The Use of Diagenetic Models in the Prediction of Sandstone Reservoir Properties: Appraisal and Development Applications
KANTOROWICZ, JOHN D., Shell U.K. Expro, London, United Kingdom
Characteristic mineral assemblages and diagenetic textures result from the processes that operate during and shortly after deposition, during burial, and during uplift and reexposure to surface groundwater. Most early diagenetic processes accentuate the textural (permeability) variations that in turn reflect depositional facies. Burial diagenetic processes usually occur ubiquitously, unless proceeded by hydrocarbon charge. The successful prediction of reservoir properties relies on combining appropriate depositional and diagenetic models. The formation of cement baffles and diminished aquifer permeabilities are potential problems that can be addresses using diagenetic models during appraisal.
Laterally extensive carbonate-cemented zones in sandstones may present barriers to fluid flow or cause reservoir compartmentalization. In alluvial sandstones, this occurs when episodic deposition creates an alteration of coarse sands with finer clastics that undergo tight pedogenic (calcrete) cementation. Isolation of individual sandbodies occurs when calcretes are eroded, creating lag deposits that form a nucleas for burial cementation. In shallow marine sandstones, episodic deposition may create compartments between "hardgrounds" formed by the oxidation of methane on the sea floor. In each case, deterministic models may be used to predict cement distributions and optimal drainage of individual compartments achieved using horizontal rather than vertical wells.
During burial, illitization of sandstones reduces permeability until production becomes uneconomic. Optimal properties occur in shallow reservoirs charged before illitization, poor properties when charge occurs later. Early charge into a good reservoir may be followed by burial illitization of the aquifer, retarding aquifer support and rendering injection uneconomic. Early recognition of the need to inject directly into the oil column will benefit development planning.
AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)