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Abstract

A compilation of average porosity and permeability data for Cretaceous petroleum reservoirs of the Middle East reveals important differences between the two main tectonic provinces present in this region. The Arabian Platform tectonic province is characterized by strong inverse correlation of average porosity with present depth in both carbonates and sandstones, whereas the Zagros Fold Belt, containing almost exclusively carbonate reservoirs, has distinctly lower porosity overall and no porosity-depth correlation. These contrasts are interpreted as reflecting the fact that Arabian Platform strata are mostly at or near their maximum burial depth, whereas Zagros strata have experienced widely varying uplift and erosion following maximum burial in mid- Tertiary time. The carbonate reservoirs show no correlation between average porosity and permeability, probably because of wide differences in dominant pore types. Average permeabilities tend to be much higher for sandstones than for carbonates, despite similar porosity for given depth.

Existence of the Arabian Platform porosity-depth correlation, despite wide diversity of depositional settings and early diagenetic porosity modifications among the individual component reservoirs, reflects the overriding importance of burial diagenesis in controlling the porosity differentiation of reservoir rock bodies. Although porosity commonly shows enormous small-scale (bed-to-bed) heterogeneity, the average pre-burial porosity of larger stratigraphic intervals tends to be very high in both carbonates and sandstones. Burial diagenesis progressively destroys this porosity by chemical compaction and associated (stylolite-sourced) cementation, such that all portions of the affected rock body move toward the zero limit as depth increases. Average reservoir porosity therefore tends to correlate inversely with depth, regardless of the complexities of depositional facies and early diagenesis.

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