Robert G. Loucks, Robert M. Reed, and William A. Ambrose
Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, University Station, Box X, Austin, Texas 78713–8924
Analysis of Pore Networks and Reservoir Quality of the Upper Cretaceous Woodbine Sandstone in the High-Recovery-Efficiency, Giant East Texas Field
Onshore Gulf of Mexico Exploration I (GRBCC, Assembly Area B)
Tuesday, September 22, 2015, 9:25 am
Reservoir quality in the Upper Cretaceous Woodbine sandstone in the giant East Texas Field is excellent and has contributed to the exceptionally high oil-recovery efficiency rate of approximately 77 percent. The sandstone has a stable mineralogy consisting predominantly of quartz with very minor amounts of rock fragments and feldspar, and it also has had a favorable diagenetic history. Primary intergranular pores are the main constituent of the pore network. Porosities within the matrix-free sandstones average 26 percent, and permeabilities average nearly 2000 md based on core analysis.
The Woodbine sandstone is predominantly a fine- to medium-grained quartz arenite with a few samples in the sublitharenite field. Granule to pebble conglomerates are common at the base of incised valleys and are composed of silicified carbonate (chert) and quartzite clasts. The material between the granules and pebbles is medium- to coarse-grained sand composed of quartz and chert. Lower-energy facies contain a clay matrix rich in nano- to micropores.
Quartz overgrowths are the most common cement; from scanning electron microscope cathodoluminescence analysis, the overgrowths appear to have been precipitated in two stages, starting at shallow burial depths of less than 2600 ft. Other cements are relatively rare and include poikilotopic calcite, siderite crystals, and chlorite coats or grain replacement. Intergranular pores between grains are the predominant pore type, and minor secondary dissolution pores are present within feldspars, volcanic rock fragments, and chert. Some of the conglomerates contain micro- to macroporous chert clasts.
Mercury-injection capillary pressure analysis shows that the high-quality sandstones have entry pressures of <6 psi and mean pore-throat sizes between 5 and 15 µm in diameter, whereas one matrix-rich sandstone has an entry pressure of 82 psi and poorly-sorted pore-throat sizes that range between 0.01 and 1 µm in diameter. Overall reservoir quality in the Woodbine sandstone is excellent and not an important risk factor in the exploitation of the East Texas Field.