Middle Pennsylvanian rocks have produced substantial quantities of oil in eastern Oklahoma and southeastern Kansas. Initial results (Table 1) of a study of possible petroleum source rocks in these units show that organic-rich shales (> 1% organic C) can be divided into four groups. Group I shales are nonfossiliferous to fossiliferous marine, and commonly pyritic. Groups II, III, and IV shales are laminated, phosphatic, and nonfossiliferous to slightly fossiliferous. Groups III and IV shales are most common in the northern part of the area, which would be closer to the paleo-shoreline. Bulk organic geochemical properties of groups II and III shales are similar to coals, suggesting that most of the organic matter was derived from peat swamps. Reworking of organic matter, particularly in shale groups I and IV, was considerable, as indicated by low inferred hydrogen contents (H index) and relatively high inferred oxygen contents (O index). For types II and III shales and coals, average H and O indices decrease, and T_max (S₂ peak - Rock-Eval) increases from north to south, indicating an increasing maturity of organic matter to the south. This increasing maturity is consistent with changes in coal rank, which increases from high-volatile C bituminous coal in Iowa to high-volatile A and medium-volatile bituminous coal in Oklahoma. Total sulfur-organic carbon plots for the four shale groups do not intercept the sulfur axis, which suggests a lack of H₂S-containing bottom waters; different slopes of these plots suggest variabilit of sulfate availability, possibly due to salinity differences or rates of sedimentation.

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