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Lack of inhibiting effect of oil emplacement on quartz cementation: Evidence from Cambrian reservoir sandstones, Paleozoic Baltic Basin

Nicolaas Molenaar^1,, Jolanta Cyziene², Saulius Sliaupa³ and John Craven⁴

¹ Institute of Environment & Resources, Technical University of Denmark, Bygningstorvet, Building 115, DK-2800 Kongens Lyngby, Denmark, and Department of Geology and Mineralogy, Vilnius University, K.M. iurlionio 21/27, LT-03101 Vilnius, Lithuania
² Institute of Environment & Resources, Technical University of Denmark, Bygningstorvet, Building 115, DK-2800 Kongens Lyngby, Denmark, and Geological Survey of Lithuania, S. Konarskio 35, LT-03123 Vilnius, Lithuania
³ Institute of Geology and Geography, T. Sevcenkos 13, LT-03223 Vilnius, Lithuania, and Department of Geology and Mineralogy, Vilnius University, K.M. iurlionio 21/27, LT-03101 Vilnius, Lithuania
⁴ Ion Microprobe Unit, Department of Geology and Geophysics, Edinburgh University, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK

Correspondence: E-mail: nicolaasmolenaar{at}gmail.com.

Currently, the question of whether or not the presence of oil in sandstone inhibits quartz cementation and preserves porosity is still debated. Data from a number of Cambrian sandstone oil fields and dry fields have been studied to determine the effects of oil emplacement on quartz cementation. The data show that the porosity distribution is not affected by the presence of oil in sandstones from oil fields and dry fields with similar porosity distribution. From this, it can be concluded that oil emplacement does not lead to preservation of primary porosity, and silica supply for quartz cementation is derived from internal sources. Rather, in spite of large variation in porosity and quartz cement content, a regular pattern of porosity decrease is related to increasing temperature or depth. The observed heterogeneity is due to local factors that influence the precipitation of quartz cement, including sandstone architecture, i.e., distribution of shales within the sandstone bodies, and sandstone thickness. Heterogeneity is inherent to sandstone architecture and to the fact that silica for quartz cementation is derived from heterogeneously distributed local pressure solution. Models predicting reservoir properties should encompass facies and architecture as important independent factors.

Key Words: oil inhibition • quartz cement • quartzarenite • heterogeneity • reservoir properties • Cambrian