Abstract
In sandstones, siliceous diagenetic overgrowths may commonly contain several generations of hydrocarbon fluid inclusions (FI's), generally of size no more than 10 µ. We successfully analyzed such inclusions in North sea sandstones from the Dunbar area, subthrusted reservoirs such as sub-Andean basins (Venezuelian and Colombian foothills) and carbonaceous series in areas of Pakistan, using synchrotron infrared microscopy. We addressed, for each case studied, molecular identification and mapping of the components, i.e. hydrocarbon content, and additional molecules such as CO2 , H2 O, in small volumes in their liquid and gas phase as well as the solid phases often associated as products or mechanically entrapped minerals. Semi-quantitative analysis leads to determination of the average composition of each inclusion, i.e equivalent aliphatic chain length (CH2 /CH3 ratios), CO2 and CH4 content. The quantitative method for hydrocarbon chain length measurements was tested first on some exceptionally large FI's (up to 500 µm) previously analyzed by gas chromatography after individual extraction. Homogeneity and /or heterogeneity of trapping as well association of oil with water or their separation is shown for each generation of FI corresponding to a specific organic fluid migration in sandstones from the Brent formation of Dunbar (North sea), and in siliceous reservoirs of subandin basins (Venezuelian and Colombian foothills). Microanalyses of solid phases closely associated within oil in such FI's indicate conditions of the dissolution-recrystallisation processes. Results are particularly promising in carbonaceous reservoirs of Northern Pakistan (Chorgali area, North Potwar basin) where first results pinpoint the effect of TSR mechanism in environments where evaporites were first deposited. PVT modeling of isochores using semi-quantitative SFTIR analyses leads to reconstruction of P-T conditions of diagenesis. Therefore by combining these results with data from organic matter (Genex modeling) and petrography, the onset of cementation was established in the Venezuelian and Colombian Foothills. The path of fluid migration along geological history was then reconstructed with Thrustpack 2D thermal modeling (IFP software), in Tunisian and Pakistani ore deposits and in the carbonaceous reservoir of Chorgali.
Highlights
Hydrocarbon bearing fluid inclusions (FI’s) can be systematically entrapped as small droplets of oil/gas in diagenetic minerals along migration paths of organic matter in numerous reservoirs
Comparison of the measured CH3/ CH2 ratios with those measured in exceptionally large fluid inclusions of known composition analyzed individually after extraction by GCMS, (Guilhaumou et al, 1990 a) allows us to define for the aliphatic parts an equivalent alkane chain length which is relative to the molecular CH3/CH2 ratio for a given compound
The combination of in situ micro FTIR analyses using a synchrotron source in the mid infrared range with Pressure Volume Temperature (PVT) modeling of included oil is very appropriate for collecting informative data from individual hydrocarbon fluid inclusions
Summary
Hydrocarbon bearing fluid inclusions (FI’s) can be systematically entrapped as small droplets of oil/gas in diagenetic minerals along migration paths of organic matter in numerous reservoirs. The molecular composition of each oil FI can be assessed by in situ FTIR microanalysis, which is a non destructive technique, and may be correlated directly to its density measured by microthermometry These combined data allow PVT modeling and calculation of the isochoric path of a single generation of fluid inclusion. A combination of trapping temperature and chemical composition of the oil with isochoric PVT modeling of oil bearing fluid inclusions lead directly to information on the PressureTemperature (PT) conditions of the fluid circulations and the associated mechanism of diagenesis occurring in the sediments during compaction and syntectonic deformation in subthrusted petroleum reservoirs (Mc Limans, 1987; Larroque et al, 1996, Guilhaumou et al, 2000). Progress in isochoric modeling using modified Equations of State (Ungerer and Batut, 1997; Mougin, 1999; Aplin et al, 1999; Guilhaumou et al, 2000; Thiery et al, 2000) leads to the possibility to calculate thermodynamic properties, i.e the phase envelopes and isochoric P-T path from analytical data of petroleum FI’s
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