Diagenesis and reservoir characterization of the Pennsylvanian Middle Atoka Formation, Sebastian and Logan Counties, west-central Arkansas.

摘要

The Middle Atoka Formation evolved from a stable passive margin during the early Pennsylvanian time to a rapidly subsiding basin, with the sedimentary fill thickening greatly southward during the middle Pennsylvanian time. The basin dips in north-south direction. An east-west anticlines and synclines were observed.;The purpose of this study is to establish the stratigraphic units, reservoir geometry and distribution, and infer the depositional environments and reservoir quality. Well correlation and petrographic studies are used to achieve the goals.;Sandstone Point Count method was used. The Point Counts was divided into five categories. They include the framework grains (quartz, feldspar, and lithic fragments), accessory minerals (muscovite and biotite), cementing materials (quartz overgrowths, feldspar overgrowths, dolomite, and calcite cements), pore-spaces (primary and secondary), and the "Other" (minerals that cannot be identified under the microscope, and matrix and pyrite).;Vertical sequences of sand bodies that are closely spaced and separated by thick marine shale intervals, and sandstones that blocky signatures and abrupt bases and tops, were correlated as genetically related sand bodies. Four stratigraphic units; Borum, Turner, Nichol, and Basham Sandstone Units were identified.;The sandstones of the Middle Atoka Formation are composed of very fine silt to coarse quartzarenites, subarkoses, sublitharenites, and litharenites. The reservoirs are heterogeneous and were divided into two: the amalgamated reservoirs (proximal and medial submarine fans), and the overbank reservoirs (levee-overbank deposits, crevasse splays, and distal lobes). The geometries of the reservoirs are elongate and radial depending on the stratigraphic units. Quartz overgrowths and clay cements are intense and advanced and variable within the two reservoirs.;Porosity loss was significantly caused by compaction and quartz cementation. Secondary porosity, where it occurs, resulted from the dissolution of the labile grains, probably from the interaction with migrating organic acids or as a result of the increasing geothermal gradient. Diagenetic processes either enhance the porosity by dissolving mineral grains, or reducing the porosity by stimulating growth of clay and quartz minerals.;Amalgamated reservoirs contain higher amount of dissolution, clay cements, and lower quartz overgrowths. Dissolutions are filled with clay cements. Dissolution when present in overbank reservoirs are better preserved than is in amalgamated portions. Dissolutions and clay cements are also higher in the south than is in the northern part. Higher clay content in the south ensures that the dissolution is almost effectively occluded by clay cements. Consequently there seem to be no net-gain in porosity despite in the area.