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The study of geopressured formations has provided considerable information on the probable pathways for subsurface fluid movement. The fluids have been traced and associated with structure, pressure distribution, salinity of formation waters, various organic and inorganic diagenetic effects, as well as local changes in the geothermal gradient and the temperature of formations. The temperature changes may be measured directly or inferred from the presence of temperature-controlled reaction products such as the modification of illite/smectites.
Clay mineral changes are detected initially at temperatures as low as 50°C and may extend to temperatures in excess of 300°C. The smectite-illite conversion is most pronounced in the range from 50°C to about 160°C. Significant changes in kaolinite and chlorite occur between 75°C and 250°C.
In shales from the Gulf Coast, the smectite-illite conversion is readily recognized, while kaolinite-chlorite reactions are most apparent in associated sands. In several places the development of kaolinite in sandstones is directly linked to the movement of high temperature fluids and the subsequent blocking of secondary porosity. Kaolinite is most abundant in those zones which experienced maximum flushing.
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