Subsurface investigation of the Pennsylvanian cross cut sandstone, TWP and Busher fields, Runnels County, Texas
Henderson, Steven Kirk
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The Upper Pennsylvanian (Missourian) Cross Cut sandstone of the TWP and Busher fields. Runnels County, Texas, is an example of the smaller hydrocarbon plays that are receiving increased attention in west- and north-central Texas. An understanding of the distributions and reservoir characteristics of these plays is necessary for ensuring the success of future exploration and development strategies. Sediment characteristics, petrophysical log responses, and sand body geometry of the TWP and Busher Cross Cut sandstone reflect deposition in a distal prograding delta environment. Gross sand isopach mapping reveals a strikeoriented sand body approximately three miles in length and one-half mile in width. These attributes are similar to characteristics of the Cross Cut sandstone in other Pennsylvanian fields, and support a delta-front interpretation. Delta-front sandstones of the TWP and Busher fields represent a southwestern distal extension of the Eastland Delta system. Log responses and sand body geometry may be employed to infer the position of distributary mouth bar and channel facies associated with the delta-front sands. Sediments of the Cross Cut sandstone were exposed to a variety of diagenetic processes that influenced the evolution and preservation of porosity and permeability within the reservoir. Early development of epitaxial quartz overgrowths significantly decreased depositional porosity. Replacive calcite may also have decreased primary porosity; however, the dissolution of this calcite produced minor amounts of secondary porosity. Late-stage diagenetic processes, including precipitation of kaolinite and ankerite cements, further occluded porosity. The types, abundances, and morphologies of clay minerals, especially kaolinite and chlorite, within the Cross Cut sandstone may influence reservoir quality, in addition to reducing initial reservoir porosity and permeability. Interaction of these clays with fluids introduced during drilling, completion, and production may cause dispersion and migration of fines and precipitation of insoluble compounds, further occluding effective porosity and decreasing permeability. Petrophysical and volumetric evaluation of the TWP and Busher Cross Cut reservoir reveals estimated recoverable reserves of 763,997 stock tank barrels of oil. Since the discovery of the Busher Field in 1956, both fields haveproduced a combined total of approximately 629,667 barrels of oil. The Cross Cut sandstone within the TWP and Busher fields is an essentially depleted reservoir.