Study On Genesis Of Formation Water And Water-rock Interaction Of Upper Triassic Xujiahe Formation, In Middle Area, Western Sichuan Depression

The member 2 and 4 of the Upper Triassic Xujiahe Formation(T3x2 and T3x4) in Middle area, Western Sichuan depression are important deep formations of natural gas exploration, so much more attention is given to the water-rock interaction of the T3x2 and T3x4 sandstones because of their seriously tight reservoirs.In this paper, the formation water genesis,the main water-rock interactions and their evolution were fully studied using geochemical, petrographic and mineralogical information on water-rock interaction from the formation water and the rock. Combined with the geological background of the study area, then the sequence of main water-rock interaction was establishesed, the water-rock interaction mechanism was also discussed. Finally, the conceptual model of water-rock about aluminum silicate and carbonate rocks was constructed in a bid to explain the water-rock interaction in the T3x2 and T3x4 sandstones respectively.First the geochemical characteristics of the formation water was analyzed, the T3x2 water salinity is bimodal distribution, the high and low salinity is mainly Cl^-～Na⁺ and HCO₃^-～Na⁺ (or HCO₃^-～Na⁺～Ca2+) type respectively. The water salinity of T3x4 is between the high and low salinity peaks of T3x2, and the water type is almost Cl^-～Na⁺. A variety of chemistry parameters, such as sodium-chloride coefficient, deterioration and desulfurization coefficients shows that the formation water was high concentration and deep deterioration, which was conducive to the preservation of oil and gas.Based on the characteristics of the formation water, 6 categories of the combination of different ions (or isotopes), whch could be used to summarize the formation water sources (or mixing), biochemical effects, and water-rock reactions were proposed. The main sources of T3x2 and T3x4 formation water ware primary sedimentary water, bromine-rich compacting fluids from the adjacent shale and coal beds, atmospheric water and marine water. The similarity of Cl^-～Br-～Na⁺ combination have been resulted from the bromine-rich compacting fluids initial mixtuxe, so the enrichment of bromine can be used as a symbol of ballast water discharged. The Caexcess～Nadeficit and Cl^-～K⁺ combinations reveal that the T3x2 formation water has remarkable marine formation water features, on the contrary, the T3x4 is obvious atmospheric water. The other combinations of Cl^-～HCO₃^-～SO₄^2-, Ca²⁺～Mg²⁺～Fe²⁺,Li⁺～Cl^-～δ18O and Caexcess～Nadeficit of formation water inddcate that many water-rock reactions that was proved to be compeletely correct by diagenetic evidence also occured in water-rock system.Burial and thermal history of T3x2 and T3x4 was recovered using Petromod 10. Based on a combination of diagenetic evolution and burial and thermal history, the evolutionary sequence of water-rock was constructed and divided into three phases: (1) compacting fluid mixing stage; (2) fluid evolution stage related to the diagenesis; (3) fluid evolution phase related to the tectonic. The fluid evolution related to the diagenessis ended as the formations reached to the maximum burial depth. The fluid underwent the extensive re-distribution and mixing assiociated with the upliftings and denudations of formations.The sources and cycles of Ca²⁺, Mg²⁺, Fe²⁺, K +, SiO2, CO2 (HCO₃^-) Br-and CH3COOH in water-rock system, the water-rock types associated with those ions above and corresponding water-rock mechanism were comprehensively analyzed and discussed to establish the conceptual models of aluminum silicate and carbonate cementation through a large number of cases of water-rock reactions in the work area and other area. In the model, mudstones do not serve as a closed role, on the contrary, is first object that the water-rock system is open to, mudstone (or coal) is also a material provider and recipient of the water-rock system, and is an integral part of the system.The most feldspar in T3x2 was effectively preserved due to high K⁺ / H⁺ activity ratio in T3x2 formation wates, but the lack of feldspar in T3x4 was main attributed to tectonic factor which controlled the source of feldspar, and the low K⁺ / H⁺ activity in T3x4 formation wate which reduced rate of the kaolinite illitization, but accelerated the dissolution of feldspar made greater difference in feldspar content between T3x4 and T3x2 than before. In study area, the characteristics of authigenous minerals and formation water indicate the potassium feldspar dissolution, precipitation of kaolinit, quartz cement and illitization were a systematic series of water-rock reaction,such that the dissolution feldspar-K + mobility - illitization in the ternary system would be controlled by the slowest rate of the three reaction steps, but not to be wheather the water-rock system was closed or not.The carbon and oxygen isotope and petrologic characteristics of carbonate cements suggest there were differences in carbonate cementation mode between the T3x4 and T3x2 water-rock reaction systems. Especially, the water-rock reaction system of the T3x4 was more open than that of the T3x2, and the T3x4 shows significantly more acidic fluid flow than the T3x2. As a result of adequate water-rock reactions in the T3x4, its formation water was rich in 18O. Other difference is the lack of organic CO2 record in T3x2 carbonate cements which is considered to be caused by the acidic fluid flow and lithology of source rock, the tectonic uplift and erosion, and CO2 migration fractionation had little impact.