Genesis and evolution of hydrocarbons entrapped in the fluorite deposit ofKoh-i-Maran, (North Kirthar Range, Pakistan)

The exceptional development of coeval hydrocarbon and aqueous fluid inclusi ons (FI) in fluorite from the MVT-type ore deposit of Koh-i-Maran, Baluchis tan (North Kirthar range, Pakistan), provides samples which are representat ive of the ore-forming fluid and which support the hypothesis of petroleum migration in the province. Primary brines at 125 degreesC (10 wt% equ. NaCl ) and secondary CH4-rich brines at 135 degreesC (7 wt% equ. NaCl), are reco gnised to be associated with oil migration in the fluid inclusions. They su pport the model of a per ascensum MVT (Mississippi Valley Type) stratabound hydrothermal deposit. A pressure-temperature path of 120-125 degreesC to 1 65-200 bars is calculated from microthermometric data and PVT modelling of hydrocarbon FI using the modified Peng-Robinson Equation of State (IFP soft ware) from primary cogenetic inclusions (oil and brines). The composition of gas and oil fractions is obtained by a combination of Sy nchrotron FTIR microanalysis and gas chromatography performed on individual fluid inclusions. The oil entrapped as a coeval primary fluid phase is a l ight aliphatic normal oil in the range C-delta-C-35 with a high CO2 content . The brown solid phase found systematically in the oil is probably asphalt ene resulting from precipitation after trapping of the heavy fraction, whic h commonly occurs by decreasing pressure and temperature and\or by CO2 inje ction. Later CH4-rich brine influx probably modified part of the oil in the primary fluid inclusions because degraded oil is observed within such incl usions. Biomarkers obtained by GC-MS analysis indicate a terpane distributi on quite similar to the nearest oil seepage in the Gokurt area. This result and the high CO2 content of organic fluid inclusions indicate a restricted /confined sedimentary environment for the source rock, which could correspo nd to the Eocene Carbonate formation with type-II organic matter. A possibl e additional input of gas from the Sambar formation is suggested as feasibl e, The link between the fluid inclusion data and the geodynamic evolution l ead us to propose a circulation of basinal fluids driven mainly by the faul t system during dewatering in the foredeep. In Pakistan, they are coeval to major compressional NW-SE Oligocene episode in the thrust belt. The origin of the fluorine may be found in the basin sediments as well as near the ba sement. The brines originated in salt structures recognized in eocambrian a t the decollement level, the source rock was already mature. (C) 2001 Elsev ier Science Ltd. All rights reserved.