--> Porosity/Depth Trends in Upper Jurassic Reservoirs, Norwegian Central Graben: An Example of Porosity Preservation at Deep Burial by Grain-Coating Micro-Quartz, by M. Ramm; #90986 (1994).

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Abstract: Porosity/Depth Trends in Upper Jurassic Reservoirs, Norwegian Central Graben: An Example of Porosity Preservation at Deep Burial by Grain-Coating Micro-Quartz

Mogens Ramm

A successful prior-to-drilling prediction of reservoir quality in deeper parts of sedimentary basins, requires an understanding of diagenetic processes and quantitative models on how porosity is related to sandstone composition and depositional facies. In the present study it is attempted to identify empirical porosity/depth trends and to relate these trends to models on mechanical and chemical compaction.

He-Porosity and petrographical data from wells in the Central Graben, North Sea, are used to assess the quantitative relations between porosity. depth and mineralogy. In this area. neither the timing of oil-emplacement nor pore-pressure and temperature variations have a controlling effect on the porosity variations. The porosity variations merely reflect differences in the framework grain stability and susceptibility for chemical compaction. The framework stability and the amount of porosity loss by mechanical compaction is dependent on clay content. Chemical compaction causes porosity reduction in many clean sandstone units during burial from about 2.8 to about 4 km. Hence, good reservoir quality beneath 4 km requires high framework stability (ie. low clay content) and retarded chemi al compaction. In some of the sandstone units, micro-crystalline quartz coatings on framework grains have efficiently hindered quartz precipitation. These sandstones have high porosity (>20%) even at 4.3 km burial depth.

A quantitative diagenetic model considering mechanical and chemical compaction is described and a set of empirical porosity depth trends is identified. A good correlation is found when the model is applied on sandstone samples that are analyzed for clay and framework grain composition by bulk-XRD.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994