Abstract: Tectonic Influence on Facies Architecture and Permeability Variation: Pennsylvanian Tyler Formation, Fryburg Field, North Dakota

Stephen D. Sturm, James C. Peterson

Tyler sandstone reservoirs in the Fryburg field represent remnants of a sand-deficient, fluvial-deltaic progradational system deposited along a 40 mi (64 km) trend in southwestern North Dakota. The heterogeneous character of these fluvial-deltaic deposits is well known, and the exploitation of these reservoirs focuses on distinguishing patterns within this variation. Anomalous reservoir pressures and distinct lateral and vertical variations in permeability, observed during field development suggest a complex reservoir architecture.

To better understand this architecture, detailed sequence stratigraphic interpretation of 46 cored wells from the Fryburg field were integrated with well logs, core analyses, and petrographic data. The results indicate the Tyler reservoirs are composed of at least three progradational deltaic parasequences. Vertical permeability baffles and barriers exist at sequence boundaries. In addition, syndepositional tectonism influenced the relative position of these parasequence units and their corresponding lateral facies changes. Movement along both northwest-and northeast-trending conjugate fault sets created the accommodation space for the Fryburg depocenter. Within the depocenter subtle recurrent movements along local structural elements controlled the position of individual distributary channels. Thus, lateral permeability variation is tectonically controlled.

Variations in bedform character, bed thickness, and grain size have the greatest affect on diagenetic processes, which in turn control permeability variations in the Tyler reservoirs. This heterogeneity is the result of variable depositional processes within the Tyler delta complex. Upward-coarsening strandplain deposits have permeability values ranging from less than 0.1 to 10 md, whereas upward-fining channel sandstone values range from 10 to 700 md. Further exploitation of the Tyler reservoir in the Fryburg field is focused on identification of these architectural elements, and the sequence stratigraphic framework superimposed upon them.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994