Origin of Peloidal Textures in Black River and Trenton Carbonate Petroleum Reservoirs in the Appalachian Basin

Jaime Kostelnik and Christopher D. Laughrey, Pennsylvania Geological Survey, Pittsburgh, PA 15222

Limestones and dolostones in the Ordovician Black River and Trenton Formations of the Appalachian basin contain normal marine skeletal assemblages and a variety of non-skeletal grains including ooids, peloids, and aggregates.  Peloids are the most abundant non-skeletal carbonate grains observed in the rocks and their origin is both complex and diverse.  Some peloids occur as original framework grains formed by physical processes, and some are diagenetically altered grains.  Most peloids in the Black River and Trenton rocks, however, are not grains, but are marine cements that precipitated in situ on or slightly below the sea floor.

Peloids that we interpret as cement are decimicron-sized and consist of a dark nucleus composed of micron-sized calcite surrounded by a rim of euhedral microspar. The nuclei consist of clots of submicron-sized opaque material, which may be organic or mineral. These characteristics in conjunction with uniform crystal size, restricted size range, consistent texture, monomineralogy, and association with hardgrounds and other marine cements support our interpretation of these peloids as cement. In limestones, the peloidal cements occur as groundmass, as internal cement within skeletal grains, as cement filling fabric-selective pores, and as a mimic replacement of allochems. In dolostones the peloidal cement occurs as ghosts within planar and nonplanar dolomite.

Precursor peloidal cement fabrics are ubiquitous in Black River and Trenton hydrothermal dolomite reservoirs. The original textures of peloidal limestones might have influenced subsequent dolomite fabrics and the development of porosity in the rocks. If so, determining diagenetic facies may be an important component in understanding and developing Black River and Trenton carbonate reservoirs in the Appalachian basin. 

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York