, an analogue for the Skagerrak 2 interval. Through investigation and interpretation of these data, the channel belt geometries and sedimentary architecture of the field were identified and interpreted. The palaeocurrent directions of the channel belts within the Skagerrak 2 interval have additionally been estimated by correlating and interpreting channel belts from the DDR data. The interpreted palaeocurrents indicates NW directions. The main architecture of the Skagerrak 2 interval changes from the west to the east across the field. The Net-to-Gross values, as well as the geometries and amalgamation of the channel belts, increase towards the west. The result from this study indicates that DDR data can be used to improve the understanding of the depositional environment of the Skagerrak 2 interval. DDR data can be used as input for reservoir models in terms of Net-to-Gross variations, channel belt geometries, -heterogeneity, and -connectivity. However, the DDR tool has some limitations, including limited depth of investigation, data quality and false resistivity response, which is essential to be aware of while interpreting DDR data. A combination of DDR data with other types of data (e.g. wireline-, core logs and seismic data) is proposed to strengthen the understanding and interpretations needed to create more realistic geological reservoir models of alluvial systems.