Rock Fabrics and Textures in Structurally Localized Dolomite Reservoirs
Structurally-localized dolomite reservoirs and temporally-related outcrops in North America and globally preserve a range of rock fabrics and textures that reflect the dynamics of emplacement processes. Replacive and void-fill saddle dolomite habits are characteristic of these settings, but genetically-related leached limestones are included. Earliest fabrics include ‘bilateral’ and ‘rimmed’ microfractures that record one or both elements of conjugate shear fracture sets, and increase in frequency proximal to extensional and/or strike-slip (and other?) faults. These shear microfractures compartmentalize expansional zebra fabrics and define boundaries of boxwork vugs, and, where closely spaced adjacent to faults, define short-segment ‘ladder’ and ‘chevron’ zebra fabrics. The microfractures are interpreted to herald stress buildup and displacement of faults, and are the initial conduits for flow of dolomitizing fluids. Extensional and transtensional spaces along faults are the preferred sites for formation of dilational breccias by hydrofracturing (?). Inclined zebra fabrics in planar and tangential geometries reflecting horizontal shear stress appear to reverse shear vector across faults. Internal dolomite sediments, variably cross-bedded, scoured and texturally-graded, also record flow dynamics within fault or related cavity systems. Limestone dissolution occurs at several phases of dolomite emplacement, in some settings after formation of ‘rimmed’ microfractures, leading to their displacement by collapse brecciation and accumulation of geopetal ‘floating’ saddle dolomite textures and cements. Microporous leached limestones often occur distal to faults, at dolomite-limestone fronts. Micro-textures include authigenic illite between ‘floating’ saddle dolomite crystals, doubly-terminated quartz crystals, and thermally-zoned kerogens with peripheral methane bubbles reflecting ‘forced maturation’ processes.
A variety of secondary pore types, including ‘zebroid’ pore systems in incompletely cemented (and/or leached?) zebra fabrics and extensive microporosity in leached limestones, contribute to many trillions of cubic feet of gas production from these reservoirs. High flow rates typically reflect later tectonic fracturing.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009