Bradley D. Ritts

Early Mesozoic Basin Development and Its Response to Thrusting in the Yanshan Fold-and-Thrust Belt, China

Mesozoic basins in the Yanshan belt of northern China record two episodes of shortening during the Late Triassic and Late Jurassic that span the transition from retroarc deformation and basin formation to continental intraplate deformation. Gravel braided river depositional systems in the Upper Triassic Xingshikou Formation show provenance from the north and become relatively more distal southward onto the North China Block. These relationships are interpreted as a foreland basin system, southward of a retro-arc, and later collisional, fold-thrust belt. The Triassic basin shows widely correlative stratigraphy, simple proximal-to-distal relationships, and uniformly southdirected paleocurrents that suggest a simple integrated foreland basin. In contrast, the Upper Jurassic Tuchengzi Formation is characterized by coarse conglomerates with rapid facies changes, lateral non-uniformity, and local provenance, which argues for deposition in a broken foreland setting, with local high-gradient depositional systems dominant.

Ordos Basin Gas Reservoir Outcrop Analogs: Permian Braided Fluvial Sandstone of the Zhuozi Shan and Helan Shan, China

Permian sandstone outcrops along the eastern margin of the Zhuozi Shan, Inner Mongolia China, represent the most geographically proximal analogs for subsurface natural gas reservoirs of the north-central Ordos basin in the emerging Permian sandstone play; they lie along depositional strike, within the same structural province 90-125 km WNW of the Sulige gas field. These units were deposited by south-flowing braided fluvial depositional systems, and are dominated by channel and macroform deposits consisting of fine to very coarse sandstone. Beds are typically amalgamated both vertically and laterally to form laterally extensive sandstone bodies that are between 8 and 60 m thick; the sandstone bodies are thinner, and less continuous laterally near the top of the Permian. Amalgamated sandstone bodies are separated by mudstones that reach 10 m in thickness, and the Permian sandstone interval overall is bracketed between Permo-Triassic mudstone above and Carboniferous mudstone below. These sedimentologic characteristics control the heterogeneity and geometry of reservoir facies at the interwell scale, and will exert influences on the productivity of reservoirs in the subsurface of the Ordos basin.