K/Ar Dating of Illite in Understanding Fault-Related Diagenesis
K/Ar dating of diagenetic illite provides quantitative information regarding the timing of major geological events within sedimentary basins. K/Ar data, however, have to be evaluated in the framework of sedimentological, mineralogical and structural configuration. These information together with other quantitative data can reveal the dynamics of diagenetic events, such as timing, condition, fluid source, and their relationship to structural/tectonic processes.
In several Rotliegendes gas fields in the North Sea basins, abundant 1M(cis) illite are found in some reservoirs. These samples are located near fracture/faulted zones. Synchrotron diffraction data and WIDEFIRE illite polytype modeling show that these illites consist of single 1M(cis) enantiomorph. Morphologically, they are platy, and form grain-coating clays oriented perpendicular to the grain surface. In these samples, detrital feldspars and lithic fragments are severely altered and pore-filling quartz cements, co-precipitated with illites, are extensive. In samples away from fractured/faulted zones, detrital feldspars are less altered and only moderate to minor amounts of 1M(trans) illites are found. K/Ar data show that illite in samples closest to the fractured/faulted regions fro ed first. Stable oxygen isotopes of illite and hand-picked quartz cements and fluid inclusions show hot, high salinity(>20 wt.%), CaCl2-rich fluids were involved in these diagenetic processes, suggestive of hydrothermal origin. Our data and other published data show that these processes may be very common in rift settings, and significant reservoir heterogeneity could be generated by fault-related diagenesis.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California