Thrust Faults Promoted Hydrocarbon Leakage at the Compressional Zone of Fine-Grained Mass-Transport Deposits

Fine-grained mass-transport deposits (MTDs), especially their compressional toe zones, are traditionally considered as effective seal in constraining the vertical fluid migration underneath. However, this study documents thrust faults at the compressional toe zone of fine-grained MTDs that could disaggregate the seal competence and promote vertical fluid flow. The investigated MTD referred to as MTD-a lies directly over a large hydrocarbon reservoir that is located within the Central Canyon of northern South China Sea, which is examined by using high-resolution 3D seismic and borehole data. Thrust faults and irregular blocks composed of coarse-grained sandstones are observed in the compressional zone of the MTD-a’s toe. More importantly, seismic evidence (e.g., enhanced seismic reflections) suggests that a large amount of hydrocarbons from the underlying reservoir penetrated through the MTD-a along these thrust faults and charged into the coarse-grained sandstone blocks. This clear evidence of thrust faults compromising the MTD’s seal effectiveness and thus facilitating the vertical fluid flow through the non-permeable strata demonstrate the importance of reassessing the seal capacity of MTD.

Diversidade de estilos estruturais em depósitos de transporte em massa

Depósitos de transporte em massa (mass-transport deposits - MTDs) resultam de importantes processos de remobilização gravitacional e ressedimentação em ambiente subaquático. Esses depósitos são recorrentes ao longo da seção estratigráfica do Grupo Itararé, na Bacia do Paraná. Portanto, são considerados importantes elementos deposicionais para compreender a evolução tectonoestratigráfica da bacia durante a deposição dessa unidade estratigráfica. A deformação é uma característica importante desses depósitos e resulta do transporte talude abaixo de sedimentos pouco ou não-litificados. Os MTDs do Grupo Itararé apresentam diferentes estruturas, tais como dobras, falhas, boudins, feições de cisalhamento, injetitos, entre outras. O estudo dessas estruturas pode fornecer informações importantes sobre evolução desses depósitos e a sua relação com os demais depósitos que compõem o Grupo Itararé. Além disso, a compreensão da anisotropia desses depósitos pode auxiliar em estudos sobre seu impacto em sistemas petrolíferos. No presente artigo, as estruturas descritas em MTDs do Grupo Itararé foram agrupadas em quatro diferentes estilos estruturais de deformação, que incluem: compressivo, distensivo, cisalhante e de liquefação/fluidificação/injeção. A diversidade de estilos estruturais parece resultar de diferentes fatores associados a relação espacial e temporal de formação das estruturas, incluindo mudanças de reologia.

Channel incision into a submarine landslide: an exhumed Carboniferous example from the Paganzo Basin, San Juan, Argentina

Emplacement of submarine landslides, or mass transport deposits, can radically reshape the physiography of continental margins, and strongly influence subsequent sedimentary processes and dispersal patterns. The irregular relief they generate creates obstacles that force reorganisation of sediment transport systems. Subsurface and seabed examples show that channels can incise directly into submarine landslides. Here, we use high-resolution sedimentological analysis, geological mapping and photogrammetric modelling to document the evolution of two adjacent, and partially contemporaneous, sandstone-rich submarine channel-fills (NSB and SSB) that incised deeply (>75 m) with steep lateral margins (up to 70°) into a 200 m thick debrite. The stepped erosion surface mantled by clasts, ranging from gravels to cobbles, points to a period of downcutting and sediment bypass. A change to aggradation is marked by laterally-migrating sandstone-rich channel bodies that is coincident with prominent steps in the large-scale erosion surface. Two types of depositional terrace are documented on these steps: one overlying an entrenchment surface, and another located in a bend cut-off. Above a younger erosion surface, mapped in both NSB and SSB, is an abrupt change to partially-confined tabular sandstones with graded caps, interpreted as confined lobes. The lobes are characterised by a lack of compensational stacking and increasingly thick hybrid bed deposits, suggesting progradation of a lobe complex confined by the main erosion surface. The incision of adjacent and partially coeval channels into a thick submarine landslide, and sand-rich infill including development of partially confined lobes, reflects the complicated relationships between evolving relief and changes in sediment gravity flow character, which can only be investigated at outcrop. The absence of channel-fills in bounding strata, and the abrupt and temporary presence of coarse sediment infilling the channels, indicates that the submarine landslide emplacement reshaped sediment transport systems, and established conditions that effectively separated sand- from mud-dominated deposits.