ABSTRACT

Mudrock samples from a well penetrating Eocene and Paleocene strata (1235-4455 m) of the Texas Gulf Coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis, and the mechanism by which detrital smectitic illite/smectite (I/S) is transformed into diagenetic illitic I/S.

Idealized smectite and illite end-member compositions for interstratified I/S are estimated to be [K_0.00X⁺¹_0.56Mg_0.39Fe_0.57Al_1.13Si_3.90O₁₀(OH)₂] and [K_0.53X⁺¹_0.18Mg_0.17Fe_0.16Al_2.28 Si_3.40O₁₀(OH)₂], respectively. There is no obvious difference in the amount of K addition to I/S in random- and ordered-interstratified I/S. Aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite-to-illite reaction is a complete dissolution-precipitation reaction rather than a solid-state [K⁺¹ + Al⁺³] for Si⁺⁴ substitution reaction. Depth-relat d mineral trends and mass-balance calculations suggest that illitic I/S may form from kaolinite, and possibly illite or mica, as well as from smectitic I/S.

Although most whole-rock element abundances are invariant over the sampled depth interval, K₂O content increases from ca. 2.0 weight percent at 1500 m of burial to ca. 3.8 weight percent at 4000 m. Some of this increase is probably caused by import of K into the shale via fluids derived from interbedded sandstones, implying that Gulf Coast mudrocks behave as open chemical systems during burial diagenesis. If this is the case, a minimum of 10₃ pore volumes of fluid must have passed through the most K-enriched shales to have introduced the added potassium, provided that the K content of present-day formation water is representative of that of ancient formation water.