Abstract: The Panna–Mukta fields host hydrocarbons in the Bassein Formation Eocene–Oligocene ramp limestones. The pore system is almost wholly secondary, comprising microporosity, mouldic porosity, vugs, solution-enlarged stylolites and fractures. Although petrographical evidence points to dissolution after extensive late cementation, the presence of a high permeability layer close to a palaeokarstic surface at the Eocene–Oligocene boundary has raised the possibility that this secondary porosity could be related to subaerial exposure. However, the Panna–Mukta reservoirs show a strong correlation between secondary matrix porosity and stylolite density measured from cores. Stylolites only developed in ‘clean’ limestones lacking argillaceous material, whereas more argillaceous limestones in the succession are characterized by dissolution seams and have poor reservoir quality. These cleaner limestones occur preferentially below the Eocene–Oligocene boundary, representing an upwards-shallowing sequence, whereas the argillaceous limestones occur further below the Eocene–Oligocene boundary in the lower part of the same shallowing-upwards sequence and in the transgressive limestones at the base of the Bassein A. This secondary porosity distribution suggests movement of corrosive fluids along pre-existing stylolites. Despite an apparent link between porosity distribution and an unconformity, secondary porosity development was mesogenetic and related to the distribution of facies that favoured stylolites that acted as conduits for the flow of corrosive fluids. The Bassein Formation reservoirs show unequivocal evidence of significant porosity development by mesogenetic dissolution but the exact process or processes by which such porosity creation occurs requires further work.

Early Pennsylvanian (309–318 Ma) paleocave sediments hosted in the Mississippian (345–359 Ma) Leadville Limestone were partly derived from long-distance (>2000 km) source areas. In addition to showing the importance of long-distant dust transport in cave sediments, because these paleocave deposits are derived from loess, their presence may document the earliest terrestrial signature of the late Paleozoic ice age in North America. The Leadville Limestone was subject to karst processes following late Mississippian eustatic sea-level fall, including formation of phreatic tubes, breakout domes, tower karst (kegelkarst), solution valleys (poljes), sinkholes (dolines), solution-enhanced joints (grikes), surficial flutes (rillenkarren), and solution pans (kamenitzas). In the Leadville Limestone, speleothems are interbedded with karst breccias and fluvial cave sediments. The overlying Pennsylvanian Molas Formation is a loessite (eolian siltstone) composed of angular quartz silt with ferruginous kaolinite rims. The U-Pb ages of accessory zircons indicate that the source areas for the eolian silt are from the peri-Gondwanan terranes and Grenville Province of eastern and southern North America, which are ~2000 km to the east. There is also a provenance signature from the rising Ancestral Rocky Mountains. The evidence suggests dust trapping on land surfaces by paleokarst topography, moisture, and vegetation. Weak paleosols in the Molas Formation suggest relatively rapid rates of dust accumulation. The high porosity and low bulk density of modern loess soils make them susceptible to groundwater piping. This mechanism may have facilitated redeposition of the Molas Formation loess into karst passageways, to be remobilized by later hydrologic events. The paleocave sediments in the Leadville Limestone can be linked to the overlying loess in the Molas Formation by compositional and textural matches. Facies analysis of the paleocave sediments documents episodic hydrologic events, producing a sequence of inundites and debrites separated by mud drapes with mud cracks. These event deposits are interbedded with flowstones and dripstones. Cave sediments are increasingly utilized as archives of geologic change. Recognition that dust is a significant component of cave sediments highlights the inherited properties from distant source areas, land-atmosphere transfer processes, land-surface deposition processes, and resedimentation processes into the karst system.