Stratigraphy and Paleodepositional Interpretation of the Lower Nacatoch Formation (Maastrichtian), Robertson County, Texas

The lower Nacatoch Formation of the Maastrichtian Navarro Group is considered to be a secondary target for energy companies exploring oil and gas reservoirs. Previous investigators of the northern and western sections of the Nacatoch Formation interpreted this unit to represent a variety of coastal depositional environments. However, investigations have been limited in the southern extent of the Nacatoch Formation, which has created a gap in the data. Well logs, drill cuttings, and core descriptions were gathered to create cross-sections and maps used to interpret geometries, sediment distribution, grain sizes, fauna, and internal stratigraphy of the lower Nacatoch. In the study area on the northwestern corner of Robertson County, Texas, the lower Nacatoch interval is composed of laminated sand and shale of variable thickness and has bar forms trending northwest to southeast. It is interpreted to represent a starved, shallow shelf storm-dominated transgressive ridge/bar deposit. Stratigraphic heterogeneity of the lower Nacatoch is significant because of the effects it has on the vertical and horizontal permeability, as well as, completion methods needed for optimal extraction.