A NEW METHOD FOR SYNTHESIZING FLUID INCLUSIONS CONTAINING ORGANIC AND INORGANIC MATERIAL

A new method has been developed for synthesizing fluid inclusions in fused-silica capillary tubing having either round (0.3 mm OD and 0.05 or 0.1 mm ID) or square cross sections (0.3 x 0.3 mm with 0.05 x 0.05 mm or 0.1 x 0.1 mm cavities). Organic and inorganic samples in solid or liquid forms were first loaded in the tube, about 6 cm long with one end sealed, and then centrifuged to the enclosed end. Gaseous samples were added cryogenically at about 0.2 MPa pressure after air was evacuated. The open end of the tube was then sealed by a hydrogen flame while under vacuum. For microthermometric measurements in a USGS-type heating-cooling stage, sample capsules of less than 2 cm in length were prepared.

The square-sectioned capsules (SSC) have the advantage of providing sample images without optical distortion. However, the internal pressures they can hold are much less than those held by the round-sectioned capsules (RSC). Our microthermometric measurements of pure H₂O samples showed that the SSC having 0.1 x 0.1 mm cavity could hold pressure of 123 MPa at 500 °C, but exploded at 550 °C and less than 159 MPa. On the other hand, the RSC having 0.1 mm ID can hold H₂O pressure of at least 333 MPa at 600 °C, and CO₂ pressure of at least 350 MPa at 650 °C.

The fused-silica capsule method is particularly useful for the study of organic samples, including natural oil and gases. Cracking of C₁₈H₃₈ was studied at various bulk densities at 350, 375, and 400 °C, and the relative amount of methane, ethane, propane, and n-butane were quantified by Raman spectroscopy as a function of reaction time. Isotopic exchanges between C₁₈H₃₈ and D₂O and between C₁₉D₄₀ and H₂O were studied at 375 °C and between 350 and 400 °C, respectively. Other studies include (1) phase relations in the Smackover oil-water-vapor system; (2) the reaction of methane with water and of ethane with water at 206 °C; and (4) characterization of Raman signals of CO₂ from room temperature up to 400 °C for various bulk densities (0.06 to 0.94 g/cm³).

When compared with the conventional method, in which fluid inclusions were formed by healing fractures in quartz chips at elevated P-T conditions, the new fused-silica capsule method has the following advantages: (1) simple; (2) large and uniform inclusions can be formed; (3) able to tolerate high internal pressure; and (4) suitable for the studies of organic material.