Diagenetic processes influencing porosity in sandstones from the Triassic Buntsandstein of the Iberian Range, Spain

Abstract

Sandstones of the Triassic Buntsandstein of Spain were studied in an attempt to quantify effects of diagenetic processes such as compaction, cementation and leaching on reservoir properties. Samples were taken from Shell well Sigüenza 44-3 some 110 km northeast of Madrid, drilled on the Sigüenza High in the Iberian Range. The sandstones are arkoses to sublitharenites of a typical Triassic redbed facies characteristic of the whole region. The present-day depth of the base Buntsandstein is 538 m, whilst a maximum burial depth of ca. 2600 m was estimated from vitrinite reflectance (0.94 to 1.14), kaolinite temperature (76°C) and illite crystallinity (Kuebler index 4.9). From an estimated average Initial Porosity of 38%, porosity was reduced to the present value of 13%. The strongest process to reduce porosity was found to be mechanical compaction causing a loss of 15%, followed by chemical compaction (pressure solution) with a loss of about 6%. The cement (11.4% R.V.) apparently reduced the Initial Porosity by about only 5%. Early carbonate and sulphate cements are interpreted to have stabili sed the rock framework working against mechanical compaction and pressure solution. This is supported by an increase in sutured grain contacts where early cements are absent or rare. The later quartz cement, interpreted to be largely the product from pressure solution, has reduced porosity in a ratio of 0.5:1. This suggests that compaction continued after the formation of the late quartz cement. Although leaching was observed especially in feldspar grains, clay and K-feldspar neomorphosis is interpreted to have counteracted the formation of substantial secondary porosity. Permeability in the Buntsandstein is generally low (average 17.4 mD) and has been strongly affected by the alteration of feldspars to clay causing a wide range of permeability from 0.11 to 73.0 mD.