Predicting porosity through simulating quartz cementation of Middle Cambrian sandstones, West Lithuania

Abstract

Quartz cementation is a major parameter controlling the reservoir properties of the Middle Cambrian quartz arenites of the central and western parts of the Baltic Basin. Marked local variations in the porosity and permeability severely complicate oil exploration and exploitation in West Lithuania. Commonly, the porosity of the oil reservoirs is 6-8%. Therefore even minor changes in the porosity have a considerable impact on the potential of oil fields. A predictive model of the quartz cementation is proposed, based on kinetic modelling results. The precipitation rate-limiting model effectively explains sharp variations in quartz cementation controlled by grain-size changes. The model was further improved by incorporating the sorting factor. Even so, the amount of quartz cementation is overpredicted by 4-7% in some intervals, implying that the precipitation rate-limiting model is too simplified. A good correlation was obtained between stylolite spacing and quartz cementation, the overpredicted quartz amount increasing with an increase in stylolite spacing. The modelling results argue against any discernable impact of the oil on the reservoir quality of the sandstones. The successful prediction of reservoir quality mainly depends on correct reconstruction of the sedimentary environment of the Middle Cambrian deposits. The evolutionary model of the quartz cementation suggests a good reservoir quality of the Cambrian sandstones during the later part of Late Palaeozoic, when most of oil was generated in the basin.