The mineral composition and petrography of sandstones, shales, carbonates, and intermediate lithologies were determined on 112 core samples of the Middle Ordovician Simpson Group in the Sunray DX Parker No. 1 Mazur well, Grady County, Oklahoma. Core was recovered from present depths of about 15,900-17,200 ft and included all or parts of the Bromide, Tulip Creek, McLish, Oil Creek, and Joins Formations. The bulk-rock mineral composition of Simpson Group rocks is diverse. The mean weighted composition of 50 sandstone samples is 66% quartz, 14% clay, and 18% carbonate, as determined by X-ray powder diffraction (XRD). Some sandstones from the Oil Creek and Tulip Creek Formations contain as much as 96% quartz. These quartz-rich sandstones were cemented early by silica. Feldspar averages 2%; some sandstones from the McLish Formation contain as much as 15% feldspar. Potassium feldspar is commonly more abundant than plagioclase; potassium feldspar overgrowths are found in some of the sandstones. Most of the shales are clay-rich and quartz-poor, averaging about 85% clay minerals, 7% quartz, and 3% feldspar, by weight, as determined by XRD. Carbonate, fluorapatite, and pyrite are present in variable amounts. Such high clay/quartz ratios are not characteristic of shales and suggest that silica has been expelled by diagenetic processes during burial. The main clay mineral in the Simpson Group at these depths is illite, although iron-rich chlorite is locally concentrated in sandstones. Illite typically makes up >90 wt. % of the clay minerals in sandstones and >95 wt. % of those in shale and carbonate. Total clay content, determined from XRD, correlates closely with total gamma-ray intensity from geophysical logs, because illite is the primary potassium-bearing phase in these deeply buried rocks. Therefore, the gamma-ray log is a good indicator of "shaliness" in potential Simpson reservoirs at similar depths. Much of the carbonate was introduced into the sandstones during burial as calcite, dolomite, or ankerite cement. Early iron-free calcite is commonly replaced by iron-bearing calcite, dolomite, or ankerite. Sandstones and carbonate rocks also contain rhombic dolomite. Many of the dolomite rhombs contain overgrowths of ferroan dolomite or ankerite, as evidenced by staining. Ankerite cementation is later and less selective than earlier dolomite and commonly replaces earlier carbonate or silica cements. Dolomite commonly replaces detrital clay and calcite. Spatial and textural relations suggest that the conversion of smectite to illite contributed, in part, to the formation of dolomite and ankerite cements. Scanning electron miscroscopy reveals that much of the diagenetic illite occurs as tabular fibers in pores or as pseudomorphic intergrowths after smectite. Most chlorite in sandstones is authigenic and occurs as a pore-lining cement or as a pseudomorphic replacement after kaolinite. Secondary porosity, formed mainly from the dissolution of intergranular carbonate cements, is best developed in sandstones from the Oil Creek and Tulip Creek Formations.