Parasequence-Scale Stratigraphic Variability of Lithology and Porosity of Mississippian Meramec Reservoirs and the Relationships to Production Characteristics, STACK Trend, Oklahoma

Abstract

Mississippian Meramec reservoirs of the STACK (Sooner Trend in the Anadarko Basin of Canadian and Kingfisher counties) play in central Oklahoma consist of mixed siliciclastic and carbonate deposits. Depositional environments range from a clay-rich distal ramp margin setting to calcite-cemented lower shoreface and tidally influenced deposits. Meramec lithologies primarily consist of calcareous and argillaceous siltstones with some silty carbonates and silty mudstones that are defined based on the relative abundance of calcite cement, silica, and clay. Combinations of gamma ray, density porosity, neutron porosity, bulk density, and deep resistivity logs were used with an Artificial Neural Network to classify lithologies in non-cored wells. Results show overall cross-validation accuracies of 88 – 91% in a training cored well and 73 – 83% in a testing cored well. User’s accuracies for the lithologies vary significantly. Lithology logs and conventional well logs were used to establish a stratigraphic framework that is characterized by a hierarchy of nine shoaling-upward, northeast-southwest strike elongate parasequences that range from 80-150 ft (24-45 m) in thickness and are capped by marine-flooding surfaces. The lower four parasequences form four retrogradational sets that back-step to the northwest. Each of these sets is capped by a flooding surface. The lower Meramec is overlain by a succession of three cycles that form a aggradational to progradational succession. These are capped by two transgressive sequences at the top of the interval resulting in an overall relative sea level deepening in the Meramec. Comparison of lithologies and mean petrophysical properties in horizontal wells show limited relationships with produced fluid volumes. Gas-oil-ratio (GOR) trends vary significantly across the study area. 30 to 180-day percent GOR changes suggest shallower, normally pressured reservoir conditions for wells in the structurally updip region of the study area. Wells producing from deeper reservoirs demonstrate potentially overpressured conditions. Reservoir pressure variability along with other factors such as fluid properties and completion techniques are likely significant controlling factors on production volumes, independent from wellbore lithology.