Controls on reservoir quality in the paleogene Kalatar Formation of the southwestern region of the Tarim Basin, China

Abstract

The Paleogene Kalatar Formation is a main target for petroleum ex125 ploration in the southwestern region of the Tarim Basin (SWRTB) and a systematical summary of controls on reservoirs of high quality has important implications for this area. According to outcrop analysis, as well as core, and laboratory experiments based on theories of sedimentology, it can be inferred that the main pore types in the Kalatar Formation are moldic, vuggy, intercrystalline and interparticle pores, fractures, and mudstone microporosity, respectively. The foreshore in the shore-shelf depositional model is the most favorable target, wherein grainstone and dolomitized grainstone are characterized by high quality in a middle ramp subfacies of the carbonate ramp model. Diagenetic factors, such as micritization, cementation, compaction, neomorphism, silicification, and pyritization, are detrimental for reservoirs. At the same time, leaching and dolomitization can improve the quality of reservoirs. Permeability of reservoirs can be greatly improved by fractures, through which quality of reservoirs can be effectively enhanced. Concentration of CO2, temperature, and humidity are affected by changes in climate, which can have an influence on depositional setting, composition and diagenesis of sediments, and eventually the properties of reservoirs. The hot and arid climate of the Paleogene was harmful to development of reservoirs, which is demonstrated in the contemporaneous diagenetic and epigenetic stages. Shallow and deep-burial diagenesis are closely related to the formation and the connate water, however these are rarely affected by paleo-climatic variation. There is possible erosion that mainly resulted from organic acid in relative well intervals according to indications of hydrocarbons in the early diagenetic stage. Fracture systems can be established when acidic fluids are emplaced during periods of uplift, by which pores can be effectively enlarged during the late diagenetic stages.