Principles of Dipmeter Interpretation in Fluvial Systems: ABSTRACT

Identification of fluvial reservoirs has become increasingly more important in exploration. Stratigraphic information provided by dipmeter data is useful in defining the fluvial facies and reservoir morphology. Field studies utilizing dipmeter data indicate that the middle Bartlesville sandstones in southeast Kansas and northeast Oklahoma are fluvial in origin and are composed of point-bar and longitudinal-bar facies. Dipmeter patterns within composite bedsets of each facies display a decrease in dip upward. This pattern indicates an upward decrease in the scale of sedimentary structures from cross-laminated to ripple-laminated End_Page 245------------------------------ sandstones as displayed in cores. The composite bedsets of the point-bar facies are significantly thicker than the bedsets of the longitudinal-bar facies. The thinly stacked nature of the longitudinal-bar composite bedsets produce an apparent random dip pattern. However, individual bedsets within the sequence show a decrease in dip upward. Azimuth-frequency plots of cross-bed dip directions yield valuable information on the reservoir morphology. A unidirectional azimuth pattern indicates a predominant paleocurrent direction characteristic of point-bar deposition. Longitudinal-bar sandstones produce a multidirectional azimuth pattern due to stream bifurcation. However, the general paleocurrent direction can be determined from a weighted average of the azimuths. The local sandstone trend of each facies is in the direction of the paleocurrent. Azimuth-frequency plots of the overlying shale drape are 90° out of phase with the paleocurrent direction, indicating that the thicker sandstones of the trend lie in the opposite dip direction of the shale drape. Paleogeographic reconstructions based on paleocurrent and shale-dr pe data show that the point-bar facies has a broadly arcuate, dip-trending morphology of high sinuosity, and the longitudinal-bar facies has a gently curving, dip-trending morphology or low sinuosity. End_of_Article - Last_Page 246------------