Basin and thermal history, geochemistry and pressure development of the Simpson Group (Middle Ordovician) based on part of Garvin, Grady and McClain counties, the Anadarko Basin, Oklahoma

The Anadarko Basin contains abnormal pressured lithologies. The presence of overpressures implies the existence of seals to maintain pressures over time. The study area and Simpson Group lithologies were selected to test for the existence of a diagenetic pressure seal.; Pressure data indicate that there is no diagenetic pressure seal at present, and modeling results show that past excess pressures are not likely to have been maintained until the present. Pressure mechanisms of hydrocarbon generation and compaction are not effective in maintaining overpressures until the present.; The thermal history of the study area can be approximated with a simple, decreasing heat flow over time. At present, the Simpson Group is in the oil and gas windows; however, kerogen data indicates little or no remaining generation potential. Total organic carbon values are insufficient for generation of hydrocarbons to produce overpressures. {dollar}rmdeltasp{lcub}18{rcub}Osb{lcub}SMOW{rcub}{dollar} and fluid inclusion temperature data indicate that diagenesis occurred in an open system at deeper burial depths. The fluid inclusion temperature data also suggest that most of the diagenetic mineralization took place at 100{dollar}spcirc{dollar}C (212{dollar}spcirc{dollar}F). This temperature and the modeling thermal history indicate that diagenesis took place between 312 to 260 Ma (Pennsylvanian to Permian), after major faulting and around the deepest burial depths for the Simpson Group. Carbon and oxygen isotope compositions indicate that diagenetic mineralization was fairly uniform, but petrographic studies show that mineralization was complex and not uniform laterally. Hydrocarbon expulsion began after diagenetic mineralization occurred.