Reservoir Quality of Lower-Middle Jurassic Sandstones within the Johan Castberg Field in the SW Barents Sea

Abstract

The study area is located in the Bjørnøya basin, the southwestern Barents Sea. It is surrounded by the Loppa High and Sub-platform to the east and southeast, the Veslemøy high to the west and to the north by the Bjørnøyrenna Fault Complex. There are two wells included in the study area. Well 7219/8-1 is located in the Bjørnøya Sør area, west of the Loppa high and 7220/7-1 is the other well, located west of the Veslemøy High. The Stø and Nordmela Formations of Lower to middle Jurassic ages are the target reservoirs in the study area where the Stø Formation is the main reservoir. Both petrophysical and petrographical analysis has been carried out using cored intervals from both wells. Well correlation and cross plots have been used in petrophysical analysis and point count analysis, scanning electron microscope, cathode luminescence and X-ray diffraction analysis has been carried out in petrographical analysis. Sandstone of the Stø Formation is quartz arenite and is texturally and minerlogically mature. On the other hand, the Nordmela Formations is quartz to sub-feldspathic arenite and minerlogically mature. Reservoir quality of the Stø Formation in well 7219/8-1 is poor due to very low porosity but porosity is good in well 7220/7-1 and act as a good reservoir. Porosity in the Nordmela Formation is good but permeability affects its reservoir quality. Quartz cementation and pressure solution are the factors controlling chemical compaction. Stø formation in well 7219/8-1 is highly compacted due to cementation and pressure solution but the Stø and Nordmela Formation in well 7220/7-1 is slightly affected by quartz cementation and having a well preserved porosity. Abnormally high intergranular volume in two of the samples from well 7220/7-1 and 7219/8-1 is mainly due to the presence high amount of carbonate cement (siderite). Siderite is present as a detrital grain during early diagenesis and then dissolution and recrystallization at deeper depth causes this increase in IGV value.