Subsurface Waters, Upper Cook Inlet, Alaska, Part I - Geochemistry

Stephen G. Franks, Zhiyong He

Pore waters in non-marine Tertiary sandstones in Upper Cook Inlet basin are mixtures of two end members. Fresh waters in deeper parts of the basin are derived from Tertiary depositional waters. These are generally low salinity (commonly <5,000 ppm TDS), HCO₃-Na, waters with very light ^dgrv¹⁸O and ^dgrvD. A second, more saline, water type is also found in the Tertiary non-marine section, especially in sands near the Mesozoic unconformity or where permeable conduits (faults or stacked sandy facies) are in fluid communication with Mesozoic marine sediments. This water is typically a Na-Cl fluid, in places significantly enriched in Ca⁺⁺, with salinities approaching that of sea water ±35,000 ppm). Its oxygen and hydrogen isotopic compo ition is relatively heavy compared to the low-salinity waters. These saline waters are derived from diagenetically-altered Mesozoic sea water which has been expelled up into the Tertiary non-marine section

Oil shows and accumulations are associated almost exclusively with the saline waters, indicating that Mesozoic oil and water used the same migration pathways. Migration of water and oil was controlled by the distribution of fine and course grained facies and by faults. There is no evidence of diagenetically-sealed fluid "pressure compartments" (Hunt, 1991). The association of Mesozoic waters with oil accumulations is compatible with interpretations, based on petroleum geochemistry, of a Mesozoic oil source. Mapping of water chemistry provides a method of tracking potential oil migration pathways in Upper Cook Inlet

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995