Influence of Fluid Pressure and Effective Stress on Quartz Cementation in Clastic Reservoirs

Abstract

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. However, the influence of vertical effective stress on chemical compaction/pressure dissolution and related quartz cementation in sandstones has been under-appreciated in many clastic reservoir studies that have favoured temperature as the key control on quartz cementation. These models suppose that quartz cementation is controlled by temperature-related precipitation kinetics and that the supply of silica is largely irrelevant. However, it is commonly considered that the main source of silica for quartz cement is from intergranular pressure solution, the rate of which is influenced by VES. This study integrates quantitative petrographic data, high spatial resolution oxygen isotope analyses of quartz cement, basin modelling, and a kinetic model for quartz cementation to understand the relevance of VES to quartz cementation by investigating clay-poor sandstones of the Upper Jurassic Fulmar Formation from Clyde, Elgin and Fulmar fields in the UK Central Graben and Paleocene-Eocene Wilcox Group from Lake Creek and Rotherwood fields in the Texas Gulf Coast. These sandstones have distinctly different histories of vertical effective stress (VES) and temperature. The study not only shows that most or all the silica for quartz cement can be derived from intergranular pressure dissolution, but that the extent of intergranular pressure dissolution and related quartz cementation correlated strongly with VES and poorly with temperature. Oxygen isotope data obtained from the quartz cements yield temperature ranges for quartz precipitation which are taken to indicate that the rate of quartz cementation is more strongly related to the history of VES rather than the history of temperature. This analysis suggests that it is the vertical effective stress history, rather than the temperature history, that exerts the greatest influence on quartz cementation. 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.