Depositional Environments and Diagenesis of Lower Pennsylvanian Caddo Formation, Stephens County, Texas

Michael Guzan, John Humphrey

Since its discovery in 1916, the Lower Pennsylvanian Caddo limestone has been one of the most prolific producing intervals in the central Texas region. Favorable reservoir characteristics, with porosities from 9 to 24% and permeabilities from 0.1 to 1,000 md in the pay interval, have enabled many oil companies to initiate successful secondary recovery waterflood projects. Since its discovery, more than 200 million bbl of oil has been recovered from this formation in Stephens County alone.

The Caddo reef trend developed during the Early Pennsylvanian, generally along the axis of the Bend flexure. Close examination of whole core and thin sections reveals the formation to be a series of small reef mounds characterized by wackestones. The wackestones consist of green algae, brachiopods, and some rugose corals, which are found on the leeward side (back reef). This sequence is commonly found in the top segment of the Caddo. Deeper into the formation the sequence grades into a packstone/wackestone, characteristic of an oceanward margin (fore reef), and is distinguished by red algae, foraminifera (fusulinids and miliolids), and crinoid hash. The basal sequence is a series of alternating lime mudstones and shales that are heavily silicified.

Diagenesis is the key to reservoir rock quality. Moldic porosity was created through dissolution of organisms composed of low magnesium calcite (brachiopods) and aragonite (corals) during periods of subaerial exposure of the back reef. During this period of low sea level, fore-reef sediments were partly cemented by CaCO₃ and were not subaerially exposed. Therefore, moldic porosity was not created in the fore-reef sediments. Porosity in the fore reef was later created in places by dissolution of this cement. Secondary dolomite was later precipitated along pore walls, and in places it occluded porosity in the pore spaces.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.