Abstract

A detailed study of the relationships between geological forcing factors and the distribution of petrophysical properties is essential for understanding the heterogeneity of reservoirs. This research seeks to describe the effect of genetic geological parameters such as composition, grain size and facies distribution in order to refine current stochastic approach for reservoir characterization and to provide a basic skeleton for subsequent reservoir model studies.

To achieve these objectives, this study used a large data set (13 cores, 150 thin sections and 100 core analysis reports) for the Late Permian Queen sandstone in the northern part of the Central Basin Platform. Four major facies and seven subfacies were distinguished in the lower part of Queen: five clastic facies (fluvial, deltaic and estuarine); a carbonate facies (tidal-flat); and an anhydrite facies (coastal sabkha).

Petrographic analysis of the texture and composition of the Queen facies and their petrophysical properties shows that reservoirs were best developed in river mouth bar (Facies 2) and tidal flat subfacies (Facies 4). The river mouth bar sands of Facies 2 are silty to very fine grained and moderately well sorted subarkosic sandstones. Facies 2 is partially cemented by dolomite and anhydrite. Facies 2 has secondary enlarged and elongated pores, created by dissolution of feldspar and carbonate cements. Rocks of Facies 2 have an average of 11.04% porosity and an average of 13.17 md permeability.

The reservoir quality of Facies 4 (tidal flat) is fair to good, with secondary enlarged intergranular porosity, and partial dissolved intra-cement porosity with intensive anhydrite cementation. The tidal flat sands are similar to the river mouth bar facies, except for intensive cementation. Even though Facies 4 has abundant secondary enlarged and elongated pores created by the partially dissolution of carbonate cements, the distribution of pore is very heterogeneous. Facies 4 has lower porosity and permeability than Facies 2. Facies 4 has an average porosity of 4.71% and average permeability of 2.72 md.

Correlation analysis of the relationship between geological parameters and petrophysical properties can determine the most important parameters for reservoir quality. Of the analyzed parameters, detrital grain size, sorting, detrital grains (quartz and feldspar), diagenetic anhydrite, and carbonate cement most influence porosity and permeability of the Queen sandstone reservoirs.