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Does Vertical Effective Stress Influence Quartz Cementation?


It is well established that the development of shallow overpressure within sedimentary basins reduces vertical effective stress (VES) and inhibits mechanical compaction, thus preserving porosity and economic reservoir quality. However, the influence of vertical effective stress on chemical compaction (“pressure dissolution”) and related quartz cementation in sandstones has been de-emphasized in several clastic reservoir studies that have favoured temperature as the control on quartz cementation. These models suppose that quartz cementation is controlled by temperature-related precipitation kinetics and that supply through dissolution of quartz at grain contacts, which may be influenced by VES, is largely irrelevant. This study is targeted at understanding the relevance of VES to quartz cementation by investigating the shallow marine sandstone of the Upper Jurassic Fulmar in the UK Central Graben. Samples have been chosen from upper shore face sands from Clyde (30/17B), Elgin (22/30C) and Fulmar (30/16) fields. These sands are essentially same facies, but have had different VES-temperature histories. All are at maximum burial depth with present day temperatures and VES as follows: Fulmar 127°C & 30MPa; Clyde 147°C & 40MPa; Elgin189°C and 10MPa. Point count data from transmitted light and SEM-CL petrographic analysis show that sands from Elgin – the highest temperature and lowest VES sample set has a lower average quartz cement content (2.0 ± 1.4%) than both Fulmar (2.8 ± 1.7%) and Clyde (3.6 ± 1.6%). The occurrence of e.g. clay coatings and micro-quartz cannot account for the differences in quartz cement. Since Elgin has been in the classic quartz cementation window (i.e. > ca. 80°C) for over 90Ma, we suggest that chemical compaction at quartz-quartz contacts and related cementation has been limited by low VES from high pore fluid pressure through much of Elgin's burial history. This work has significant implications for understanding how overpressure and VES influence porosity preservation in high pressure- high temperature (HPHT) reservoirs, and would also aid the development of better reservoir quality predictive models for prospective HPHT reservoirs.