Correlating diagenetic facies with well logs (conventional and image) in sandstones: The Eocene–Oligocene Suweiyi Formation in Dina 2 Gasfield, Kuqa depression of China

Abstract

The lithology, mineralogy, texture, pore systems, and diagenesis of the Eocene–Oligocene Suweiyi Formation sandstones in Dina 2 Gasfield of Kuqa depression were investigated by integrating thin sections, scanning electron microscope (SEM) images with X-ray Diffraction and routine core analysis. The results reveal that carbonates and clay minerals are the dominant pore-filling constituents, while quartz cements and evaporate minerals occur in only minor amounts. Compaction played an important role in reducing intergranular porosity, especially in the clay-rich or poorly sorted successions. Framework grains dissolution greatly enhance total porosity. The pore systems consist of intergranular and intragraular dissolution porosity, micropores and minor intergranular pores. Four diagenetic facies are identified according to lithologic characteristics, diagenesis, as well as diagenetic minerals, and they are: (1) tightly compacted facies (high degree of compaction but low degree of cementation and dissolution); (2) carbonate (calcite) cemented facies; (3) clay mineral (illite and mixed-layer illite/smectite) filling facies; and (4) dissolution (of framework grains) facies. Diagenetic facies are correlated with the conventional well logs, and the well log expressions for diagenetic facies are summarized, then the diagenetic facies in uncored intervals and wells can be predicted based on the qualitative well log characteristics. Image logs were used to derive the porosity spectrum (porosity distribution histogram) by applying the typical Archie's formula to the flushed zone. Then the porosity spectrum is integrated with the conventional logs to predict the diagenetic facies, and the results can be evidenced by thin section observation, and SEM analysis. Dissolution facies, which has the highest reservoir quality, tend to have bi-modal porosity spectrum, and the porosity spectrum is very broad or even contains tail distributions. In contrast, the porosity spectrum of the carbonate cemented facies and tightly compacted facies are narrow and dominantly of uni-modal behaviors, indicating the absences of large pores and a poor reservoir quality. The porosity spectrum of clay mineral filling facies can be bi-modal or uni-modal, but the porosity spectrum is narrow and contains no tail distributions. The diagenetic facies and reservoir quality in sandstones can be predicted through correlating the diagenetic facies to conventional well logs integrated with image logs.