A Research of High-Quality Sandstone Reservoirs in Deep Formation in the Central Depression of the Fergana Basin, Central Asia

Abstract

Abstract In this study, the outcrops, core-data, well-logging, and rock thin-film were used. Based on the core lithfacies, single-well and multi-wells sedimentary facies, and the structure texture, the Mesozoic and Cenozoic reservoirs were studied in the central depression zone of the Fergana Basin. In this paper, the petrological characteristics, pore structure and diagenesis characteristics of the reservoirs were systematically studied on the base of thin-film (slice), scanning electron microscopy, X-ray diffraction of clay minerals, physical property analysis and constant velocity mercury intrusion. The reservoir differentiation in this area was analyzed. The genesis and the formation conditions of relatively high-permeability high-quality reservoirs were surveyed, it is believed that high-quality sandstone reservoirs are mainly developed in relatively high-structured sandstones, and the reservoirs are strongly squeezed or stretched to increase the original porosity. The presence of ultra-high pressure (pressure coefficient > 2.0) allows the original relative high porosity to be well preserved. The high energy and thick-layer river sand bodies in the delta front are the basis for the formation of relatively high-porosity and high-permeability reservoirs in this study area. The high-pressure and ring-side cementation formed by rapid deposition facilitates the preservation of intergranular pores and is the native of relatively high-porosity and high-permeability reservoirs. The key to pore preservation, in addition, the dissolution contributes to the formation of relatively high porosity and high permeability reservoir pores. The study suggests that the high-quality sandstone reservoirs in the central depression of Fergana are formed by the following factors: high-yield reservoirs are mainly controlled by sedimentary features, structural features, channel sandstones, pore structures, ultra-high pressure, and lithologic characteristics. According to these control factors, the author divides the reservoirs in this study area into three categories: Class I—high-porosity and high-permeability reservoirs with porosity above 17% and permeability of 87-2417md; Class II—medium-hole high-permeability reservoirs, porosity is 8-17%, permeability is 1.2-116md; Class III—low porosity and low permeability reservoir, porosity is less than 8%, permeability is less than 1.2md; the main reservoir of the study area is clearly defined The characteristics of the main strata, the characteristics of the high-quality sandstone reservoirs, and the distribution facies. This study is a kind of exploration of reservoir porosiry of the central depression in the Fergana Basin, it analyzes the relevant laws and characteristics of reservoirs in the deep formation of this area, and provides suggestions for the research and development of the region.