Glacio-Eustasy, Depositional Environments, Diagenesis, and Reservoir Character of Goen Limestone Cyclothem (Desmoinesian), Concho Platform, Central Texas

Abstract

The Goen Limestone (Desmoinesian) of Concho and Runnels Counties, central Texas, consists of three stacked transgressive-regressive sequences. Each cycle consists of a thin basal transgressive unit (clayey facies) deposited in a deeper water lower ramp setting, and thick regressive limestones (algal, foraminiferal, and bryozoan facies) representing middle to upper ramp-mound, mound-associated, and back-mound environments. Glacio-eustatic sea level fluctuations were the dominant control on the observed cyclicity and facies distribution. The major porosity forming process in the Goen Limestone was early leaching by freshwater lenses that migrated in response to glacio-eustatic sea level changes. The dominant porosity reducing processes included early cementation in a meteoric-phreatic environment and late-stage saddle dolomitization in a deeper burial connate environment. The hydrocarbon-producing regressive middle and upper ramp units (foraminiferal and bryozoan facies) underwent the most complex diagenetic histories and contain extensive secondary pores. The presence or absence of penetrating freshwater lenses and the distribution of green phylloid algae and secondary pore-filling cements were the major factors controlling Goen reservoirs. Numerous cyclic carbonate reservoirs with depositional and diagenetic histories similar to the Goen are present in the Desmoinesian section of central Texas (e.g., Caddo, Odom, Gardner, Capps, and Dog Bend limestones). Late Paleozoic mound-associated and back-mound carbonate reservoirs may be of greater importance in petroleum exploration than algal mounds, one of the most frequently exploited late Paleozoic carbonates of North America.