The Geochemical Characteristics Of Formation Water And Preservation Condition Of Oil And Gas In Middle Area,western Sichuan Depression

Formation water is an important part of fluid in the oil and gas basin, the formation and displacement regular of which has a very close relationship with the generation and transport of oil and gas and reservoir formation, preservation and destruction. Western Sichuan Depression is the foreland basin since the Late Triassic foreland basin evolved gradually. Depositing a thick Late Triassic-Cretaceous stratum. Many gas-bearing series have been found in the middle part of western Sichuan depression, in which highly mineralized formation brines is ubiquitous. In geological history, there are prevalent exchanges of matter and energy among formation water, surrounding rock and oil-gas. And geochemical characteristics of formation water directely records these reaction process, including much information about the oil-gas generation and preservation. This article is based on datas of formation water in the Upper Jurassic(J3), the Mid-Jurassic(J2) and the member 2 and 4 of the Upper Triassic Xujiahe Formation(T3x2 and T3x4), detailedly analyzing basic geochemical characteristics of formation water in the study area. Combining with the geological background, this essay comrehensively elaborates the causes and initial source of fornation water, discussing the hydrogeological preserve condition.At first, the article distinguishes the property of the water out of gas wells in the study area, defining the water sample the TDS content of which is less than 1000mg/l as consendsate water, and defining the water sample the TDS content of which is more than or equal to 1000mg/l as formation water. By comparing formation water salinity characteristics of each series of strata, we conclude that salinity of the Upper Triassic formation water significantly greater than the Jurassic. And the Upper Triassic formation water are mainly brine, but the Jurassic water are saline. In the Upper Triassic strata, thr formation water salinity of T3x2 is greater than that of T3x4; In the Jurassic strata, the Middle Jurassic formation water salinity is greater than the Upper Jurassic. Overall the study area has a water salinity increased with the depth of the normal evolution of the characteristics.The main type of formation water are CaCl2 and and small amounts of MgCl2 type and NaHCO3 type, the more amounts of type of Na2SO4 could be only found in the Upper-Jurassic. From the Upper Jurassic to the member 2 of the Upper Triassic Xujiahe Formation, Water by Na2SO4 type gradual transition to CaCl2 type, salinity gradually increased. And the main cationic is Na+, the main anion is Cl- in formation water. Different layers produced formation water content cation size relationships are expressed as Na+>Ca2+>K+>Mg2+, the relationship between the size of formation water anion content of the member 2 and 4 of the Upper Triassic Xujiahe formation and the Middle Jurassic shows Cl->HCO3->SO42-, but the formation water anion of the Upper Jurassic shows Cl->SO42->HCO3-.By comparison with the oceans, lakes and rivers in the trace element content, the study area shows obviously enrichment of trace elements Li, Sr, Ba, Br and Fe, which Indicates that the study area has experienced a degree of formation water evaporated and water-rock interaction effects. The Xujiahe formations specially enrich Br, the main reason may be enriched in organic matter source.Based on detailed geochemical characteristics and different causes of formation water, the fomation of NaHCO3 water is most likely associated with the CO2 of organic origion in the twice and fourth member of Xujiahe formation. And the type of NaHCO3 and Na2SO4 reflect primary terrestrial formation water mostly. The type of the Na2SO4 may be the mixture of the CaCl2 andthe NaHCO3 in the 2 member of Xujiahe formation. The type of the MgCl2 of the twice member of Xujiahe formation is more likely to represent the original marine formation water features, and the type of the MgCl2 in the Upper Jurassic is the mixture of the CaCl2 in the lower Xujiahe and the NaHCO3 of the Upper Jurassic. The type of CaCl2 of the Upper-Mid Jurassic mainly origins from the high salinity water of the lower Xujiahe.The chemical formation feature of formation water reflects that the water all come from seawater, and evaporation degree is week, not reaching phase of gypsum percipitation. The components of formation water are characterized by lower seawater evaporation, and the Jurassic formation water is the mixwater of terrestrial fresh water and the external water in the Upper Triassic.The analysis of hydrogen and oxygen isotopic shows that the study area’s fomation water is mainly mixorigion of seawater and atmospheric water. The formation water of the Upper Triassic is mixwater, and Jurassic formation water features rich seawater. Influenced by isotope exchange between formation water and carbon dioxide, little Upper Triassic formation water shows obvious δ18O negative decreasing, presenting cause character of atmosphric water. But actually it is still mixwater. The Jurassic formation water are mainly mixwater, the Middle Jurassic water is richer seawater components, and some Jurassic formation water is still characterized atomosphic water.Formation water chemical character parameters, such as chloride coefficient, coefficient of desulfurization, carbonate balance coefficient and coefficient of magnesium chloride, etc, shows that most of the formation water features high concentration degree and deep metamorphic gradein the study area. The Jurassic strata may have certain openness, especially the Upper Jurassic exists lots of formation water of the Na2SO4 type, and the oil-gas preserve conditons are relatively poor. Otherwise, the 2th and 4th member of Xujiahe formation are relatively closed, contributing to oil and gas preservation.