- Devonian Strata overlay the Proterozoic and Cambrian rocks over most of the eastern Holbrook Basin. North of the Holbrook Basin (HB), Devonian sands (McCracken) are productive where they were deposited around pre-Devonian topographic relief along the northwest margin of the Defiance-Zuni Uplift. Approximately 100,000 barrels of oil have been produced from these basal Devonian sands in northeastern Arizona at the Walker Creek Field (Rauzi, 1996). South of the Holbrook Basin, basal Devonian sands (Beckers Butte, BB) were deposited on a surface of as much as 300 feet of local relief along the Mongollon Rim and in Salt River Canyon (Huddle and Dobrovolny, 1952). The isopach pattern of Devonian Rocks indicates that similar pre-Devonian relief is present at depth in the eastern Holbrook Basin. Local depressions and embayments with similar basal sand deposits are probable along the entire length of the southwest margin of the Defiance-Zuni Uplift in the eastern Holbrook Basin.
- Drifting plankton and algae, abundant flora of psilophytes (primitive land plants) and layers of lime mud rich in organic matter probably filled the local depressions and embayments (Teichert, 1965). As a result, these areas may contain source rocks and significant potential for stratigraphic and subtle structural traps in the pre-Permian strata in the subsurface of the eastern Holbrook Basin. Basal Devonian sands, 10 to 20 feet thick, usually occur below a thick sequence of dark brown, petroliferous limestone in outcrops along the Mogollon Rim (Huddle and Dobrovolny, 1945). In like manner, basal sands deposited in local depressions and embayments along the southwest margin of the Defiance-Zuni Uplift may contain trapped hydrocarbons generated from organic-rich source rock. Such hydrocarbon source rock at depth in the eastern Holbrook Basin may have geochemical analyses similar to or better than Devonian mudstones that crop out in Salt River Canyon. The Devonian mudstones in Salt River Canyon have a total organic carbon content of 2.81 percent and are within the oil generating window (Desborough & et al.).
- The Mississippian Redwall Limestone maintains a fairly consistent thickness of about 100 feet across most of the Holbrook Basin and wedges out between Devonian and Pennsylvanian rocks along the southwest margin of the Defiance-Zuni Uplift Zone (DZUZ). Mississippian rocks are not present in a broad area northwest of Heber, probably because of late or post Mississippian uplift and erosion (Havenor and Pye, 1958). More than 800,000 barrels of oil and 385 million cubic feet of helium-bearing gas have been produced from Mississippian carbonate units in northeastern Arizona (Rauzi, 1996). As a result, the hydrocarbon and helium potential of Mississippian rocks in eastern Holbrook Basin should not be dismissed, especially along the southwest margin of the Defiance-Zuni Uplift. Mississippian rocks may contain hydrocarbons or helium-bearing gas, especially where truncated between underlying Devonian hydrocarbon source rocks and overlying impermeable Pennsylvanian shales.
- The fossiliferous Pennsylvanian strata at depth in the eastern Holbrook Basin, therefore, may very well have generated and trapped hydrocarbons, especially along the southwest margin of the Defiance-Zuni Uplift, where Tertiary volcanic/intrusive activity may have enhanced the hydrocarbon generation and potential of Pennsylvanian strata, much as it has enhanced the generation and production of hydrocarbons in northeastern Arizona. Most of the shoreline clastic rocks of Pennsylvanian age in the eastern Holbrook Basin are shales, calcareous siltstones, and silty limestones, indicating that the southwest margin of the Defiance-Zuni uplift remained relatively low relief, at or slightly above sea-level through Pennsylvanian time.
- The Pennsylvanian Naco Formation grades from unfossiliferous red beds in the western part of Holbrook basin into fossilferous carbonate beds in the eastern and southeastern part of the basin, where they are lithological similar to the “Bough” zone of probable upper Pennsylvanian age in southeastern New Mexico (Kottlowski et al, 1962). The bough zone produces oil in three fields and appears to have similar relationships to the Matador arch as the Naco has to the Defiance-Zuni uplift (analog).
- Massive to nodular fossiliferous limestones of Pennsylvanian age crop out in the southeastern most part of the Holbrook Basin on the North side of Escudilla Mountain in 28-7n-31e. These outcrops indicate that a fairly thick sequence of Pennsylvanian marine strata was deposited in this portion of the Holbrook Basin and is present at depth beneath the White Mountain volcanic field. A well drilled into the Permian Supai in 1993 on the south side of Escudilla Mountain shows extensive volcanic rocks forming in the White Mountains are not extensive at depth and have not been detrimental to the oil and gas potential for this region. This volcanism, in fact, may have locally enhanced the potential for oil and gas generation and accumulation as it has in northeastern Arizona. Bleeding oil from Permian Carbonate units in the hole drilled south of Escudilla Mountain attests to the presence of hydrocarbons at depth beneath volcanic rocks in the White Mountain Area (Rauzi, 1994).
- The large, organic rich reefs and associated lagoonal deposits, suggested at depth by the fossilferous Pennsylvanian rocks that outcrop on the north side of Escudilla Mountain, have significant potential for generation, accumulation and production of oil and gas along this part of the southwest margin of the Defiance-Zuni Uplift. Clearly the White Mountain region of the Holbrook Basin should not be overlooked, it has favorable paleogeography and potential for hydrocarbon production.