Significance of Reservoir Diagenetic Alteration for Petroleum Exploration, Gulf of Alaska Tertiary Basin: ABSTRACT

Abstract

The Tertiary clastic section of the Gulf of Alaska sedimentary basin is considered an important potential future petroleum province. Considerable work in the onshore area has been carried out by industry preparatory to a projected sale of federal leases on the continental shelf. Examination of numerous samples collected from measured sections in Oligocene to Pleistocene strata has shown that the sandstones are mineralogically immature and unstable. Diagenetic alteration of these potential reservoir sands at shallow to intermediate depths of burial has resulted in the ubiquitous formation of authigenic clay rims and coats around detrital grains and of pore-filling zeolite cements. These diagenetic alterations adversely affect both porosity and permeability and will provide a major limitation on the thickness of the sedimentary section than can be considered prospective. A compaction versus depth gradient, which is related to the sandstone porosity gradient, can be determined in offshore areas by combination of seismically derived interval, velocity depth profiles, with a velocity-density calibration based on well data from the Hecate Strait (a geologically similar Pacific Margin basin lying off British Columbia). The resultant density configuration can be cross checked by gravity modeling along the same seismic line. Work on a line off Cape Yakataga shows that the prospective section is characterized by a high velocity and density gradient, indicating rapid loss of reservoir porosity with increasing depth of burial, as predicted by the onshore diagenetic m del. Semiquantitative evaluation of the compaction profile using the onshore reservoir data suggests that even in the youngest sections porosity decreases to less than 20% at depths of 5,000 to 8,000 ft. End_of_Article - Last_Page 1898------------