Abstract

Hybrid dolomitization of Permian carbonate strata by 1) Permian reflux-mechanical compaction dolomitization and 2) Late Eocene -Early Miocene tectonic-topographic driven-hydrothermal re-dolomitization (recrystallization) enhanced reservoir porosity-permeability-connectivity of strata, and developed residual oil zones (ROZ’s)-tilted oil/water contacts in Permian Basin reservoirs.

This two-step process of 1) dolomitization and 2) re-dolomitization (recrystallization) improved reservoir productivity. Down side is that residual oil zones (ROZ’s)-tilted oil/water contacts were created. This process extended through ramp/shelf strata and down-dip into open-system basinal lithofacies.

Finale was cave development in the Guadalupe Mountains, with Carlsbad Caverns the most famous.

Reflux-Mechanical Compaction Dolomitization: Permian reflux-mechanical compaction dolomitization of Permian strata during initial-shallow burial produced clear, E-planar dolomite, and preserved original rock fabric. Porosity-permeability was preserved in grain-rich strata, with additional porosity created as brine(s) became depleted with respect to Ca²⁺ and CO₃^2-, which caused partial dissolution (↑poro-perm) of carbonate grains during dolomitization.

Dolomitizing brine(s) sourced from a distant, broad, inner ramp/shelf lagoon environment. During burial, up-dip Mg²⁺ rich brine was expelled by mechanical compaction to dolomitize down-dip carbonate strata.

Relative sea level fluctuations (late highstand-early lowstand) restricted the inner ramp/shelf and formed Mg²⁺ rich brine. Middle Permian Grayburg Formation ramp strata contain inner ramp anhydritic dolostone and evaporite strata 1.25->3 times wider than down -dip carbonate strata, while upper Middle Permian inner shelf evaporite strata are wider than down-dip shelf margin carbonate strata.

Reflux-mechanical compaction dolomitization provided a more efficient way to dolomitize carbonate ramp/shelf margins.

Tectonic-Topographic Driven-Hydrothermal Re-dolomitization (Recrystallization):

Late Eocene-Early Miocene uplift formed Southern Rocky Mountain Epeirogen (SRME) via crustal heating as plutons and volcanism emplaced Trans-Pecos magmatic province and North American Cordilleran alkali igneous belt (tectonic).

Hot, High Pressure, High Volume Meteoric Recharge (Late Eocene-Early Miocene) By 38-35 Ma an erosional surface extended across New Mexico, an immense upland recharge area into the Permian Basin (topographic driven). Meteoric water heated to 113°-224° C by contact with plutons (hydrothermal). Meteoric recharge partially dissolved Permian dolomite (↑poro-perm) and precipitated inclusion-rich dolomite (↓poro-perm), containing hydrocarbon-aqueous inclusions due to oil columns being in-place. Once moveable hydrocarbons were swept out of structural-stratigraphic traps, dissolution of Permian dolomite (↑poro-perm) precipitated clear, inclusion-poor, E-planar, limpid dolomite (↓poro-perm). Continued dissolution etched dolomite crystals and improved lateral connectivity of strata (↑poro-perm). In cathod-oluminescence, dolomite crystals contain a “buck shot” appearance due to multiple dissolution-reprecipitation events. Meteoric recharge dissolved K-feldspar grains in siliciclastic strata, producing secondary porosity (↑poro-perm). Dissolution of evaporite cement and carbonate (↑poro-perm) repositioned strati-graphic traps further up-dip. Meteoric recharge entered open-system basinal debris flows.

Area of meteoric recharge was 130 miles wide, contained an enormous head-of-energy, and partially-completely swept Permian Basin structural-stratigraphic traps (oil fields) of primary and secondary recovery mobile oil to residual oil saturation to waterflood (S_row).

Cool, Low Pressure, Low Volume Meteoric Recharge (Middle Miocene-Present) Rio Grande Rift extension, Middle-Late Miocene, destroyed the massive recharge area and allowed oil columns to partially-completely re-saturate with oil-gas. Some oil columns did not re-saturate. Residual oil zones (ROZ’s) (brownfields-greenfields) and tilted oil/water contacts are preserved in oil fields and fairways. Meteoric recharge was from small mountain ranges still attached to the Permian Basin.

Reduced meteoric recharge allowed hydrogen sulfide (H₂S) to migrate out of Delaware Basin source rocks. H₂S mixed with meteoric recharge to generate sulfuric acid that dissolved into Capitan Limestone to form >300 caves (↑poro-perm) along Guadalupe Ridge as the water table fell between 12-4 Ma.