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During the last fifteen to twenty years, our understanding of the geologic significance of the clay minerals has increased from nearly zero to the point where we can begin to make some practical application of our knowledge. Following is a brief list summarizing our general knowledge.
1. We know the clay mineral composition of most of the formations in the United States.
2. Pre-Middle Mississippian clay suites are less complex than those in younger sediments.
3. The composition of clay suites may change drastically over an interval of a few feet or remain nearly constant throughout 5,000 feet of section.
4. Both source and environmental information can be obtained.
5. Expanded clays are contracted by burial.
6. The clay suites in limestones are as variable as those in shales.
7. Clay minerals in sandstones are more apt to have been altered after deposition than the clays in other rocks.
8. Once the clay mineral distribution pattern is established, reasonable environmental identifications can commonly be made.
9. If the environmental significance of the clay suites can be established by other means, the clay data can be used independently to identify environments near the control area.
10. Clays can best be used for environmental determinations in young sediments where deposition is relatively slow and environments are materially different.
11. Clay can be used for approximate "time correlation" where bentonite beds or unconformities are present and where there is a change in source material.
12. Relatively minor clay differences can be correlated for 10-50 miles.
In the Upper Mississippian and Lower Pennsylvanian of the Mid-Continent, the clay suites reflect the source areas and tectonic history. Many formations in this interval contain distinctive clay suites which can be used for identification. Detailed correlations can be made by making detailed studies.
Both in recent and ancient sediments (shales, limestones, and sandstones) clay mineral suites are related to depositional environments. Clays should be of value in locating the more effective energy barriers. In the recent, both sandstone bars and limestone reefs appear to act as barriers and cause different clay suites to be deposited on the landward and seaward sides. Environmental studies are best made by investigating the whole basin and determining the distribution pattern of the various clay facies.
It has been shown that clays can be used to subdivide formations and to locate formation boundaries more accurately than can be accomplished by other techniques.
Regional (1,000-2,000 miles) clay mineral zones occur in the Lower and Middle Ordovician and the Upper Mississippian.
In the Ouachitas, kaolinite can be used to differentiate the geosynclinal and foreland facies. The relative sharpness of the X-ray diffraction peak of illite affords a good measure of the degree of metamorphism in this area.
The available data suggest that there may be some relation between hydrocarbon production and expanded clay minerals. Most of the major producing formations in the United States contain an abundance of expanded clays (montmorillonite, mixed-layer-illite-montmorillonite, and chlorite-montmorillonite). Expanded clays retain their pore water to greater depths of burial than do the other clays. If hydrocarbons are formed at any appreciable depth, the shales containing expanded clays would be the main source of water to carry the hydrocarbons from the shales.
Comparison of clay data and velocity data for Tertiary and Paleozoic shales of similar composition (montmorillonite) indicates that the rate of collapse of the expanded clay layers is related to depth and is independent of time. However, the loss of pore water (as shown by density and velocity data) from shale is closely dependent on time.
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