--> ABSTRACT: Depositional Sequences and Porosity in the Salem Limestone (Mississippian), Colborn Field, Southeastern Illinois, by Arthur H. Saller, Hutch Jobe; #91020 (1995).

Datapages, Inc.Print this page

Depositional Sequences and Porosity in the Salem Limestone (Mississippian), Colborn Field, Southeastern Illinois

Arthur H. Saller, Hutch Jobe

Most of the Salem Limestone was cored in a well in southeastern Illinois, providing insight into its depositional facies, sequences, diagenesis and porosity evolution. Two types of shallowing-upward cycles, open-marine and lagoonal, were observed. Open-marine cycles contain burrowed fossiliferous wackestone/packstones, passing upward to burrowed packstone/grainstones and crossbedded grainstones, representing deposition in low-energy, open-marine environments at the bottom and passing upward to tidal grainstone shoal complexes at the top. Lagoonal cycles consist of (from bottom to top): (1) burrowed peloid mudstone/wackestones deposited in lagoons, (2) laminated mudstones, wackestones, packstones, and/or grainstones deposited by tidal currents on intertidal flats and lagoo al shoals, and either (3a) fenestral packstones deposited in high-energy intertidal and supratidal environments, or (3b) algal-laminated mudstones and wackestones deposited on low-energy tidal flats. Ten shallowing upward cycles were identified in the 480-foot-thick Salem section. Open-marine cycles dominate in the lower and upper Salem. Lagoonal cycles dominate in the middle Salem.

Two main pore types were observed in the Salem: intergranular pores in lime grainstones and intercrystalline pores in dolomitized wackestones and packstones. Grainstones were originally composed of calcitic grains, mainly ooids, peloids, bryozoa, and echinoderms. Only 30% of the lime grainstones have reservoir-grade porosity (5-12%). Porosity occurs in bryozoa grainstones and other grainstones which had early, scattered calcite cement which retarded compaction. Burial compaction obliterated intergranular porosity in many grainstones without early cement. Lime grainstones with 5-12% porosity have permeabilities of 5-30 md, pore throat diameters of 10-30 µm, and oil. Dolomitized wackestones and packstone have substantial porosity (10-16%), but their permeabilities are low (0.2-2 md , pore throats are small (1-2 µm in diameter), and they do not contain oil even above the main oil/water contact.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995