--> --> Using Pore System Characterization to Subdivide the Burgeoning Uteland Butte Play, Green River Formation, Uinta Basin, Utah

AAPG Rocky Mountain Section Meeting

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Using Pore System Characterization to Subdivide the Burgeoning Uteland Butte Play, Green River Formation, Uinta Basin, Utah

Abstract

Over 100 horizontal wells have been drilled in the large and growing Uteland Butte play in the past decade, with decidedly variable results. The best wells cum north of 300K barrels in the first 12 months of production, with conservative EUR’s well above a million barrels per well. Conversely, the poorest Uteland Butte wells IP less than 10K in the first year and will never pay out their drilling costs. Pore pressure, oil viscosity, well length and frac size are recognized as important controls on well productivity, but the variable reservoir types within the Uteland Butte play is perhaps less understood. Defining each sub-play by its pore system, there exists four distinct sub-plays within the Uintah Basin; 1) A proximal play dominated by siliciclastic and carbonate grainstone pores, mostly normally pressured and charged by migrated hydrocarbons. This play has an average maturity of 0.5-0.7 VRo and produces highly viscous black wax with very low GOR’s. To date, these wells have not been successful. 2) An intercrystalline-pore dominated play, with dolomites acting as the best reservoirs and at normal pressure or slightly above charged by mostly migrated hydrocarbons. This play was the first to be drilled horizontally because the dolomites are the most conventional targets. The Uteland Butte is quite thick in this fairway, consisting of over 130’ carbonates and black shales, but only a fraction are high-porosity dolomites. This play has an average VRo of 0.6 to 0.8 and produces a black wax with low GOR’s. 3) A mixed intercrystalline-organic porosity play, with significant overpressure and largely self-charged or with very short migration of hydrocarbons. This sub-play has the thickest gross section, but much less net dolomitic reservoirs. However, high TOC carbonates and shales contribute significant production. Maturities in this play vary from 0.8 to 1.0 VRo and produces a yellow to grey wax with moderate GOR’s 4) an organic porosity dominated play, highly overpressured and completely self-sourced. Relatively little carbonate exists in this fairway and almost no reservoir-quality dolomites. Productive reservoir consists of organic porosity largely contained in bitumen mats that are expelled at lower maturities, then continue to thermally degrade with higher maturity, converting to zones of interconnected organic porosity. Maturities range from 1.0 to 1.2 VRo and produces a bright yellow wax with relatively high GOR’s. By recognizing the important differences these pore systems exert on best development practices and then accurately mapping them across the basin, operators, interest owners and regulatory agencies can better plan operations.