Subsurface Fluids _mostly water_

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					                                                       Pore Space
           Subsurface
                                    • Sedimentary rocks have 0 - 40% open space
             Fluids                   (porosity)
                                    • Typically 5 - 20%
            (mostly                 • Filled with:
                                       – Mostly water
             water)                    – Rarely oil, natural gas, CO2, etc




Types of Water                                        SEM images


          •Free Water
                                                                        Quartz Sandsonte
          •Irreducible Water
             •Bound to grains by
             surface tension
             •In clays


                                   Kaolinite (clay)




Clay Structure
                                                  Sources of Water
                                    • Meteoric
                                    • Connate Water
                                    • Juvenile Water
               Meteoric Water
                                                                           Connate Water
•   Surface water
                                                   •   Basin water trapped during sedimentation
•   Fresh (low salinity)                           •   Starts as seawater
•   Oxidizing                                      •   Saline
•   Acidic (organic acids)                         •   Reducing (low Oxygen)
•   High bicarbonate (HCO3 -)                      •   Alkaline
                                                   •   High Na+, K+, Cl-
•   Low Ca, Mg




             Water Chemistry
                                                                           Juvenile Water
                                                   •   From magmatic or metamorphic sources
                                                   •   Not common in petroleum systems
                                                   •   Important for metallic mineral deposits
                                                   •   Distinguished by O isotopes




                                 Selley, 1998




                                                                         Salinity vs. depth
                     Salinity
                                                                           Salinity (ppm)
                                                                     0
•   Total dissolved solids in ppm or mg/liter
                                                                                  Meteoric water circulation
•   Seawater is 3.5% or 35,000 ppm salinity
                                                        Depth (km)




                                                                     1
•   Oilfield brine up to 64 % salt (640,000 ppm)
                                                                                  Rapid increase with depth
•   Usually less than 350,000 ppm
                                                                     2

                                                                     3
                                                                                      Reducing environment,
                                                                                      Salinity uniform with depth
                                                                     4
                       Applications of Water Chemistry
Salinity of            • Clues to petroleum system
Pennsylvanian
Cherokee sandstones,
                       • Evolution of porosity
Oklahoma                 (cementation/dissolution)
                       • Calibration of electrical resistivity logs
                          – Calculations of petroleum saturation



  Dickey, 1979

				
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