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Palo Pinto (Missourian) Algal Limestone Buildups as Hydrocarbon Reservoirs, Conley Field, Hardeman County, Texas

Steven E. Lovell, Wayne M. Ahr

Mississippian and Pennsylvanian carbonates are important hydrocarbon reservoirs in the Hardeman basin of north Texas. The basin axis extends northeast-southwest, and stratigraphy at Conley field indicates that the area was structurally high during both the Mississippian and Pennsylvanian. The Palo Pinto Formation at Conley field is composed mainly of platy algal packstones with subordinate fossiliferous shales. The algal limestones thicken over the crest of the ancient structural high, where the low lime-mud content and the abundance of platy algae indicate that the depositional environment for the Palo Pinto was shallow (less than 100 ft in depth) and current-swept marine in character. Local thickening of the Palo Pinto can be explained as growth of platy algal bafflesto es which nucleated on the crests of paleostructural highs. The shales indicate periodic inundation accompanied by renewed terrigenous clastic influx. The highest parts of the carbonate buildups were altered by freshwater diagenesis during episodic periods of emergence. Dissolution of metastable allochems led to the formation of moldic porosity. Subsequently, during burial diagenesis, occlusion of pore space by calcspar cements, neomorphism, pressure solution, and some replacements occurred. The best porosity and permeability occur in packstone and grainstone facies, indicating that the reservoir mainly is controlled by depositional processes and only altered to an extent by diagenesis. Fracture porosity is not a major factor in this reservoir.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.