Abstract: Impact of Diagenesis on Rock Properties and Fluid Flow in Sedimentary Basins

BJØRLYKKE, KNUT

The properties of sedimentary rocks change continuously during burial due to diagenetic processes. Prediction of porosity, permeability, thermal conductivity, and rock mechanical properties requires a clear understanding of diagenetic processes. Data from Jurassic reservoirs in the North Sea Basin and the Mid-Norwegian Shelf show that porosity and permeability vary greatly at similar burial depths and fluid pressures. Adjacent sandstones with the same burial history commonly have very different porosities and permeabilities, illustrating the effect of primary mineralogy and textural relationships. Thus, predictions of reservoir quality must be linked to provenance and facies.

Feldspar dissolution and precipitation of kaolinite due to meteoric water flushing are common in shallow marine and fluvial facies, but rare in turbiditic sandstones. The amount of diagenetic illite in reservoir sandstones shows a strong increase at depths greater than 4 km, when both the precursor minerals (K-feldspar and kaolinite) have been present.

Quartz cementation is strongly influenced by textural relationships such as coatings of chlorite on quartz grains, and the observed local variations in porosity and stylolites support a local source of silica. Mass transport is mainly driven by local dissolution and diffusion over shorter distances and advective transport is in most cases not significant during deeper burial diagenesis.

Compaction of sandstones, limestones, and shales deeper than 2--3 km is mainly chemical, involving dissolution and precipitation of minerals. In North Sea mudstones the porosity and seismic velocity varies greatly mainly as a function of mineralogy. Both mechanical and chemical compaction of larger shale volumes is very slow, and episodic expulsion of water is constrained by the rate of compaction.

During basin subsidence the confining stress increases and open fractures are not likely to develop except when overpressures approach fracture pressure. Fluid flow along faults and fractures is then usually not significant, particularly in the shallow ductile sediments, except at high overpressures. The average fluid fluxes in sedimentary basins are controlled by the rate of compaction and phase changes like dehydration of clay minerals and generation of petroleum, which are driven by temperature and to a much lesser extent by effective stress (overpressure). The pressure gradients are secondary effects of the permeability distribution determined by the lithology. The fact that inorganic and organic diagenesis determines the rate and direction of fluid expulsion and the thermal conductivity of the rocks has important consequences for basin modeling.

AAPG Search and Discovery Article #90943©1996-1997 AAPG International Distinguished Lecturers