Ramp-scale geomodel for reservoir and stratigraphic analysis of the Hugoton field (Wolfcampian, midcontinent U.S.A.)

Abstract

The full-field model of the 70-year-old Hugoton field (largest in NA) is a comprehensive lithologic and petrophysical view of a giant reservoir system in a 108-million cell model covering 10,000-mi2 (26,000-km 2). It is a quantitative basis for evaluating remaining gas, particularly in low-permeability intervals, and will aid field management and enhance ultimate recovery. The model is also a tool for developing depositional models and for understanding controls on sedimentation. Both the knowledge gained and the techniques and workflow employed have implications for understanding and modeling similar reservoir systems worldwide.

Accurate representation of lithofacies in the model is critical because water saturation from wireline logs is inaccurate due to filtrate invasion. Lithofacies-based petrophysical properties are used to estimate water saturation. Neural-network prediction of lithofacies using wireline logs and two geologic variables is effective in predicting lithofacies at wells. Between wells, lithofacies and wireline-log porosity, corrected by lithofacies-dependent algorithms, are reliably represented by stochastic methods. Permeability, water saturation, and gas in place at the cell level are calculated by lithofacies- and porosity-dependent petrophysical transforms. Based on the model, 963 billion m3 (34 tcf) of the produced gas represents 65-70% of original gas in place. The reservoir is a layered, differentially depleted system, and most remaining gas is in intervals having lower permeability.

The model illustrates shifting sedimentation patterns related to glacioeustacy on a large, stable, gently sloped ramp. The 160-m reservoir comprises thirteen upward-shoaling carbonate cycles vertically stacked in a low-relief setting. Lithofacies bodies are laterally extensive and reservoir storage and flow units, mostly grain-supported marine carbonate, exhibit broad lateral continuity. Carbonate cycles are separated by fine-grained siliciclastic strata (mostly loess) deposited in a savannah-like setting. Climate variability controlled sediment supply and delivery. Relatively dry conditions and low vegetative cover during low sea level allowed fine siliciclastic sediments to be delivered to the ramp by eolian processes where they were stabilized by vegetation in an aggradational landscape. During high sea level wetter conditions and increased vegetation curtailed siliciclastic supply to a flooded, carbonate-dominated ramp. The results illustrate new climate-controlled mechanisms for cyclicity in fine-grained siliciclastic strata.