8 Chimney Rock Shale Member, Paradox Formation, Utah: Paleozoic, Shallow Carbonate-Dominated Shelf-to-Basin Billion-Barrel Source Rocks

Abstract

ABSTRACT This chapter illustrates the expression of sequence-stratigraphic units and surfaces in the Paleozoic, relatively shallow-water carbonate-dominated setting of the Chimney Rock and associated members of the Paradox Formation, Paradox basin, Utah. These Pennsylvanian source rocks and reservoirs form part of the petroleum system of the billion-barrel Aneth field complex and are well exposed in outcrops along the San Juan River in southern Utah. The Chimney Rock member accumulated in a broad, relatively shallow, marine bight surrounded by uplifts at moderately low latitudes under semi-arid to arid conditions during a cool climate (“icehouse”) mode. The depositional area spanned proximal to distal, storm-wave– and tidal-current–dominated shelf environments. Our dataset included descriptions and samples from seven outcrop sections, ten conventional cores, and wireline logs from more than 387 wells, along with the many excellent published studies of the area. Details of mudstone distribution and character were elaborated within the well-known sequence-stratigraphic framework of the enclosing coarser grained carbonate and siliciclastic facies. The study interval was subdivided into parasequences 1.5–4.6 m (5–15 ft) thick that can be correlated in detail over more than 6400 km2 (2,500 mi2) based on outcrop, core, well-log, and geochemical data. At the parasequence scale, the highest enrichment in organic-matter content occurs not only just above the basal flooding surface but also a short distance above in some more distal areas. The intervals with the highest organic-matter content occur in various bathymetric settings within the transgressive and basal highstand systems tracts; the study intervals were influenced by the full combination of production, destruction, and dilution. This study offers an opportunity to examine mudstone accumulation in a shallow benthic-carbonate–dominated setting where organic-matter enrichment varies significantly from standard models of their stratigraphic distribution.