Sedimentology, Diagenesis, and Porosity Development of Middle Silurian Lockport Dolomite, Johnson County, Kentucky: ABSTRACT

A complete core of the Lockport Dolomite reveals the presence of five distinct lithofacies. Proceeding upsection from the underlying littoral Keefer Sandstone, these units are: (1) extensively burrowed and sparsely fossiliferous mudstone-wackestone (initial transgression); (2) thinly bedded, crinoidal packstone-grainstone (open shelf); (3) stromatoporoid-coral-crinoidal packstone (open shelf); (4) oolitic-peloidal packstone-grainstone (shoal); and (5) finely laminated mudstone (tidal flat). The middle to upper sections reflect a shoaling-upward sequence that grades into the overlying Salina Formation sabkha sediments. Original rock textures have been veiled for the most part by dolomitization. Petrographic analysis indicates that dolomite occurs in two forms: (1) as a fine to medium crystalline replacement mineral, and (2) as a coarse crystalline cement (including saddle dolomite) filling vugs and fractures. The first variety is eogenetic, having formed primarily as a result of freshwater/seawater mixing during occasional subaerial exposure of sediments. The coarser dolomite is of late (mesogenetic) origin. Historically, the Lockport has been an important oil and gas producer in eastern Kentucky. Secondary porosity development is significant and End_Page 1441------------------------------ appears to have been a function of sedimentary facies and early dolomitization, coupled with freshwater invasion and solution. Intercrystalline (dolomitic) and vuggy (solution) porosity are present throughout the section but are particularly conspicuous in the open-shelf and shoal facies where they reach values of 6-10%. These particular facies had a higher relative abundance of mineralogically metastable skeletal grains and accommodated greater pore-water flow during early diagenesis. Reservoir quality in these facies has been significantly enhanced by good fracture permeability. End_of_Article - Last_Page 1442------------