Mudstone diagenesis in the Cenomanian–Turonian Eagle Ford Group in the San Marcos Arch area. PART I: Chemostratigraphy, early diagenesis, bitumen expulsion and migration pathways

The Cenomanian–Turonian Eagle Ford Group (EFG) is composed of massive, laminated, and/or burrowed limestones, marls, and lime mudstones deposited below storm-wave base on the drowned south Comanche Shelf. Also, eolian deposited volcanic ash layers are common being preserved as individual ash beds or reworked into the sediment. An investigation of diagenesis relative to burial delineated a complex diagenetic history as a result of changes in burial history and associated changes of water chemistry and redox conditions. The first part of this comprehensive study sheds light on initial depositional redox conditions, early mineral diagenesis, and expulsion and migration pathways of generated bitumen and oil from kerogen of the EFG in the study area (maximum burial depth <6000 ft [1828.8 m]). The EFG followed a diagenetic pathway from the deeper marine directly into the burial environment (not affected by meteoric diagenesis). Original depositional porosity of the EFG marl could be as high as 70–80%, whereas present-day porosity values have been decreased to around 10% because of burial related diagenesis. At shallow burial depths, compaction was the mechanism responsible for most porosity loss followed by cementation. The texture and fabric of these cements increase mechanical strength of the EFG mudrocks significantly. The shallow, immature EFG limestones and marls have abundant interparticle micropores, high porosity, permeability, and water saturation with relatively low resistivity. With thermal maturation, bitumen and hydrocarbon generation increases oil saturation and alters wetness of the rock. The migration and filling of the bitumen and possibly oil would limit further cementation of calcite and quartz. The interaction between diagenesis of mineral matrix and organic matter (OM) evolution is significant and notably impacts cementation in mudstones at relatively shallow burial depth. The impact of OM is also more remarkable in mudrocks compared to that in sandstone and carbonate counterparts.