CONVENTIONAL WISDOM PREVAILS IN UNCONVENTIONALLY EXPLOITED OIL PLAYS: AN EXAMPLE FROM THE GIANT MISSISSIPPIAN LIME CARBONATE SYSTEM IN OKLAHOMA

In north central Oklahoma, a thick sequence of Mississippian fractured carbonates hosts a regionally extensive system of hydrocarbon accumulations in water-free oil traps and their associated transition and waste zones. Structural traps align along deep basement-involved fault zones where reservoir enhancement via alteration of limestone to chert is genetically related to the upward movement of granite-derived fluids along near-vertical reverse faults. Stratigraphic traps associated with exposure and truncation cover multi-county areas. These conventional traps were discovered and exploited in the 20^th century using vertical drilling and simple completion practices. Unconventional drilling and completion techniques, initially established for developing gas shale fairways, are now being used to explore for and exploit the waste zones and transition zones. Because the system is regionally heterogeneous and fraught with natural fractures, fluid ratios are variable and water content is generally very high throughout the transition and waste zones. While unconventional technology, including horizontal drilling and multistage fracking are required to commercially exploit the water-rich portions of the system, the oil is not trapped unconventionally; that is, in situ within the source rock sequence. Instead the occurrence of water-free oil and oil-free water, and the distribution of all permutations in between, are a function of the interplay of pore geometry and structurally controlled height above free water. Regardless of the technology used, the factor that governs oil and water distribution throughout these giant heterogeneous carbonate systems, including many in the Permian and other onshore basins, is the local battle between buoyant forces and entry pressure. Successful exploitation of these plays will require application of sound conventional trap models prior to the use of expensive unconventional drilling and completion technology.