When a clay or shale with fixed cation exchange capacity is saturated with salt water and the solid matrix is slowly squeezed, so that equilibrium is maintained between the remaining pore water and successive increments of squeezed-out water, these successive increments will contain salt of decreasing normality L N = --- [1 - (n/2L)2], 2 provided the water is conducted away from and out provided the water is conducted away from and out of diffusional contact with the clay as it is produced. Here, n is the cation exchange capacity per unit pore volume and L is a constant determined so that pore volume and L is a constant determined so that this equation correctly relates the initial concentration of solution in equilibrium with the clay, No, and the initial exchange capacity per unit pore volume, no. The pore water remaining in the clay or shale at any point in the squeezing process will contain positive and negative ions of normality: 1 N = -- [L n + n2/4L). 2 In the pores, the positive-ion concentration increases steadily while the negative-ion concentration and N decrease to zero for n 2L. The squeezed-out pore water, if collected and mixed, will contain pore water, if collected and mixed, will contain salt of decreasing normality: 1 N = ---- [L - non/4L]. 2 In deriving these equations, it is assumed that the ratio of activity coefficients of salt inside and outside the porous material is unity, and that there is no association between cations and either anions or exchange sites. Association of cations and exchange sites is treated briefly. Approximations used are thatan increment of salt water passing out of the porous medium does not change volume andthe matrix material does not compress during compaction. The process must be carried out so slowly that the changing porosity and salinity remain uniform throughout the porous material. The given thermodynamic results correspond to a common experiment in which a water-saturated clay cake is compressed and the squeezed-out water is conduced away through a short capillary of minimal volume and dropped into a receiver. The results also approximately describe some geologic situations in which compaction water from shales drains into comparatively thin sands that conduct the water away. Introduction A number of authors have reported progressive changes in the salinity of water squeezed out of clay cakes. The trend of the data is for the salinity of the squeezed-out water to decrease generally as the porosity is reduced. Chilingar et al. reported that the salinity decreases if the compaction is very slow, but can be made to increase if the compaction pressure increases abruptly. Experimental objectives have been to understand salinity changes in shales and sands in geologic settings. Knowledge of expected salinity changes can be applied to quantitative interpretation of electric logs, to interpretation of the direction of hydrodynamic flow over geologic time in compacting sand-shale sequences and to determining whether the more rapid water influx into producing petroleum reservoirs may have come from surrounding shales. Fowler has interpreted detailed data from the Chocolate Bayou field, Brazoria County, Tex., in terms of the last two phenomena. Patchett has shown that logged shale conductivities in wells from a number of sedimentary basins, when converted to a common temperature basis, fall within a narrow range for any given porosity. This is in accord with a finding here that ion concentrations in compacting shales should evolve to values depending on cation exchange capacity but not on the initial salt concentration in the depositional environment. SPEJ P. 377

Shales are the most abundant class of sedimentary rocks, distinguished by being very fine-grained, clayey, and compressible. Their physical and chemical properties are important in widely different enterprises such as civil engineering, ceramics, and petroleum exploration. One characteristic, which is studied here, is a systematic reduction of porosity with depth of burial. This is due increases in grain-to-grain stress and temperature. Vertical stress in sediments is given by the overburden less the pore fluid pressure, $\sigma$, divided by the fraction of the horizontal area which is the supporting matrix, $(1-\phi )$, where $\phi$ is the porosity. It is proposed that the fractional reduction of this ratio, $\Lambda$, with time is given by the product of ${\phi }^{4m/3}$, ${(1-\phi )}^{4n/3}$, and one or more Arrhenius functions $Aexp(-E/RT)$ with m and n close to 1. This proposal is tested for shale sections in six wells from around the world for which porosity-depth data are available. Good agreement is obtained above 30–40 °C and fractional porosities less than 0.5. Single activation energies for each well are obtained in the range 15–33 kJ/mole, close to the approximate pressure solution of quartz, 24 kJ/mol. Values of m and n are in the range 1 to 0.8, indicating nearly fractal water-wet pore-to-matrix interfaces at pressure solution locations. Results are independent of over- or under-pressure of pore water. This model attempts to explain shale compaction quantitatively. For the petoleum industry, given porosity-depth data for uneroded sections and accurate activation energy, E, paleo-geothermal-gradient can be inferred and from that organic maturity, indicating better drilling prospects.

The grain-to-grain stress vertically in sediments is given by the overburden less the pore fluid pressure, σ, divided by the fraction of the horizontal area which is the supporting matrix , (1 − φ), where φ is the porosity. It is proposed that the fractional reduction of this ratio, Λ, with time is given by the product of φ 4m/3 , (1 − φ) 4n/3 , and one or more Arrhenius functions A exp(−E/RT ) with m and n close to 1. This proposal is tested for shale sections in six wells from around the world for which porosity-depth data are available. Good agreement is obtained above 30-40 C and porosities less than 0.5. Single activation energies for each well are obtained in the range 15-33 kJ/mole, close to pressure solution of quartz, 24 kJ/mol. Values of m and n are in the range 1 to 0.8, indicating nearly fractal pore-matrix spaces and water-wet interfaces. Results are independent of over- or under-pressure of pore water. This model explains shale compaction quantitatively. Given porosity-depth data and accurate activation energy, E, one can infer paleo-geothermal-gradient and from that organic maturity, thus avoiding unnecessary drilling.