Sandstone Diagenesis and Reservoir Quality: Models, Myths, and Reality

Taylor, Thomas R.¹, Melvyn R. Giles², Lori Hathon¹, Neil Braunsdorf¹, T. N. Diggs¹, Gino v. Birbiglia³, Calum Macaulay¹, Mark G. Kittridge¹, Irene S. Espejo¹ (1) Shell International E&P Inc. - Technology Applications & Research, Houston, TX (2) Shell International Exploration & Production, Rijswijk, Netherlands (3) Sarawak Shell Bhd, Miri, Indonesia

The success of many exploration opportunities depends on finding sandstone reservoirs with sufficient porosity and permeability to support commercial development. Assessing reservoir quality risk is especially important in plays and prospects where the targeted sandstones have been exposed to elevated temperatures (>~120ºC) and/or high effective stresses for significant periods of geologic time. Anomalous high-quality reservoir sandstones that deviate from “normal” porosity/depth trends are attributed to processes/conditions that have limited compaction and/or cementation, or to porosity enhancement by dissolution of grains or cements. Calibrated numerical models that forecast sandstone porosity and permeability by modeling compaction and quartz cementation as a function of thermal and effective stress histories have been successfully applied globally. However, some key factors influencing the evolution of porosity and permeability (e.g. grain coats, carbonate cements, authigenic clays) are important model inputs and must be constrained via use of analogue data or diagenetic concepts.

Diagenetic models that are commonly used to explain anomalous, high-quality sandstone reservoirs include: 1) inhibition of quartz cement precipitation by grain coats, 2) preservation of porosity due to emplacement of hydrocarbons, 3) influence of overpressure on sandstone compaction, 4) suppression of quartz cementation in subsalt settings, and 5) porosity enhancement by dissolution of framework grains and/or cements. All too commonly, uncritical applications of such models are inconsistent with quantitative rock data and observations. Based on our analyses, recommendations are made as to the viability and applicability of each of the models in terms of reservoir quality prediction.