Clay minerals, and silica in chalk reservoirs with respect to source area, diagenesis and reservoir properties
The amounts of silica and the different types and amounts of clay minerals are investigated in a number of samples from the Upper Cretaceous-Danian chalk for determination of stratigraphical and geographical variations.
Recent studies indicate that reservoir chalk may comprise large amounts of quartz (up to 60% of the rock) without changing appearance or porosities as compared to pure chalk. However, the high contents of quartz results in a distinct change in the rock mechanical properties. Analyses in 1997 have shown that the high quartz content may be related to precipitation of abiogenic silica of nanno-size. Recent analysis also indicate that content of large quartz grains corresponds to a high content of kaolinite and a low content of smectite. Smectite and smectite/mixed-layer clays may give an important indication of the diagenesis, whereas kaolinite primarily is detrial and relates to erosion from high-lying areas. This means that local structural conditions may give rise to special reservoir conditions in a limited area. The aim of the project is to map areas and intervals with different types of clay and quartz. The clay mineralogy will be used to support the diagenetic history of the chalk and relate the content and composition of the clay minerals and quartz to reservoir quality. A secondary aim will be a comparison of the variation in clay mineralogy and the diagenetic conditions with cyclicity and magnetic susceptibility
The presence of silica and clay is attempted to be related to the reservoir quality of the chalk. New and advanced methods for investigation nano-size quartz and of illite-smectites are applied to the Danian and Upper Cretaceous samples. A high countet of quartz in the North Sea chalk is according to atomic force microscopy present as nano-quartz. This nano-quartz has formed by precipitation in the free-water phase but in some samples the nano-quartz has to some degree been modified by diagenesis. High content of this quartz result in increased strength of the chalk and low permeabilities in horizons of almost pure nano-quartz. The reservoir quality is highly dependent of the content of clay. The dominating clay mineral in the Chalk sequence and especially the Danian) is kaolinite originating probably from exposed Lower Cretaceous shales nearby. The content of kaolinite varies both stratigraphically and geographically. Comparison with regional data shows that local highs and not the Ringkøbing-Fyn High must have supplied the kaolinite found in the Danian in the South Arne, the Gorm, the Valhall and the Ekofisk fields. The illite-smectites and smectites are dominating in the Campanian and Maastrichtian chalk demonstrating some variation in the amount of illite layers in the mixed-layer illite-smectite. In addition, corrensite formed by interaction of Mg-containing reservoir brines, is found with the illite-smectites. The variation in illitem-smectite and the formation of corrensite represents differences in the diagenetic alteration of the chalk. The chalk sequence exhibits cyclic behaviour and a correlation to insoluble residue has been undertaken using magnetic susceptibility #kappa#. The magnetic susceptibility of the samples primarily reflect the clay mineralogy, and consequently the cyclic variations of #kappa# is a result of small changes in clay mineral composition or small variations in the amount of clay minerals
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Geologisk Institut. Academy of Science, Moskva (RU) | ||
| Scottish Univiversities Research and Reactor Centre, Glasgow (GB) | ||
| Aarhus Universitet |
Contact
Thoravej 8
DK-2400 København NV, Denmark
Lindgreen, Holger (seniorforsker), 38142000, geus@geus.dk
Øvr. Partnere: Geologisk Institut. Academy of Science, Moskva (RU); Scottish Univiversities Research and Reactor Centre, Glasgow (GB); Århus Univ., Kemisk Institut. Centeret for fast-stof NMR spectroscoopi