Abstract: Deltaic Depositional Environments of Permian Admire Sandstone, El Dorado Field, Butler County, Kansas

Gary Flesch

Analysis of seven slabbed cores from the Admire ("650 ft") sandstone was undertaken to interpret the environment of deposition and to determine rock parameters controlling porosity and permeability of the reservoir. This study is in support of a tertiary oil recovery project in El Dorado field, Butler County, Kansas. The reservoir is shallow (650 ft deep) and thin (11 to 23 ft thick) and has produced 40.2 million bbl of oil since the discovery in 1915.

The vertical sequence consists of (in ascending order): Unit 1, predominantly claystone unit (17 ft thick) with lenticular laminations of sandstone, siltstone, and carbonaceous material; Unit 2, sandstone (Admire, the reservoir), medium-grained, with trough cross-stratification, climbing ripples, claystone laminations; and Unit 3, fossiliferous, fine-grained carbonate rocks interbedded with claystone and lignite.

Interpretation of the depositional environment is based on (1) internal sedimentary and biogenic structures, (2) external unit geometry, (3) unit lithology, and (4) the vertical sequence.

The environmental setting is deltaic with close intermixing of swamp, bay, and distributary-channel subenvironments in the vertical sequence. The three lithologic units have the following genetic interpretations: a basal (Unit 1) interdistributary bay overlain by (Unit 2) distributary channels with variations in the major channel positions, overlain by (Unit 3) a swamp or marsh and a marine open bay. Lateral and vertical changes in position of the distributary channels occurred through time causing variations in current flow and in resulting sedimentary structures. These changes had a marked effect on the porosity and permeability of the Admire (650 ft) sandstone.

Porosity and especially permeability of the reservoir vary in direct proportion to the lithology and sedimentary structures, which in turn are dependent on the depositional environment. Medium-grained, trough cross-stratified sandstone is typical of the distributary-channel facies and is the most favorable. Sandstone facies which originated as natural levees, subsidiary channels, and channel margins contain abundant claystone laminations, climbing ripples, and fine-grained sandstone, all of which contributed to reducing permeability. Diagenetic alteration by iron oxide or calcite cementation had only a minor effect on porosity and permeability.

APG Search and Discovery Article #90974©1975 AAPG Mid-Continent Section Meeting, Wichita, Kansas