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Relationships Between Diagenesis and Reservoir Quality in Upper Tomma Formation, Haltenbanken, Offshore Mid-Norway

S. N. Ehrenberg

The principal hydrocarbon reservoir of Haltenbanken is the upper member of the Middle Jurassic Tomma Formation, a shallow marine to fluvial sandstone, generally 40-110 m thick, with medium to fine grain size and very low contents of mica and detrital clay. Upper Tomma Formation cores from 21 wells display good overall correlation between mean porosity and depth, but there is considerable scatter in the mean values of both porosity and permeability for the 9 cores from depths greater than 3.5 km. Better correlations are obtained by plotting reservoir quality vs. indices of thermal maturity, either calculated time-temperature index or measured vitrinite reflectance. There appears to be a threshold value of thermal maturity equivalent to VRo 0.7-0.8 above which th primary pore system is largely destroyed.

This destruction occurs by compaction and quartz cementation, whereby the intergranular pores are reduced in volume, and by illitization, whereby remaining macroporosity is converted to microporosity. The illitization involves extensive replacement of earlier authigenic kaolinite and dissolution of detrital potassium feldspar, resulting in large secondary macrospores and a lowered whole-rock potassium/aluminum ratio. Because of the feldspar alteration in the cores with higher thermal maturity, we see a positive correlation between total porosity and feldspar content and a negative correlation between total porosity and secondary porosity when all of the deeper Tomma cores are plotted together. A major unresolved question is the timing of quartz cementation and illitization with respec to both the burial history of the Tomma Formation and the emplacement of hydrocarbons.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.