FIRST APPROACH TO A SEQUENCE STRATIGRAPHIC FRAMEWORK OF THE SILURIAN OF THE PERMIAN BASIN: INTEGRATION OF WELL LOGS AND BIO-CHEMOSTRATIGRAPHIC DATA

Silurian rocks in the Permian Basin record a complex syn- and post-depositional history of sea level fluctuations with subaerial exposures and multiple episodes of dolomitization. Correlation is therefore difficult and compromises identification of oil and gas target zones. This research presents a preliminary sequence stratigraphic framework within Silurian units based on well logs, lithological descriptions, conodont biostratigraphy, and carbon isotope chemostratigraphy. The chosen sections are in the southern Permian Basin, where the strata are deeper water facies with less dolomitization. The data set allows interpretation of a 3-tract sequence stratigraphic model linked to relative age control.

The analysis demonstrates at least nine sequences within the Silurian. The Fusselman Formation records stratigraphically thicker sequences due to carbonate factory development in shallower water conditions. The Wristen Group records progressive deepening of the basin with eventual drowning of the carbonate platform represented by thinner sequences developed in shalier deeper water facies. The identification of sequence boundaries (SB) and maximum flooding surfaces (MFS) allow the sequence stratigraphic framework definition and the interpretation of the LST, TST, and HST tracts. SB represents the effects of subaerial exposure of the platform on the deeper facies, and is marked by the inflection point between coarsening-upward to fining-upward in GR log, confirmed with lithology changes and gravity flows. MFS shows a linkage between organic enrichment and laminated detrital sediments visualized in the highest GR values.

The sequence stratigraphic framework is integrated with relative time using existing conodont-carbon isotope bio-chemostratigraphy. The lower Fusselman is an Ordovician sedimentation system with an unconformity between it and the upper part, which contains an additional four sequences. There is a hiatus between the Fusselman and Wristen Group, where sedimentation resumes around the Llandovery-Wenlock boundary as represented by the Ireviken excursion in the Wink Formation. Finally, the Wink contains five sequences, including a major sequence boundary during the Mulde excursion. Sequence boundaries may be correlated to karsted target intervals in shallow facies.