Abstract: Shale Dehydration, Salinity, Cracking, and Migration

Charles E. Weaver

Data indicate that, at least on a statistical basis, there is a relation between the occurrence of montmorillonite and oil generation and production. Montmorillonite layers contract during burial and the released interlayer water is available to aid in hydrocarbon migration. Layer contraction and water release starts 600 to 1,500 mm above the depth where the 17-A peak disappears and continues to 300 to 1,500 mm below. Over this interval approximately 50% of the expanded layers collapse and 5% water is released to the pores. Montmorillonite layer collapse starts at 50 to 60°C and persists to 120 to 150°C. These temperatures show little variation with time. This is the same temperature range in which most oil is generated and produced.

Over this interval, not only is interlayer water released, facilitating oil migration, but layer collapse provides an environment favorable for cracking by increasing the availability of H⁺. Conversely the uptake of H⁺ by the organic material should accelerate the conversion of montmorillonite to illite.

Interlayer water has a cation concentration 9 times that of seawater. When layers collapse to 10 A these cations are released to the pores. Release starts much shallower than the depth at which the 17 A disappears. The released cations can dominate the pore-water chemistry.

The density and amount of interlayer water have little to do with the development of high pressures.

When a mixed-layer illite-montmorillonite clay attains a ratio of 7.3 (120 to 150°C) no further dehydration occurs until the temperature reaches 220 to 250°C. "Illites" commonly contain 20 to 30% of 7:3 mixed-layer material. When shales containing 7:3 mixed-layer clay are exposed to temperatures of 220 to 250°C the last interlayer water is released. This water can cause secondary migration and redistribution of hydrocarbons.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma