Diagenetic Aspects of Lower Morrowan, Pennsylvanian, Sandstones, Northwestern Oklahoma

Diagenesis has profoundly altered the reservoir capacity of some lower Morrowan sandstones in the Anadarko basin. Definitions of sandstone geometry, depositional environment, etc., comprise only part of the information necessary to explore intelligently for hydrocarbons in these rocks. Most of the lower Morrow sandstones were deposited in a broadly transgressive system. Two basic types of sandstone are recognized: (1) Type A is a very nearshore, clean, well sorted, non-glauconitic, non-calcareous sandstone deposited in a high energy environment; (2) Type B is a seaward facies of Type A, deposited under lower energy conditions and is normally poorly sorted, “glauconitic,” calcareous, and here and there shaly. Petrographic analyses indicate Type A sandstones have either (1) extreme pressure solution and no permeability, or (2) moderate to minor pressure solution with secondary quartz overgrowth cement. These rocks are generally permeable when medium- to coarse-grained and impermeable if fine-grained. Upper and lower edges of some sand bodies are tightly cemented with CaCO3 possibly through ionic impedance at the sand-shale boundary. Type B sandstones have been diagenetically altered by minor secondary quartz overgrowth and pressure solution, extensive carbonate cementation, and some authigenic clay. Commercial permeability is associated with secondary removal of carbonate cements. Such reservoirs have random shapes and are difficult to predict or evaluate. A qualitative petrographic rock fabric classification based on relative degree of pressure solution, type of grain outlines and contacts, amount and type of cement and porosity is useful in mapping their geographic distributions. Integration of this type data into normal subsurface exploration methods is helpful for the understanding of the lower Morrow in the area studied.