Development of Organic and Inorganic Porosity in the Cretaceous Eagle Ford Formation, South Texas
Fishman, Neil; Guthrie, John M.; Honarpour, Matt
Petrographic and SEM, along with RockEval pyrolysis analyses were used to constrain the nature of organic material (OM) that contains porosity in the Cenomanian-Turonian Eagle Ford Formation, South Texas, where the formation is in the oil/condensate window (Ro ~1.2%). Samples used were from a well that contained intervals of both 1) foraminiferal mudstones with high (up to 8 wt%) total organic carbon (TOC) contents, deposited within the trangressive system tract (TST) or near maximum flooding surface (MFS) intervals, and 2) limestones with relatively lower TOC (<1 up to 6 wt%) contents, deposited largely in the overlying high stand systems (HST) track interval.
In mudstones, early diagenetic processes resulted in precipitation of euhedral-subhedral authigenic minerals (e.g., calcite, pyrite, kaolinite) that filled foraminifera and coccosphere tests (intraparticle pores) and partially filled interparticle pores between other detrital grains. In limestones, recrystallization of bioclastic material resulted in euhedral-subhedral microsparry calcite crystals between remaining interparticle pores. In both lithologies, OM coats the euhedral-subhedral minerals and locally fills intraparticle and interparticle pores, but this superposition relationship is particularly well developed in mudstones whereas OM is less common in limestones. Pores in the OM range in size from <0.1 μm to ~1 μm across, and are variably round, elliptical, or irregularly shaped. For both lithologies, OM was clearly emplaced after authigenic mineral precipitation, and porosity development subsequent to its emplacement. For TOC-rich mudstones in the TST, RockEval pyrograms generated on the same samples before and after solvent extraction indicate the presence of a relatively greater amount of extractable phase (i.e., bitumen), observed as a shoulder on the S2 peak of the pyrogram. In contrast, the TOC-lean limestones from the HST contain a relatively lower amount of the extractable phase (S2 shoulder) and a greater amount of "free" hydrocarbons measured as the S1 peak from Rock-Eval relative to the TOC-rich mudstones.
Given its inferred mobility and relative post-depositional timing of emplacement, the OM that coats authigenic minerals is presumed to be the bitumen identified from RockEval analyses. As such, organic porosity in the Eagle Ford appears to be spatially linked to the dispersal of bitumem, whereas free hydrocarbons appear to be lithologically controlled.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013