LINKING DIAGENESIS WITH DEPOSITIONAL ENVIRONMENTS AS IT BEARS ON PORE TYPES AND HYDROCARBON STORAGE—AN EXAMPLE FROM THE CRETACEOUS EAGLE FORD FORMATION, SOUTH TEXAS

Integration of sequence stratigraphy, diagenesis, and geochemistry provides a comprehensive understanding of the nature, distribution, and connectivity of pores in the hydrocarbon-productive Cretaceous Eagle Ford Formation, South Texas. For this study, samples were gathered from two wells that contain 1) foraminiferal mudstones with high (up to 8 wt%) total organic carbon (TOC) contents, deposited in the transgressive system tract (TST) or near the maximum flooding surface (MFS), and 2) limestones with relatively low TOC (<1 to 6 wt%) contents, deposited largely in the overlying high stand systems tract (HST). The Eagle Ford differs in thermal maturity between the wells, with the formation at ‘low’ maturity (Ro ~0.7%) in one and at higher maturity (Ro ~1.2%) in the other.

Early diagenesis in TST/MFS mudstones resulted in precipitation of euhedral pyrite, quartz, and kaolinite. These minerals filled foraminifera tests (intraparticle pores) and partially filled interparticle pores between detrital grains. In HST limestones, euhedral microsparry calcite precipitated from recrystallization of abundant foraminifera and coccoliths; interparticle pores remained between calcite crystals. In both lithologies, bitumen coats all precipitated minerals. Bitumen occludes pores in mudstones, whereas it lines pores and only locally occludes them in limestones. Subsequent porosity development in the bitumen (limited connectivity) was observed only in the high maturity well and principally in mudstones. Based on laboratory measurements and inferred from focused-ion-beam scanning electron microscopy, good connectivity exists between interparticle pores in limestones, which is consistent with higher hydrocarbon yield (S1 peak in RockEval analyses) from limestones relative to mudstones, and indicates that hydrocarbon storage is significant in limestones.

Microsparry calcite in the limestones is also responsible for their brittle nature compared to the more TOC-rich, ductile mudstones. Thus, rock brittleness (and related natural fractures) and pore connectivity, which were ultimately controlled by depositional and diagenetic processes, are key factors in hydrocarbon producibility from the Eagle Ford, which is why HST limestones are critical elements in hydrocarbon production from the formation.