Twenty-seven samples of feldspathic phyllarenite were obtained from three 1 m grids in a large, homogeneous distributary-bar sand stone body with the goal of elucidating bed-scale diagenetic het erogeneity. Significant variability in the content of both detrital and diagenetic components is present in the sample set. This variation is within the range of values predicted from comparison with a large regional data set from the same formation, but nonetheless, it is large enough to preclude successful prediction of diagenetic prop erties from consideration of average composition and thermal his tory alone. Grain size is one important local control on the overall abundances of cement and porosity. Grain size operates to control the overall degree of cementation (in these particular rocks, indi rectly) through its influence on lithic grain content and, hence, on compaction. Relatively minor carbonate cement is an exception to this trend and appears to be distributed somewhat randomly, with out regard to the size or composition of the detrital components.

Diagenetic heterogeneity of the magnitude documented in this example is correlated to differences in porosity and permeability and therefore has implications for reservoir simulation and explo ration risk assessment. Prediction of rock properties through dia genetic modeling is more successful if the textural controls on dia genetic heterogeneity are considered.