PACROFI VI - Electronic Program


Gases in Fluid Inclusions in Veins Cutting the Bald Eagle and Adjacent Formations, Central Pennsylvania, USA

Barker, Colin, Sullivan, George and Underwood, William


Mass spectrometric analysis of individual fluid inclusions, together with microthermometry, were used to study compositional changes in the fluids that moved through an Ordovician clastic sequence in central Pennsylvania. The section consists of the Reedsville shale, which is overlain by the once uniformly red Bald Eagle and Juniata sandstones. The lower portion of the Bald Eagle has been altered to a green color, with the color boundary varying as much as 200 meters above the Reedsville but having no lithostratigraphic significance. The color change was probably caused by the passage of reducing fluids. Joint-frequency analysis indicates that the green sandstones are more fractured than the red sandstones suggesting that the fluids and fracturing were related (Lacazette, 1991).

Gases in individual fluid inclusions are released by heating a ~10 mg mineral sample in a vacuum system that forms the inlet to a pair of high speed, computer-controlled, UTI 100C quadrupole mass spectrometers. As temperature rises, phase relationships in the inclusions change in such a way that the pressure rises rapidly and ruptures the inclusions, releasing the gas for analysis. For the geometry of our vacuum system the burst of gas lasts about 25 -30 msec. The mass spectrometers scan continuously and take 18 msec per mass spectrum. The analog signals are digitized (16-bit) at 200 kHz, and the data transferred to CD-WORM for subsequent off-line processing. During processing the fluid inclusion burst is recognized by the abrupt increase in mass spectrometer response, and the immediately preceding spectra are subtracted to remove background. Data can be displayed as individual mass spectra or, for groups of inclusions, as ternary diagrams. The system has been described by Barker and Underwood (1992).

Analysis of gases in individual fluid inclusions in quartz and calcites that filled fractures shortly after fracturing show a systematic compositional trend of decreasing methane from the Reedsville shale up through the Bald Eagle sandstone and into the Juniata sandstone. Fluid inclusions compositions in quatrz from the Reedsville shale range from methane-rich with minor water, to water-rich with minor methane. There are no higher hydrocarbons or hydrogen sulfide. In contrast, the two-phase inclusions in the Juniata are exclusively water-rich. Quartz and calcite from the intermediate Bald Eagle have water-rich inclusions with minor methane. Little methane appears to have reached the Juniata sandstone even though fractures with green, reduced halos are observed. The accumulated data support the concept that lithostatically-pressured, methane-rich fracturing fluids from the organic-rich Reedsville shale fractured and reduced the overlying Bald Eagle sandstone before pressures returned to hydrostatic as the fractures intersected the meteoric groundwater system. Fluid inclusions in the Juniata tend to be less saline and poorer in methane than the fluids in the underlying Bald Eagle and Reedsville.

We are pleased to acknowledge the help and cooperation of Dr. Lacazette who kindly provided sample materials and supporting documentation. In addition helpful discussions improved our understanding of the geology of the area.

This research was supported by the DOE (DE-FG05-85ER13417).

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Figure

Examples of 15 amu - 17 amu - 18 amu ternary diagrams (i.e., methane (15) - water (17/18), and selected mass spectra for individual inclusions.

Top : Ternary diagram with data for 311 individual fluid inclusions from quartz in the Juniata sandstone showing a very water-rich composition.

Middle : Ternary diagram with data for 1846 individual fluid inclusions from quartz in the Bald Eagle sandstone showing a composition dominated by water but with some methane-bearing inclusions.

Bottom : Ternary diagram with data for 406 individual fluid inclusions from quartz in the Reedsville shale with a wide range of methane-water ratios. Note that the trend does not go through the 15 amu corner (CH3), but is displaced towards 17 amu due to the contibutions from 13CH4 (with some CDH3).