Xuan Liu

In the western Tarim Basin, Carboniferous granular dolostones deposited on a carbonate platform contain a small amount of terrigenous materials of sand-size fraction, agglomerated clay minerals, or similar phases. However, the role of terrigenous materials on dolomitization is still unclear. The aim of this study was to reveal the dolomitization mechanism. The granular dolomites have small crystal size, earthy yellow color, and fabric-retentive texture, with relatively good order. These features indicate dolomites precipitated during early diagenesis. The ratio of rare earth elements (RREs) abundance of the stable isotopes ⁸⁷Sr/⁸⁶Sr relative to Post-Archean Australian Shale (PAAS) normalized patterns was used to study the source of the dolomitizing fluids. The composition of REEs is characterized by heavy rare earth (HREE) enrichment (average Nd_SN/Yb_SN = 0.83). There is a positive (La/La*)_SN anomaly and slightly positive (Gd/Gd*)_SN and (Y/Y*)_SN anomaly; δ¹⁸O of seawater in fractionation equilibrium with granular dolostones was from −2.8‰ to 1.7‰ PDB, implying the dolomitizing fluid was contemporary, slightly evaporated seawater. The granular dolostones on the relatively thick shoals were subject to subaerial exposure before pervasive dolomitization, with evidence that the input of detrital kaolinite predated the formation of dolomites. Higher ⁸⁷Sr/⁸⁶Sr values and ∑REE in granular dolostones than the values in equivalent limestones indicate that dolomitization was related to terrigenous materials. Within the terrigenous materials, the negative-charged clay minerals may have catalyzed the dolomitization, resulting in dramatically decreased induction time for precipitation of proto-dolomites. A greater amount of terrigenous materials occurred on the shoals at the sea level fall, resulting from enhanced river entrenchment and downcutting. As a result, after subaerial exposure, the penesaline water flow through the limy allochems sediments lead to dolomitization, with the catalysis of illite on relatively thick shoals.

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.