Origin and Geochemical Evolution of Formation Water in the Dameigou Shale Gas Reservoir in Northern Qaidam Basin

Abstract

The origin and geochemical evolution of formation water trapped in continental shale of the Dameigou Formation in the Northern Qaidam Basin have been investigated for selecting unique signatures of formation water compared with shallow groundwater and conventional oilfield brines (COB). The formation water, which is trapped in a closed shale with no halite and sylvite in it, has a higher Cl/Br ratio (1183.43) than that of seawater (657.89). Furthermore, δ 2 H (−67‰) and δ 18 O (−8.1‰) values of the formation water lies near the local evaporation line. These suggest that the formation water originated from evaporated rainwater. The temperature (>120°C) of the Dameigou Formation and the vitrinite reflectance values (1.0-1.3%) of the shale indicate that thermochemical sulfate reduction (TSR) was responsible for the depletion of SO 4 in the formation water. The increased CO 2 concentration and Mg/Ca ratio in formation water leading by TSR caused kaolinization of plagioclase feldspar and dolomitization, respectively. The formation water depletion in Ca and HCO 3 was caused mainly by calcitization. The Na/Ba ion exchange and dissolution of continental borate occurring in the shale-formation water system were vital to the geochemistry of trace elements in formation water. All these geochemical processes have given the formation water distinctive δ 34 S-H 2 S (25.9‰), 87 Sr/ 86 Sr ratio (0.713842), Ba/Ca ratio (2.60×10 -2 ) and δ 11 B (1.1‰) signatures compared with COB in shallow groundwater.

Sensitivity Assessment of Boron Isotope as Indicator of Contaminated Groundwater for Hydraulic Fracturing Flowback Fluids Produced from the Dameigou Shale of the Northern Qaidam Basin

“…These indicate that the δ 11 B of groundwater is expected to have a significant response to continental HFFF contamination in the study area. The boron isotope characteristics of the HFFF were mainly evolved by the mixing of a shale formation water in which dissolution of non-marine borates (−30.1 to 10.2‰ with mean value of 4‰) has occurred during the formation of the Dameigou Shale, and the water-rock interactions were dominated by the dissolution of boron-bearing alkali feldspars during hydraulic fracturing and subsequent flowback periods [42]. Conversely, the boron isotope characteristics of the shallow groundwater were mainly imparted by the local rainwater (δ 11 B = 16.7‰), which is the major source of groundwater recharge, and groundwater in specific aquifers (NG2) was further altered by clay adsorption [43,44].…”