Hybrid Model of Dolomitization, Permian Basin

Abstract

A hybrid model of dolomitization is responsible for dolomitization of Permian carbonate strata via: 1) reflux-mechanical compaction dolomitization during the Permian; and 2) tectonic and topographic driven dolomitization during the Late Eocene to Early Miocene.

Reflux-Compaction Model:

Study of >3000 thin sections and >150 cores revealed that dolomitization of Permian strata was during initial burial as strata experienced mechanical compaction. Dolomitizing brine was sourced from a distant, broad, inner ramp lagoon-intertidal environment. During burial, Mg²⁺ rich brine was expelled by mechanical compaction to dolomitize down-dip carbonate strata. Proof was mechanical compaction of grains and mud prior to lithification by dolomitization and is herein called the reflux-mechanical compaction model of dolomitization.

Dolomitization formed cloudy, inclusion-rich, E-planar, highly etched dolomite. Porosity was preserved in grain-rich strata, with additional porosity created as the brine became depleted with Ca²⁺ and CO₃^2-, with partial dissolution of carbonate grains during dolomitization.

Relative sea level fluctuations during late-highstand–early-lowstand restricted the inner ramp to form Mg²⁺ rich brine favored density-driven circulation of brines. Inner ramp strata are 1.25-3+ times wider than down-dip carbonate strata. A problem is the speed with which brine could move from up-dip to down-dip.

Tectonic and Topographic Driven Models:

During the Late Eocene-Early Miocene, uplift formed the Southern Rocky Mountain Epeirogen (SRME) and Rio Grande Rift (RGR) via crustal heating as intrusive plutons and extrusive volcanism formed the Trans-Pecos magmatic province and larger North American Cordilleran alkali igneous belt.

By 38-35 Ma an erosional surface extended across New Mexico to form an immense recharge area into the Permian Basin. Meteoric water heated to 113°-224° C by contact with plutons. During recharge, undersaturated meteoric water partially dissolved Permian dolomite and precipitated clear, inclusion-poor, E-planar, limpid dolomite cement. Area of meteoric recharge was 130 miles (209 km) wide and partially to completely swept Permian Basin oil fields of primary and secondary recovery oil to residual oil saturation to waterflood (S_row), forming residual oil zones (ROZ’s). Extension of RGR, during Middle-Late Miocene, destroyed the massive recharge area and allowed oil columns to partially to completely resaturate with oil or gas.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018