Origin and occurrence of illite clay mineral in Unayzah Sandstone reservoirs in Central Saudi Arabia. 9th Middle East Geosciences Conference, GEO 2010.

Abstract

The Unayzah (Late Permian) sandstone reservoirs in Central Saudi Arabia are important sources of light sulfur-free crude oil and gas. However, the quality of the reservoirs can vary significantly based on the amounts of clay minerals (especially of illite) and quartz cement present in the reservoirs. It has also been observed that illite clay in amounts as little as 2-3 wt% can cause a precipitous decline in the permeability and productivity of a reservoir. In order to evaluate the nature and amount of illite clay in the Unayzah reservoirs, 69 core plugs from 25 wells spanning a depth (temperature) range of 6,200 to 15,500 feet were analyzed by X-ray diffraction (XRD) and environmental scanning electron microscope (ESEM). The results show that illite clay mineral occurs as domains, aggregates, pore linings or infillings, coatings around stable grains, and bridges between grains. Those illite clays can be classified into five types based on petrographic analysis: (1) matrix illite; (2) illuviated illite; (3) illite coating; (4) illite from illitization of kaolinite; and (5) fibrous illite. Types 1, 2 and 3 are detrital in origin whereas types 4 and 5 are diagenetic. Among the five types of illite clays, the fibrous illite is more important than others as it is typically diagenetic in nature and grows into pore space during burial diagenesis. The XRD and ESEM results indicate that up to 11 wt% diagenetic illite is present in the cores. However, the data do not show any definite illite trend with depth. The data suggest a large increase in the amount of fibrous illite between 14,000 and 14,500 ft, but then the trend appears to reverse itself below 15,000 ft, where the amount of illite is reduced by 50%. The study revealed that diagenetic illite in Unayzah is mainly related to K-feldspar-kaolinite reaction. However, at shallower depths it appears that the illitization reaction has not gone to completion, which results in non-equilibrium assemblages of illite, kaolinite and K-feldspar. In the samples enriched with detrital illite coatings, although kaolinite is converted to illite, there is still significant amount of K-feldspar present in the rocks. This suggests that detrital clays may be blocking pore fluids from further reaction. It may be possible to predict illite precipitation using a kinetic model based on Arrhenius approach. This will lead to better correlations of illite cement with reduction in porosity and permeability and in identifying potentially good quality reservoirs in areas yet to be drilled.