Permeability values decrease upward in ten to twelve foot thick cycles within the lower sand units of the middle Miocene Oficina Formation in Uracoa Field, Venezuela. Within these cycles, permeability loss of up to two orders of magnitude have been observed. While these cycles do not correspond to variations in Helium porosity, a relationship among grain size variations, the presence of certain diagenetic phases, and decreased permeability has been observed through comparison of petrographic, air permeability, and X-ray diffraction data. Samples containing sparse kaolinitic grain coatings display permeability values in the thousands of millidarcies. As the abundance of grain coating clay increases upward within a cycle, permeability values decrease to hundreds of millidarcies. Similarly, correlation has been noted between quartz overgrowth abundance and decreased permeability in these cycles. While an overall fining upward trend is visible in each cycle, these order-of-magnitude differences i n permeability have also been observed in samples of uniform grain size among which the only visible difference is the abundance of these diagenetic phases.

Though fine grained kaolinite suggests a marine origin for the lower Oficina Formation sands, the significance of cyclical variations in clay abundance with respect to depositional environment in Uracoa Field is under investigation.