Characteristics and controlling factors of the Lower–Middle Jurassic sandstone reservoirs in Amu Darya right bank area, Turkmenistan

Abstract

Based on laboratory analysis and core description, characteristics and the controlling factors of the Lower–Middle Jurassic reservoirs in Amu Darya right bank area are examined in this article. The study reveals that fine-grained lithic sandstones, feldspar–lithic sandstones, and siltstones are the main types of reservoir rocks in the central area, while fine-grained lithic–quartzose sandstones are predominant in the eastern area. The major pore types are intergranular and intragranular dissolved pores; the predominant pore throats are lamellar throats. Fractures develop poorly in the study region, but relatively well in the eastern area. The reservoirs are pore-dominated sandstone reservoirs with limited fractures, characterized by very low porosity and permeability. Reservoir evolution is jointly controlled by sedimentation, diagenesis, and tectonic movements. Sedimentation laid the foundation to porosity evolution. Subaqueous distributary channels and mouth bars are the beneficial lithofacies for reservoir development. Poor compressive strength of sandstones, deep paleoburial depth, and destructive subsidence style of the strata led to strong compaction. Primary pores are almost entirely destroyed by compaction and eogenetic calcite cementation. Ferroan dolomite cements and quartz overgrowths occupied the remaining intergranular pores furtherly. However, dissolution played an effective role on the generation of secondary pores, and the influences of authigenic clay minerals are minor on reservoirs. The Neogene–Quaternary tectonic movements caused intensive lateral compressional stress thus made further damage to reservoir pore structure. Nevertheless, fractures generated by tectonic movements increased reservoir permeability and provided valid migration pathways for the late dissolution fluids, therefore improved reservoir physical properties obviously.