ABSTRACT: Depositional Environments, Diagenetic History, and Porosity Development, Vacuum San Andres Field, Lea County, New Mexico

Jeffrey W. Robertson

The Permian San Andres Formation was deposited in environments ranging from supratidal to subtidal, protected shallow marine to open shallow marine. Pelletoid-skeletal packstone/wackestone and pelletoid-oolite-skeletal packstone/grainstone facies of the composite subtidal, marine environments were concentrated in the southern half of the field where grainstone deposition was linked to a paleotopographic high. Deposition in the northern half of the field was dominated by the pelletoid wackestone/mudstone facies of the supratidal, restricted shallow-marine environment.

Porosity is mainly diagenetic in origin. San Andres rocks passed through the marine phreatic, the mixed phreatic, the meteoric phreatic, and the deeper subsurface diagenetic environments. Matrix dolomitization in the marine and mixed-phreatic environment produced intercrystalline porosity in all the facies. Leaching of nondolomitized grains in the mixedphreatic and meteoric-phreatic environments created large moldic pores. Diagenetic patterns follow structural and depositional trends so that suites of characteristic pore types exist for each facies. Individual pore types consist of moldic, intergranular, intragranular, vuggy, and intercrystalline categories. Intercrystalline pores are present nearly everywhere, and predicting the abundance of moldic pores is the largest variable in pr dicting porosity values for a given stratigraphic interval. Highest porosity exists in the southern half of the study area where grainier facies were deposited on a paleotopographic high, and grains were partly dolomitized and subsequently were removed by leaching, leaving behind a composite intercrystalline-moldic pore network. Total porosity values do not correspond with facies boundaries, so porosity mapping was accomplished by dividing the San Andres into 20-ft-thick slices and computer contouring total porosity greater than 6% in each slice.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990