266 / 2019-07-01 15:47:21

Thickening-upward cycles in deep-marine and deep-lacustrine turbidites

Yangchang Formation,thickening-upward cycles,deep-marine turbidites,deep-lacustrine turbidites,Ordos Basin

Deep-marine and deep-lacustrine reservoirs have been targets for conventional and unconventional oil and gas exploration development for decades. Thickening-upward cycles (TUCs) in deep-marine Carboniferous Ross Sandstone outcrops in western Ireland and deep-lacustrine Triassic Yanchang Formation outcrops in southeast Ordos Basin are studied. Thickening-upward cycles (TUCs) consist of, from bottom to top: (1) Laminated shales/ shales with fine siltstone beds; (2) Interbedded sandstones/siltstones and mudstones; (3) Structureless, massive sandstones, are well recognized in these outcrops. A continual prograding lobe-element model is proposed to account for the repeated occurrence of thickening-upward cycles. TUCs developed within deep-marine and deep-lacustrine environment are highly comparable in terms of sedimentary structures, sheet-like geometries and amalgamation features, whereas they differ from deep-marine and deep-lacustrine channels. Frequent and strong degree of amalgamation is developed within the massive sandstone at the top of each TUC, suggesting a layer-by-layer depositional manner. Field observations and comparison with deep-marine counterparts support the origin of turbidity flow in Yanchang Formation, Ordos Basin.