Risk factors of CO2 geological storage in the Baltic sedimentary basin

Abstract

The Baltic sedimentary basin is relatively little affected by tectonic structuring owing to cratonic setting of the region. However, the fault network cutting the sedimentary layers is identified. The distribution of faults is highly variable. It is notable that the major local uplifts prospective for CO2 geological storage are associated mainly with those faulted structures. Therefore the risk of the CO2 leakage from the potential storage sites should be evaluated. The main prospects of the CO2 geological storage are related to the Cambrian siliciclstics that represent the basal part of the Baltic sedimentary basin. In terms of the hydrostatic pressure and temperature conditions, the most part of the Baltic sedimentary basin is favourable for CO2 storage in the Cambrian saline aquifer. This prospective area is characterised by a rather thick (in access of 0.5 km) shaly package of the Ordovician– Silurian age that provides a reliable seal which is a very important parameter for safe storing CO2 in Cambrian sandstones. The main faulting of the sedimentary cover of the Baltic basin took place during the latest Silurian - earliest Devonian referred to as the Caledonian phase of the tectonic activity. Different types of faults are defined that show different geometries which, in turn, may affect the tightness of faults. The compressional reverse faults of predominating NW–SE orientation show rather simple geometries, while flower structures are typical for the transpressional faults striking mainly in sub-latitudinal direction. Accordingly, the former type of faults is considered of lower risk in terms of CO2 leakage compared to the latter type of faults. Furthermore, three types of local uplifts prospective for CO2 storage were defined, i.  e. (1)  uplifts not associated with any faulted structures, (2)  uplifts associated with fault(s), and (3) uplifts dissected by faults. The second type of structures predominates in the Baltic sedimentary basin. It is therefore not surprising that majority of the largest and most prospective local uplifts are located in Latvia that is affected by the largest scale Liepaja–Saldus fault zone. Analysis of fifteen major uplifts of Latvia and two potential storage sites of Lithuania indicates that only two structures (Degole and Liepaja) do not have any evident association with faults. The rest uplifts were formed along the dominating fault or the intersection of several faults of either transperssional (higher risk) or compressional (lower risk) types. The other important parameter that may affect the isolation of the storage structure relates to drilling. The wells are mainly 30–40 years old therefore some might lose the integrity. The prospective storage structures are drilled to a different degree. The number of wells ranges from 1 to 15 wells; in most cases structures were investigated by 2–4 wells.