ABSTRACT

Authigenic minerals, especially clay minerals, control the petrophysical properties relating to reservoir quality, particularly permeability and irreducible water saturation. Pore size and shape also are altered significantly by the development of diagenetic clay minerals. However, the most significant result of sandstone diagenesis is the development of a very large surface area within the pore system. The extremely large surface area available for current flow in these pore systems often causes them to appear "wet" on resistivity logs.

Electric log evidence for diagenetically altered zones which may actually be productive or which may be serving as diagenetic traps is very subtle. The main reason is that a very small percentage of diagenetic clays in a reservoir can cause a negligible decrease in SP response while simultaneously causing a significant decrease in the true resistivity values recorded by electric logs (of which the induction types are most significantly affected).

Presently, there is no logging program that definitely can detect a productive zone containing diagenetic clays which appears to be "border line" or "wet." Nor is there a program to determine if a nonproductive "wet" zone is in truth a diagenetic seal or a hydrocarbon accumulation. Until a fast well-site method can be developed to evaluate reservoirs containing diagenetic clay minerals, we must depend on petrographic and scanning electron microscopic analyses to aid us in our search for hydrocarbon production.