Midtbø, Ruth Elin A.1, Trond Lien2, Ole Martinsen3
(1) Norsk Hydro, 5020 Bergen, Norway
(2) Norsk Hydro Research Center ASA, Bergen, Norway
(3) Norsk Hydro Research Centre, N-5020 Bergen, Norway
ABSTRACT: Prediction of Reservoir Quality in Deep-water Depositional Systems, Norwegian Sea: Facies and Temperature History as Controlling Factors
Exploration in Deep-marine Sediments has in the last years become increasingly important offshore Norway, and prediction of reservoir quality is a major challenge. Thick and sand-rich deep-marine sediments were deposited in the Norwegian Sea during Late Cretaceous and Paleocene. Due to the general deeply buried setting and the variable burial history and heat flow, reservoir quality is often poor and difficult to predict. In addition, the source area and the depositional processes vary and have great influence on the porosity and permeability.
The variation in grain size and total clay content has a major influence on the reservoir quality. A clay rich source area normally gives high clay content in the sediments, especially if deposited by low energy processes. High energy processes, such as high-density turbidity currents and strong sustained bottom currents, will deposit coarser and less clay-rich sediments and thereby have a better reservoir quality. Dewatering will further improve the reservoir quality.
Different facies give different porosity and permeability trends with burial depth. For massive sandstones, there are, as expected, a close to linear decrease in porosity with increasing burial depth. This trend is similar to trends observed in shallow-marine Jurassic sandstones. However, in some wells the massive sandstones show a porosity trend with a parallel displacement equal to 1000m deeper burial. Thin section analyses show that sandstones with divergent porosity contain more quartz cement than sandstones following the normal trend. These sandstones have, at some stage, been exposed to temperatures equal to a minimum of 1000m deeper burial.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.