Xiaoqi Ding

The He 1 Member of the Xiashihezi Formation (Upper Paleozoic) in the Ordos Basin represents typical tight sandstones. However, against the extensive tight sandstone background of the He 1 Member in the southern basin, conventional reservoirs (Φ>12%, K>1 mD) occur locally. Elucidating the genetic mechanism of these conventional reservoirs is critical for evaluating gas reservoirs in this region. Based on core descriptions and systematic sampling from cored wells, reservoir types are classified according to pore types and porosity in sandstones. Depositional microfacies, petrology, and diagenesis of each reservoir type are then investigated to ultimately elucidate the genetic mechanism of conventional reservoirs. Results demonstrate that intense compaction and quartz overgrowths are the primary controls on the development of the He 1 Member tight sandstones. Alteration of volcanic lithic fragments and volcanic ash matrix generated abundant intragranular dissolution pores and micropores within matrix, while simultaneously producing substantial illite-smectite mixed-layer clays and chlorite clays. Additionally, this process supplied silica for quartz overgrowths. Moderate amounts of chlorite coatings can inhibit quartz overgrowths, thereby preserving residual intergranular porosity. Conventional reservoirs exhibit low lithic fragment content and coexisting intergranular and intragranular dissolution porosity. Their formation requires weak compaction, intense dissolution, and weak quartz overgrowths. These reservoirs develop within high-energy transverse bars that are sealed by overlying and underlying mudstones. Such transverse bars constitute closed intrastratial-diagenetic systems with restricted mass transfer during burial. This study provides a compelling example of diagenetic heterogeneity induced by variations in sandstone architecture within fluvial successions.

The He 8 sandstone unit in the Western Sulige field is one of the most prolific gas‐producing intervals in the Ordos Basin. Sedimentologic and reservoir characteristics analysis were conducted to investigate the reservoir's petrological, diagenetic, petrophysical properties, and production characteristics. Three sandstone types and four lithofacies were identified. The two most commonly observed lithofacies are labelled lithofacies C and D. Lithofacies C is composed of fine‐ to medium‐grained litharenites (Sandstone I); lithofacies D includes coarse‐grained sublitharenites (Sandstone II) and coarse‐grained/gravelly litharenites (Sandstone III). Compared with Lithofacies C, Lithofacies D was deposited as a series of laminated sand sheets in a higher energy fluvial system. The tightness of Sandstone I is a result of an episode of intense compaction of the rock framework causing early‐diagenetic dissolution of grains and deformation of ductile rock fragments. The processes that led to the low permeability of Sandstone II and Sandstone III are more complex. In addition to intense compaction, the growth and emplacement of quartz and clay cements are a critical factor. Diagenesis and porosity evolution analysis show that the petrophysical properties of these sandstones were better at the gas charging time. The porosity and permeability values of Sandstones II and III are similar at surface, but significantly different at formation pressure. This leads to differences in gas production rates. Sandstone II has higher gas production rates and cumulative gas production than Sandstone III. The presence or absence of Sandstone II controls the distribution of sweet spots within less productive tight gas reservoirs.