Diagenesis of upper Pennsylvanian (Missourian) hoxbar sandstone from the Anadarko Basin and its impact on reservoir quality (Caddo County, Oklahoma)



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The Anadarko Basin, located in the foreland of the Wichita Mountains in Oklahoma, is one of the deepest and most prolific hydrocarbon basins in North America and contains approximately 3-4 km of Pennsylvanian strata within its deepest regions. Consisting primarily of marine and non-marine shale and sandstone, these strata present multiple major hydrocarbon reservoirs in the Anadarko Basin. The upper Pennsylvanian (Missourian) Hoxbar Group sandstone is a thick sedimentary unit composed of alternating sequences of black shale, argillaceous sandstone, shaly interbedded sandstone, and occasional limestone thought to be deposited in a tide-dominated deltaic environment. Over 160 m (552 ft) of cored materials taken from two wells drilled in northwest Caddo County, Oklahoma allowed for a multi-analytical investigation into the diagenetic nature of this sandstone. The multifaceted approach utilizes: (1) polarized light microscopy, (2) QEMSCAN®, (3) XRD analyses of both whole-rock and clay (<2 μm) fractions, (4) SEM-EDS investigations of pore-filling mineral constituents, and (5) ICP- MS whole-rock geochemistry.

Investigations revealed detrital illite, Fe-rich chlorite, and several generations of authigenic mixed-layered illite-smectite (I-S) and mixed-layered chlorite-smectite (C-S) as dominant pore-filling clay phases. Albitization of detrital K-feldspar is common, and minor occurrences of authigenic kaolinite fill remnant detrital K-feldspar grains as well as intergranular pore space. Additionally, sandstone of the Missourian Hoxbar Group was pervasively cemented by calcite and siderite, likely attributed to the abundant bioclasts and prolonged residence time of the sediments under certain geochemical conditions. Thus, it was deduced that eogenetic alterations in the Hoxbar sandstone include: (i) precipitation of smectitic precursor minerals, (ii) dissolution of feldspars and kaolinite, (iii) illitization, chloritization, and albitization, and (iv) cementation by calcite. Mesogenetic alterations that are encountered in the Hoxbar sandstone include: (i) continued illitization and chloritization of mixed-layered clays, mud intraclasts, and mica; (ii) albitization of feldspars; (iii) cementation by calcite; and (iv) quartz overgrowths.

It was found that early diagenetic processes leading to an increase in porosity through the creation of secondary porosity by the dissolution of detrital grains (primarily feldspars) or cement (carbonate) has been all but destroyed by later diagenetic events including the pervasive formation of mixed-layered clay minerals and quartz cementation. A conceptual model was constructed, illustrating the distribution of diagenetic alterations in Missourian Hoxbar Group sandstone, and estimation of the maximum burial temperature within this region of the Anadarko Basin was determined to be 100-120°C at a depth of ~4.0 to 4.5 km. Overall, the findings provide new insights and ideas at both regional and reservoir scale that will assist the future exploration and development of hydrocarbons in these reservoirs.



Anadarko Basin, Hoxbar, Diagenesis, Paragenesis, Reservoir quality, Clay mineralogy, Mixed-layered clay minerals, Illite-smectite, Chlorite-smectite, Thermal burial history analysis, QEMSCAN, X-ray diffraction (XRD), ICP-MS, SEM-EDS