Textures of sandstones and carbonate rocks in the world's deepest wells (in excess of 30,000 ft. or 9.1 km): Anadarko Basin, Oklahoma

Abstract

Accelerated exploration in the deep Anadarko Basin has resulted in the need to understand the effects of deep burial at depths in excess of 30,000 ft. (9.1 km) on the textural characteristics of carbonate rocks and sandstones. The purpose of this study is to determine how deep burial affects the development of diagenetic textures in sandstones and carbonates. Textures found in carbonate rocks of the Hunton Group (Late Ordovician to Early Devonian) in the Lone Star Baden well, one of the world's deepest boreholes (9.16 km deep), and in basal sandstone sequences from the Simpson Group (Early to Middle Ordovician), from the Lone Star Bertha Rogers well, the world's deepest borehole (9.58 km deep), indicate that textures are not significantly different from diagenetic textures found in similar strata buried to shallower depths within the same basin. Mean vitrinite reflectance ( R O) values from the Lone Star Bertha Rogers well were plotted versus depth as a means of developing a ranking parameter to characterize the thermal history of the deep Anadarko Basin. Temperatures of 200°C and higher had no apparent effect on the development of diagenetic textures or formation of new minerals, even though these temperatures approach those found in the zone of greenschist facies metamorphism. Diagenetic textures which developed formed in response to increases in stresses and strains that accompany deep burial. Textures related to the effects of strains in carbonate rocks of the Hunton Group include the development of various kinds of twinning, and the mechanical and multiple displacement along twin planes within single crystals, as well as cataclastic textures, such as granulation and mortar structure. Among sandstones of the Simpson Group only the development of concavo-convex and sutured boundaries and undulose extinction of quartz grains can be directly related to stresses and strains developed during deep burial. Mechanical adjustment occluded pores. Both limestones and standstones are tight.