Abstract
Deep-marine and deep-lacustrine reservoirs have been targets for conventional and unconventional oil and gas exploration and development for decades. Thickening-upward cycles in the deep-marine Carboniferous Ross Sandstone Formation outcrops in western Ireland and the deep-lacustrine Triassic Yanchang Formation outcrops in southeast Ordos Basin have been investigated and correlated in this study. Typical thickening-upward cycles consisting of, from bottom to top: (1) laminated shales/shales with interbedded siltstone beds; (2) interbedded sandstones/siltstones and mudstones; (3) structureless massive sandstones, are well recognized in these outcrops and are interpreted as turbidite lobes. A continuously prograding lobe-element model is proposed to explain the repeated stacking of thickening-upward cycles. Thickening-upward cycles developed within deep-marine and deep-lacustrine environments are highly comparable in many aspects, such as sedimentary structures, sheet-like geometries and amalgamation features. A frequent and strong degree of amalgamation is developed within the massive sandstone at the top of each thickening-upward cycle, suggesting a layer-by-layer depositional manner. Field observations and comparison with deep-marine counterparts support the occurrence of turbidity flows in the Yanchang Formation, Ordos Basin.
Highlights
Deposits resulting from deep-marine gravity flows can be identified into a “thickening-upward cycle (TUC hereinafter)” pattern in a vertical profile (Mutti and Normark 1987; Macdonald et al 2011; Grundvag et al 2014), with an upward transition of shales to interbedded shale-sand to thick massive sandstones
3.1 Thickening-upward cycles in deep-marine environment Within the Ross Sandstone Formation exposed at the Kilhaba Bay, thickening-upward cycles can be recognized continuously in a vertical profile (Figs. 3, 4)
1) Thickening-upward cycles have been recognized within the deep-marine Ross Sandstone Formation, Clare Basin and the deep-lacustrine Yanchang Formation turbidite deposits, Ordos Basin, consisting of, from bottom to top: (1) Laminated shales/shales with fine-grained siltstone beds; (2) Interbedded sandstones/siltstones and mudstones; (3) Structureless massive sandstone with a strong degree of amalgamation and load structures at bases
Summary
Deposits resulting from deep-marine gravity flows can be identified into a “thickening-upward cycle (TUC hereinafter)” pattern in a vertical profile (Mutti and Normark 1987; Macdonald et al 2011; Grundvag et al 2014), with an upward transition of shales to interbedded shale-sand to thick massive sandstones. In the Ordos Basin, a single deep-lacustrine gravity flow reservoir in the Triassic Yanchang Formation has an area of more than 3000 km, with the estimated geological reserve of more than 1 × 108 tons (Zou et al 2012). In the Songliao Basin, deep-lacustrine turbidite lobes can extend 70 km in length, with an area of more than 2100 km (Feng et al 2010), suggesting that despite the much shallower depth than deep-sea, gravity flows in deeplakes can form depositional bodies of similar size
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