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Publication authorized by Director, Bureau of Economic Geology, The University of Texas at Austin.

Abstract

A complex of reefs, banks, tidal bars, channel fills, and stabilized grainflats accumulated along the Lower Cretaceous shelf margin in South Texas. The wide range of energy levels along this shelf-margin complex resulted in deposition of numerous facies, some with initial porosities as great as 30 to 40 percent. Subsequent marine cementation, meteoric phreatic diagenesis, and deep subsurface cementation have generally filled pore spaces and blocked permeability within the limestone. Only four facies commonly have greater than 5 percent porosity and 5 milli-darcys permeability--the algae encrusted miliolid-coral-caprinid pack-stone, mollusk grainstone, rudist grainstone, and coral-stromatoporoid boundstone.

Intraparticle porosity is common in several wackestone facies but only locally reaches 20 percent; permeability, however, is generally low in these intervals. Interparticle and fracture porosity are also locally significant. Where interparticle porosity is greater than 5 percent, permeability reaches 10 millidarcys. Higher permeability always occurs where fractures are common.

Stuart City gas fields are facies-controlled stratigraphic traps. Gas is present within common intraparticle and moldic porosities, which are best preserved in the four depositional facies listed above. Fractures provide the effective gas collecting network in the Stuart City reservoirs. Relatively rapid decline in gas production from many wells, response to fracture enhancement, and decreased water production with time supports the importance of fractures to gas production.

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