Regional stratigraphy and proximal-to-distal variation of lithology and porosity within a mixed carbonate-siliciclastic system, Mississippian strata of northern and central Oklahoma

Abstract

Mississippian carbonate and silica-rich reservoirs of northern and central Oklahoma formed along a regionally extensive carbonate ramp to basin transect. The stratigraphy, lithology, and porosity characteristics of the Mississippian Meramec and Osage series significantly vary as older ramp carbonates prograde southward and transition into younger calcareous and quartz-rich silt deposits of the Anadarko Basin. Lithofacies identified within the northern carbonate-dominated portion of the system commonly include altered chert, skeletal grainstones, peloidal packstones-grainstones, bioturbated wackestones-packstones, bioturbated mudstones-wackestones, glauconitic sandstones, and siliceous shale. Lithofacies within the southern siliciclastic-dominated portion of the system include structureless to bioturbated sandstones, siltstones, and laminated mudstones, each with varying degrees of carbonate content. We have grouped these core-based lithofacies into dominant lithologies/rock types, which tie to well-log properties. Electrofacies classification methods including the artificial neural network (ANN) and k-means clustering predict lithologies in noncored wells. ANN yielded the highest overall prediction accuracy of 85% for lithologies. Core, well log, and lithology log data establish the regional stratigraphic framework. The Mississippian interval of interest subdivides into 16 stratigraphic zones. A depositional-dip-oriented cross section and the associated reservoir models illustrate the proximal to distal and stratigraphic variability of the lithology and porosity. Lithology trends moving from north to south, from older to younger strata, reveal a carbonate-dominated succession capped by diagenetically altered chert northward shifting into a siliciclastic-dominated interval, which increases in clay content southward. Northward, prospective conventional reservoirs developed near cycle tops within diagenetically replaced cherts and cherty limestones associated with subaerial exposure and sea-level fluctuations. Southward, higher total porosity associates with increased clay content linked to the suppression of calcite cement, forming prospective unconventional targets near the bases of depositional cycles.