Petrologic, geochemical, and scanning electron microscope (SEM) studies were undertaken on the Cretaceous Eagle Ford Formation in South Texas, to evaluate the nature and distribution of porosity as it bears on understanding hydrocarbon storage and producibility. Samples were analyzed from two cored wells at different levels of thermal maturity. One core is in the oil to condensate window (Ro ~1.2%) in the Eagle Ford and the second well, which is updip from the first well, is in the oil window (Ro ~0.7%). In both cores, samples were analyzed from similar lithologies including
foraminiferal mudstones with high total organic carbon (TOC) contents (up to 8 wt%), deposited in the lowermost trangressive system tract (TST) or near maximum flooding surface (MFS) intervals, and
limestones with relatively low TOC contents (< 1 to 6 wt%), interbedded with and deposited in the overlying high stand systems tract (HST) interval.
In both wells, early diagenesis in mudstones resulted in precipitation of euhedral authigenic minerals (e.g., calcite, pyrite, kaolinite) that partially filled foraminifera fossils (intraparticle pores) as well as interparticle pores. Bitumen subsequently coated (i.e., post-dates) earlier-formed authigenic minerals. In the high maturity well, porosity (7 to 10%, with pores observed by SEM ranging in size from < 0.1 ???m to ~1 ???m across) is present in the bitumen (organic porosity), whereas no organic porosity was observed in the low maturity well. Focused ion beam-SEM (FIB-SEM) analysis of foraminiferal mudstones revealed low interconnectivity of organic porosity (permeability ~50 nD) in the high maturity well. Early diagenesis in the limestones resulted in recrystallization of biogenic material into microsparry calcite crystals, between which interparticle pores (<0.1 to ~1 ???m across) remain. Bitumen lined the walls of many interparticle pores and only locally fills others in the limestones. Compared to the mudstones, the porosity of limestones (6–7%) is slightly lower but permeability (up to 1000 nD) is greater. Furthermore, FIB-SEM analysis reveals much higher pore connectivity in the limestones than in the mudstones. Programmed pyrolysis analysis reveals a higher hydrocarbon yield in interbedded and HST limestones compared to TST/MFS foraminiferal mudstones, which indicates that there is better hydrocarbon storage in the limestones. The pyrolysis data corroborates the petrographic, SEM, and FIB-SEM data.
Because bitumen hosts organic porosity only in the higher maturity well, the distribution of such porosity is directly linked to the post-depositional dispersal of bitumen and, subsequent higher levels of thermal maturity. However, HST limestones in the high maturity well contribute more to hydrocarbon producibility due to greater pore connectivity than underlying TST/MFS, TOC-rich mudstones with lower pore connectivity.