Geochemistry of Formation Water and Implications for Ultradeep Tight Sandstone of DK Gas Field in Kuqa Depression

Abstract

Formation water is produced during gas well production, which can seriously affect gas well productivity, especially in deep and ultradeep tight sandstone gas reservoirs. In this paper, we have studied the formation water production and geochemical properties of the ultradeep tight sandstone gas reservoir in the Cretaceous Bashijiqike Formation of the DK gas field in the Kuqa Depression. The results indicate that the formation water can be classified as condensate water, gas-water transition zone water, mixed water, and isolated formation water, all of which are acidic and of CaCl2 type, with increasing salinity or chloride ion content. Sodium/chloride ratio (ρNa+/ρCl-), metamorphic coefficient (ρ(Cl--Na+)/ρ(Mg2+)), desulfurization coefficient ( 100 × ρ S O 4 2 − / ρ C l − ), and trace elements concentration parameters show that the formation water was formed in a closed and reductive environment and experienced strong water-rock interaction. The formation water may have been influenced by early seawater, but it is not significant and is more likely to have been influenced by water seepage from the overlying gypsum strata. The Sr, H, and O isotopes reveal that the different types of formation water have good homology and are not affected by atmospheric precipitation, which is propitious to the preservation of natural gas.

Characteristics of orderly hydrocarbon accumulation of deep reservoirs in Kuqa Depression and its exploration implications

“…It is located tectonically between the southern Tianshan Fold Zone and the Tabei Uplift. Influenced by the strong uplift of the southern Tianshan Mountains, a series of basements were involved in the thrust faults that developed in the northern piedmont belt of the Kuqa depression, and then most of the basements tilted inward to the basin (Figure 1) (Guo et al, 2022; Liang et al, 2018). The Kuqa depression includes four structural belts, three depression belts and one uplift belt.…”

“…The Kuqa depression includes four structural belts, three depression belts and one uplift belt. They are the northern monoclinal belt, the Kelasu tectonic belt, the Yiqikelike structural belt, the Qiuritage structural belt, the Yangxia Sag, the Baicheng depression, the Wushi depression and the southern slope belt (Guo et al, 2022; Liu et al, 2016; Xie et al, 2020; Yang et al, 2022). The sedimentary formations revealed by the drilling include the pre‐Triassic, Jurassic, Cretaceous, Palaeogene, Neogene and Quaternary (Figure 2).…”