Petrologic and Petrophysical Differences Between Sandstones In the Middle and Lower Parts of the Upper Jurassic Bossier Formation, East Texas Basin, Texas: Implications for the Source of Produced Gas

The diagenetic histories of gas-producing sandstones in the Upper Jurassic Bossier Formation, East Texas Basin, Texas, were studied to determine if the source of gas was from interbedded mudstones (currently overmature, with <1% TOC) or from other formations. Analyzed core samples from the middle part of the Bossier came from wells (n=3) in Freestone County and from the lower Bossier in a well in Limestone County.

Sandstones from the middle Bossier are A) fine- to very-fine-grained sublitharenites; B) cemented by quartz (<26 volume percent) as syntaxial overgrowths (containing no observable petroleum-bearing fluid inclusions), with lesser ferroan-dolomite (< 22%), illitic clays (< 5%), or pyrite (<1%); and C) characterized by low porosity (range 1.01 to 7.07%) and permeability (empirically-calculated range of 0.0012 to 0.075 mD), with median pore-aperture diameters (range 0.232 to 0.458 μm), based on high pressure (< 60,000 psia) mercury porosimetry. In contrast, sandstones in the lower Bossier are A) very-fine grained sandstone or siltstone (litharenite or fossiliferous litharenties); B) cemented by microsparry calcite (< 28 volume percent), and minor pyrite (0-3%); and C) characterized by very low porosity (range 0.717 to 3.85%) and permeability (empirically-calculated range 0.000052 to 0.00459 mD), with median pore aperture-diameters (range 0.0281 to 0.132 μm) based on mercury porosimetry.

Significantly different diagenetic histories between sandstones in the middle and lower Bossier point to intraformational fluid compartmentalization, at least for part of their post-depositional history. Fluid inclusion evidence indicates that gas accumulation in middle Bossier sandstones post-dated quartz cementation. If fluid compartmentalization persisted during gas accumulation, Bossier mudstones, despite being lean, could have charged reservoirs in close hydrologic connectivity to them. Although an external source cannot be eliminated, such would require cross-formational gas migration through low porosity/permeability rocks to charge multiple Bossier reservoirs; however, petrologic data are lacking to substantiate any cross-formational gas movement.