Microporosity Estimation of the Upper Jurassic Arab-D Carbonate Reservoir: Outcrop Analog Approach, Central Saudi Arabia

This study investigates the microposity distribution and prediction utilizing an outcrop reservoir analog of the upper Jurassic Arab-D carbonate reservoir in Saudi Arabia. The petrographical examination of the studied samples from the outcrop of Arab-D reservoir analog showed that porosity point counts of all thin sections have values of zero percent. This could be attributed to the sub areal meteoric cementation which totally filled all primary and secondary porosity. However, quantitative measurement of the porosity values for the same samples, using water saturation method, gives considerable percentages of microporosity. This microporosity occurred also in the sub surface Arab-D reservoir and affected reservoir quality estimation and formation evaluation. A high resolution geostatistical analysis for the outcrop section was carried out, using 14 outcrop profiles, to assess microporosity variation and distribution. Quantitative analysis of the results, based on water saturation, showed that microporosity values range from zero to 5% with an average microporosity of 2%. These values represent approximately 13% to 30% of the actual total porosity of the Arab-D reservoir in the subsurface. Thin section and high resolution SEM images showed that most of the microporosity is found around grains as micrite envelopes and between macrocrystalline calcite cementation. Although the Arab-D outcrop analog section reveals seven different carbonate lithofacies, there is no remarkable change of microporosity type and percentage due to the lithofacies change. It is suggested that the main controls on microporosity distribution are grains to matrix ratio and micrite to cement ratio. The geostatistical analysis confirmed the results and also proposed that dolomtization is also other possible factor that affected microporosity construction. The high resolution evaluation of microporosity at outcrop scale provides better understanding and prediction of the reservoir quality in the subsurface.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California