ABSTRACT

The Mississippian Greenbrier Limestone is a major gas reservoir in the Appalachian basin, but its complex porosity patterns often deter active exploration. In southern West Virginia, the reservoir consists of oolitic tidal bars that are composites of smaller shoals. Porosity trends closely follow the ooid-grainstone facies that occupied shoal crests where coarse-grained, well-sorted ooid sand was generated with either unidirectional or bidirectional cross-beds. Nonporous packstone occurred in adjacent tidal channels, and a transitional grainstone/packstone facies of marginal porosity was situated along the flanks of the shoals. The key to drilling successful wells is in understanding the complex internal geometry of Greenbrier ooid shoals. A well penetrating the oolite with good porosity and bimodal cross-beds should be offset perpendicular to the dip directions; that is, parallel to the shoal axis. However, a well penetrating thin, porous limestone with one dominant cross-bed azimuth should be offset opposite to that dip direction; that is, up the flank of the ooid shoal. Shaly interbeds characterize the edges of the shoals and mark the limit of productive wells. Schlumberger's Formation MicroScanner log, which provides data on both lithology and cross-bedding, has proven to be a useful tool in predicting the distribution of oolite porosity.