DIAGENESIS AND RESERVOIR CHARACTERIZATION OF THE PENNSYLVANIAN MIDDLE ATOKA FORMATION OF SEBASTIAN AND LOGAN COUNTIES, WEST-CENTRAL ARKANSAS
The purpose of this study is to establish the stratigraphic framework, reservoir geometry and distribution, and depositional environments; and to document the quality of the reservoir. Well correlation and petrographic studies are used to achieve the set goals.
Blocky Gamma-Ray signatures with abrupt tops and vertical sequences of sand bodies that are closely spaced and separated by thick marine shale intervals are correlated as genetically related sandstone deposits that resulted from one depositional cycle. Four stratigraphic units are recognized on this basis. They include the Borum, Turner, Nichol, and the Basham Sandstone Units respectively from the bottom up.
Two structural patterns, north-south dip and east-west structural anticlines and synclines are observed. The geometry of the reservoirs is both elongate and radial. They are best developed in the north and in the central portion of the study area.
The sandstone is very fine silt to coarse sand, poor to well sorted, deepwater sandstones. They are chaotic interbeds of sandstones and shales deposited by turbidity currents and mass-transport processes in the slope-basin environment, and composed of quartzarenites, sublitharenites, litharenites, and subarkoses sandstones. The amount of quartz overgrowths and clay cements are intense and advanced.
Quartz overgrowths increase as sorting increases and verse versa. It ranges from moderate to high in the reservoirs. Dissolution and clay cementation increases as sorting decreases. The amalgamated reservoir has a lower amount of quartz overgrowths, and a higher amount of dissolution and clay content. Overbank deposits are composed of higher amounts of quartz overgrowths, and lower dissolution and clay cements. Porosity value is higher in the north than in the south due to the variations in the amount of clay cementation between the two ends.