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An Unconventional Platform-Top Carbonate Reservoir: The Bend Formation in Bryant-G Field, Midland Basin, West Texas


Bryant-G Field in the Midland Basin is a legacy asset currently being developed as part of the Wolfberry Play, in which vertical wells target 6 zones whose production is comingled. The Bend Formation is a deeper, Lower Pennsylvanian (Atokan) target. The Bend is a continuous reservoir, which is not self-sourcing but has no water leg and requires stimulation and factory drilling to be economic. The long history of production makes the Bend a useful reservoir to evaluate single zone production and the efficacy of different stimulation techniques within the Wolfberry play. The Bend consists of a stack of mixed carbonate-siliciclastic m-scale high-frequency sequences (HFS). HFS are bounded by subaerial exposure horizons (e.g. karst, paleosols), reflecting the high-amplitude glacio-eustatic sealevel fluctuations dominant in the early Pennsylvanian. Siliciclastic facies include paralic to marine mudstones and a prominent coal horizon, which persists across the field. Carbonate facies include both heterozoan and photozoan assemblages. Heterozoan wacke/pack/grainstones are crinoid-rich, with brachiopods and bryozoa. Photozoan pack/grain/boundstones are dominantly composed of beresellid algal thalli and/or coated grains. Depositional facies exert a strong control on diagenesis and ultimate reservoir quality: macro-porous, oil-stained photozoan facies have 3 times the porosity and 2 orders of magnitude higher permeability than the tight, microporous heterozoan facies. Macro-porosity varies from moldic and intraparticle pores, which locally appear related to meteoric diagenesis, to intercrystalline and stylolite-related pores, which formed during burial. The Bend in Bryant-G field has production stretching back >20 years, with peak monthly rates for individual wells that range from ∼50-200 BOEPD. Several stimulation techniques have been employed, with variable results. Legacy wells were stimulated with matrix acid or acid fracs, but current wells typically employ sand or acid fracs. Identifying the most effective stimulation techniques presents a significant opportunity to optimize the exploitation of deeper targets in the Wolfberry Play.