Carbon and Nitrogen Isotope Measurements of Gas-Bearing Fluid Inclusions — A Tool For Tracing Gas Sources and Maturity of Source Rocks
Since gases in reservoirs may be accumulated from different sources it is of importance to trace regional migration paths especially in unexplored areas or commercially dry fields where no information about the composition, origin and fate of migrated gases can be obtained in due to the lack of petroleum samples from reservoirs or gas tests. In such frontier areas fluid inclusions can provide pointers to commercial hydrocarbon accumulations because they may contain considerable amounts of gases, such hydrocarbons, CO2, and/or N2.
For the past decades, several studies of isotope ratios in fluid inclusions have been performed using different techniques for the extraction of gases from inclusions and for the determination of isotopic ratios. Most of the studies used off-line or online sample preparation, either by thermal decrepitation of fluid inclusions or mechanical crushing. Subsequent measurements of isotope ratios of released gases were either done by using isotope ratios mass spectrometry (IRMS) or gas chromatography-mass spectrometry (GC-MS). The amounts of sample material being used typically varied between 1 to 10 g, or even more.
Here we describe a new analytical method for simultaneous measurements of natural gases in fluid inclusions using a sample crusher and an Elemental analyser (EA) combustion-IRMS system, including an EA, ConFlo III and DeltaplusXL mass spectrometer. The use of such an analytical setup provides a means to analyze the isotopic composition of simultaneously released N2, CH4, and CO2 from fluid inclusions. Even small amounts of sample material (0.1 - 0.5 g) produce enough gas to obtain reproducible values.
A case study being performed in the North German Basin demonstrates the use of gas-bearing fluid inclusions, as remnants of gas migration in the unexplored southern part of the Lower Saxony Basin, to obtain information about probable source rocks and their maturities. It turned out that gases in fluid inclusions are either be derived from type II or type III kerogens. Strong variations in carbon isotopic composition of methane-bearing inclusions indicate that the parent source rocks had achieved different degree of maturity in local parts of the basin at the time when the gases where released. Furthermore, the data obtained from simultaneous measurements of carbon isotopic composition of methane and nitrogen isotopic composition in fluid inclusions yielded evidence of different origin of nitrogen in some reservoirs.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California