Small- and large- scale soft-sediment deformations in a Triassic lacustrine delta caused by overloading and seismicity in the Ordos Basin, central China

The instability of an Upper Triassic lacustrine deltaic system, in the southwestern Ordos Basin, central China, is examined based mainly on detailed measurements and descriptions of core data. Nine lithofacies associations are identified, including undeformed and deformed channel deposits, sandy and muddy slumps, sandy debrites, sandy dykes, undeformed and deformed fringe silty deposits, and muddy deposits. Quantification of the vertical and horizontal spatial distribution of these sediments reveals the deformed sediments can comprise more than one third of the entire cored interval in more than half of the wells, indicating the significance of soft-sediment deformation structures as essential components of this deltaic system. A depositional model is constructed, which highlights the location of deformed and remobilized sediments and their relationships with undeformed sediments. In the proximal part of the delta, the undeformed and deformed channel deposits and sandy slumps dominate the sandy sediments, which are interbedded to form thick amalgamated sandstones. In areas distant from these massive sandstones, undeformed and deformed fringe fine-grained deposits are well developed, which could generate muddy slumps. In the distal part of the delta, besides undeformed and deformed fringe fine-grained deposits, sandy debrites are present as isolated sandbodies. Large-scale sandy dykes formed between thick sandstones and adjacent mudstones. Two possible trigger mechanisms for initial deformation development are identified - sediment overloading induced by sudden deposition of remobilized sediments and seismicity which caused large-scale remobilization in the study area. The high percentage of soft-sediment deformation structures in this delta influences the reservoir heterogeneity of the three types of sandstones (undeformed channel deposits, sandy slumps and sandy debrites). Porosity-permeability analysis reveals that reservoir quality of these three types varies significantly, and thus distinguishing deformed sediments from undeformed deltaic sediments is essential for hydrocarbon development in this area.