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Petrology of High-Maturity Samples of the Barnett Shale

Milliken, Kitty L.*1; Esch, William L.2; Reed, Robert M.1
(1) Bureau of Economic Geology, University of Texas, Austin, TX.
(2) ExxonMobil Upstream Research Company, Houston, TX.

Integrated petrographic and bulk compositional analysis of a suite of high-maturity (Ro 1.52 - 2.15) samples from the eastern side of the Fort Worth Basin reveals considerable lithologic heterogeneity. More heterogeneity is apparent from this integrated analysis than is apparent from either petrography or geochemical analysis alone. Primary detrital composition in these rocks arises as 4-component mixing system comprising two grain size populations (silt + sand versus clay) each of which includes compositional variants relating to extrabasinal versus intrabasinal derivation of the particles. Quartz is present in all four fractions and locally dominates as many as three fractions, even in a single sample, thus limiting the use of bulk analytical methods for accurate assessment of this important mineral component.

Authigenic minerals in these samples are most evident as grain replacements and include quartz, dolomite, and chlorite. Dark-luminescing microcrystalline (clay-size) quartz closely intergrown with clay minerals forms argillaceous chert and is notable within samples that contain less and finer extrabasinal siliciclastic debris.

Organic matter is observed in the form of silt-size particulate material of terrestrial origin and also as material filling sub-micron intergranular spaces. Both OM types develop secondary pores, but where intimately admixed with clays the intergranular type of OM (interpreted as pyrobitumen) is especially porous.

The integrated bulk analytical and petrographic approaches used here contributes to a more detailed understanding of primary versus diagenetic causes of lithologic heterogeneity that we hope will prove useful for ultimately building predictive paradigms for relating rock properties to a mappable lithostratigraphic framework.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California