Abstract:

Discoveries have been made in the Paleogene hydrocarbon exploration in the Mahaidong area on the northern margin of the Qaidam Basin. However， the current productivity is low primarily due to the limited understanding of the characteristics of micro-pore structures， restricting hydrocarbon exploration and exploitation in the area. To reveal the microscopic characteristics of the Paleogene sandstone reservoirs in the Mahaidong area， we investigate the petrology， physical property characteristics， micro-pore structures， and heterogeneity of low-permeability sandstone reservoirs in the Mahaidong area using techniques such as the micro-petrographic observation， X-ray diffraction （XRD） analysis， and high-pressure mercury injection （MICP） experiments. Additionally， we quantitatively characterize the degree of reservoir heterogeneity using the coefficient of variation. The findings reveal that the Paleogene sandstone reservoirs consist primarily of feldspathic litharenites， with calcite identified as primary interstitial fillings. The porosity of the reservoirs decreases in the order of sandstone member Ⅰ of the Lulehe Formation， sandstone member Ⅱ of the Lower Ganchaigou Formation， and sandstone member Ⅱ of the Lulehe Formation. The reservoir rocks have undergone diagenetic processes such as compaction， cementation， and dissolution. Consequently， the reservoir heterogeneity decreases in the order of sandstone member Ⅱ of the Lulehe Formation， sandstone member Ⅱ of the Lower Ganchaigou Formation， and sandstone member Ⅰ of the Lulehe Formation. Low-permeability sandstones rich in plastic detrital lamina or matrix are identified as tight reservoirs， with calcite cementation and deformation of plastic clayey debris as the predominant factors contributing to reservoir tightness.

Key words: reservoir heterogeneity, microscopic pore structure, low-permeability sandstone reservoir, Paleogene, Mahaidong area, Qaidam Basin