Clayey rocks are host to oil pools in many parts of the world. A producing formation in West Siberia is the Upper Jurassic Bazhenov. There are differences in opinion as to how these clayey rocks have develop satisfactory reservoir properties. Some attribute it to fracturing associated with faulting, and others to hydrofracture from anomalously high formation pressure.
A mechanism is proposed here that involves specific changes in clayey rocks during catagenesis that lead at certain temperatures and pressures to decompaction. In this connection it is interesting that all known clayey rocks that have reservoir properties, regardless of age, composition, or structure, occur in the same temperature zone at 60-120 degrees C in the 1.7-3 km depth interval.
Such temperature selectivity is not fortuitous. Studies have disclosed that anomalously decompacted rocks are associated genetically with the top and bottom of the middle zone of catagenesis. Various rock properties change abruptly in the sub-zones where temperatures are 60-70 degrees C and pressures are 20-36 MPa; also at 90-120 degrees C and 50-60 MPa. For example, the orientation of clay particles decreases, and porosity of clays, which generally decreases with depth, increases. Strongly compacted clays become unstable in water, their strength decreases considerably, and velocity of elastic waves drops.
Experiments indicate that water in clayey rocks at temperatures in the middle sub-zone not exceeding 55-60 degrees C cannot exist in the combined state and on the whole passes into the free state. Some of the remaining combined water is given off at 95-120 degrees C, where clays are converted into shale. As a result of this process, clays that occur in the sub-zone of middle catagenesis should be more permeable than the same rocks occurring above and below this sub-zone. Confirmation of greater permeability of such clays is the anomaly in values of geothermal gradient, which decreases sharply below the 60-70 degree C isotherm.
Water that has been transformed into the free state has aggresive properties because of its high capacity to dissolve. As a result of the appearance of transformed combined water, the equilibrium between the composition of the pore water and the host clayey rock is disrupted. Not only can easily soluble salts be dissolved but also less soluble minerals as well as calcite and various aluminum silicates - feldspar, mica, clays.
Numerous phenomena such as disappearance of fine-grained
carbonates; presence of de-cemented sandstones, changing them
into sands; cavities in limestone and dolomite - all indicate instability
of rocks in anomalously decompacted horizons. Scanning electron
microscope studies show traces of such solution.
Taken from Krivosheyeva and Sokolov (1980); digested in Petroleum
Geology, vol. 18, no. 1.
Copyright 2001 James Clarke. You are encouraged to print out this
News Letter and to forward it to others. Earlier News Letters are
available at our web page: http://geocities.com/internetgeology/
You are invited to receive this News Letter. Please send your e-mail
address to: jamesclarke@erols.com
For information on the journal Petroleum Geology, please telephone
703 759-4487.