No abstract

A B S TRACT: The SEM, XRD, FFIR and DTA analyses of different size-fractions of clay material from sandstone reservoirs which have experienced a large range of burial conditions have been used to examine the different steps of the depth-related kaolinite-dickite reaction. Dickite progressively replaced kaolinite within a range of burial depths estimated between about 2500 m and 5000 m. The kaolinite-to-dickite reaction proceeds by gradual structural changes concomitant to crystal coarsening and change from booklet to blocky morphology. The crystallization of dickite proceeds by two distinct paths: (1) Accretion of new material from either dissolution of smaller unstable kaolinite crystals and/or detrital minerals (chiefly feldspars), on early-formed coarser metastable kaolinite crystals which exert extended morphological control on the growing crystals.(2) Neoformation of ordered dickite which will continue to grow by a dissolution-crystallization process. The kaolinite-todickite reaction is kinetically controlled and anomalies in the kaolinite/dickite ratio observed in certain sandstone reservoirs may be used to assess the timing of invasion by hydrocarbons.

Extensive magmatic activity took place in the VÖring Basin, o¡shore Norway, related to the Early Cenozoic rifting.The break-up of the North-Atlantic at the Palaeocene^Eocene transition induced strong volcanism.There are numerous magmatic sills below 3 km depth in the area.They are predominantly layer parallel and thin compared with their lateral extent. Igneous intrusions, sills and dykes a¡ected the temperature history, and thus need to be taken into account in petroleum prospect analysis.We have calculated the temperature and maturity e¡ects in the sedimentary layers in the Gjallar area associated with the emplacement of single sill and sill complexes. A 120-m-thick sill produces a theoretical vitrinite re£ectance (%R 0 ) 0.8% higher than normal at a distance of 100 m from the sill.Vitrinite re£ectance changes caused by a swarm of seven sills varying from 8 to 80 m in thickness were calculated. It is shown that the calculated thermal pro¢le can account for the observed shift in vitrinite re£ectance in the well. A two-dimensional section crossing the Gjallar Ridge, consisting of numerous magmatic intrusions, is also modelled.The modelled geological development and temperature history over the pro¢le show that there are signi¢cant maturation e¡ects in the interval under investigation. Based on this work, the sill swarm observed in the area could more than double the fraction of the kerogen that has been transformed to petroleum at the (present) depth of 4 km.

The Longyearbyen CO2 Lab of Svalbard, Norway was established to estimate the potential for geological carbon sequestration at Spitsbergen. Several monitoring wells were drilled in and around the planned CO2 injection site. These revealed a Triassic to Cretaceous stratigraphy consisting of (from top to bottom) a zone of permafrost, the aquifer, the caprock shale, and the upper, middle and lower reservoir. This paper uses two tools to investigate fluid communication within and between these entities: 87 Sr/ 86 Sr of formation waters extracted from cores using the residual salt analysis (RSA) method, and the δ 13 C of gases, principally methane and CO2, degassed from core samples. The Sr RSA data reveal that the upper reservoir rocks have very constant formation water 87 Sr/ 86 Sr in wells several kilometres apart, suggesting good lateral communication on a geological timescale. However, there is a distinct barrier to vertical communication within the middle reservoir, indicated by a step change in 87 Sr/ 86 Sr, corresponding to thin but presumably laterally extensive (>1.5 km) lagoonal mudrocks. The aquifer, which shows a gradient in 87 Sr/ 86 Sr, is also interpreted to have some degree of vertical internal communication on a geological time scale. The caprock shale shows large-scale (over 350 m) smooth vertical gradient in 87 Sr/ 86 Sr. This is indicative of an ongoing mixing process between high-87 Sr/ 86 Sr waters within the caprock and lower-87 Sr/ 86 Sr waters in the underlying reservoir. Diffusion and flow modelling of the Sr data suggest that at some time in the past shale, fluid transport properties were enhanced by the formation of temporary pressure escape features (fractures or chimneys) during deep burial and uplift or cycles of glaciation. Nevertheless, the smooth compositional gradient in the caprock indicates that fluid mixing has subsequently taken place slowly, dominated by diffusion. This 3 interpretation is supported by the gas isotope data, where systematic variations in gas δ 13 C values also indicate slow and incomplete diffusional fluid mixing. These are positive indicators for caprock effectiveness during a CO2 injection project.

No abstract