Fault seal analysis in unconsolidated sediments: a field study from kentucky, USA

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Abstract The Eastern Kentucky Coalfield contains Pennsylvanian aged strata deposited into the Central Appalachian Basin, eastern USA. These strata are deltaic to shallow marine facies and represent prograding sediment accumulated into a foreland basin, from an emergent and uplifting Appalachian landmass to the east. The sediments are predominantly unfaulted away from the Appalachian thrust front; however, spectacular examples of metre to decimetre-scale syn-sedimentary growth faulting are observed. The growth faulting was caused by sedimentary differential loading of underlying mobile shale units. For this reason these fault systems develop soon after deposition while the sediments are semi- to unconsolidated. The fault systems are comprised of listric to planar, down to the basin (regional) and landward (counter-regional) faults as observed in modern deltaic environments such as offshore Texas, Gulf of Mexico and the Niger Delta. Observations made on the structure and architecture within the fault zones identify three critical deformation styles that will affect predicting the fluid flow characteristics of fault systems formed in this environment. These deformation styles are as follows. (1) Faults formed whilst the sediments are unconsolidated show evidence of sands being sheared down the fault plane forming disaggregation bands parallel to the fault plane. (2) Faults propagating through an inter-bedded sequence of sands and shales experience a large strength contrast at the sand-shale interfaces leading to fault plane refraction. Further fault development tends to ‘smooth’ the fault's surface producing slivers (riders) of sandstone, which are incorporated into the fault zone. (3) Fault motion during soft sediment deformation can lead to sands injected along the fault plane and along planes of weakness in the footwall to increase sand connectivity. Traditional fault seal analysis techniques such as the Shale Gouge Ratio (SGR) assumes the percentage of shale in the fault zone, its petrophysical properties and hence its sealing potential is controlled by the ratio of sand to shale passing a point on the fault. The incorporation or injection of sand into the fault plane is not accounted for. Thus use of the SGR method for assessing intra-reservoir faulting formed by syn-sedimentary processes can overestimate the sealing potential of the system.