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Dolomite Textures and Porosity Development in Trenton and Black River Formation (Middle Ordovician) HTD Carbonate Reservoirs, Appalachian Basin, USA

Christopher D. Laughrey1 and Jaime Kostelnik1

1Pennsylvania Geological Survey, Pittsburgh, PA 15222

Two distinct dolomite textures – planar and transitional to nonplanar – occur in fractured carbonate petroleum reservoirs of the Trenton and Black River Formations in the Appalachian basin. Planar dolomites consist of micron- to centimicron-sized, euhedral to subhedral crystals that selectively replace limestone matrix in all depositional facies. Less common planar-p and planar-e dolomite locally replace allochems, and rare planar-c dolomite fills pore space. Planar textures in peritidal facies might have formed during reflux and/or mixing zone diagenesis. Planar textures in subtidal facies likely formed in association with compaction-driven fluid flow in burial environments. Planar textures also occur where hydrothermal dolomite replaces limestone adjacent to fractures and faults.

Transitional to nonplanar dolomites are entirely hydrothermal. They consist of decimicron- to millimeter-sized crystals in fractured rocks adjacent to faults. Transitional and nonplanar-a dolomites occur as an obliterative replacement of allochems, matrix, and cement in limestones, and as neomorphic recrystallization of planar dolomite. Nonplanar (saddle) dolomite occurs as pore-lining and pore-filling cement.

Distinctive pore textures in Trenton and Black River reservoirs vary with carbonate rock type. Porosity is partially related to depositional texture in productive dolograinstones and dolopackstones. These rocks consist of planar-s to nonplanar-a and saddle dolomite. Mesoporosity is fabric selective, consisting of moldic and intercrystalline voids. Macroporosity is not fabric selective, and consists of small to medium vugs and fractures. Microporosity is both intercrystalline and intracrystalline. Porosity formed through a combination of fracturing, selective dissolution of allochems, and dissolution of calcite and dolomite cements.

Porosity in productive dolowackestones and dolomudstones is not fabric selective. Macroporosity consists of vugs, channels, fractures, and breccia. Vugs formed through selective dissolution of allochems followed by corrosive enlargement of the initial void. Vugs also formed through dissolution of calcite, dolomite and other cements. Channel, fracture, and breccia porosity developed in conjunction with hydraulic fracturing and brecciation by hydrothermal fluids. Some channel porosity formed through reopening stylolites.

Hydrothermal dolomitization accompanied porosity development in Trenton and Black River reservoirs. It did not provide or increase it. Porosity in all dolostones was modified by late-stage precipitation of nonplanar dolomite, calcite, quartz, feldspar, sulfides, sulfates, and oxides. Pyrobitumen lines and fills most pores.

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