Source-rock evaluation and depositional environment of black shales in the Triassic Yanchang Formation, southern Ordos Basin, north-central China

Source-rock samples from the Upper Triassic Yanchang Formation in the Ordos Basin of China were geochemically characterized to determine variations in depositional environments, organic-matter (OM) source, and thermal maturity. Total organic carbon (TOC) content varies from 4 wt% to 10 wt% in the Chang 7, Chang 8, and Chang 9 members — the three OM-rich shale intervals. The Chang 7 has the highest TOC and hydrogen index values, and it is considered the best source rock in the formation. Geochemical evidence indicates that the main sources of OM in the Yanchang Formation are freshwater lacustrine phytoplanktons, aquatic macrophytes, aquatic organisms, and land plants deposited under a weakly reducing to suboxic depositional environment. The elevated [Formula: see text] sterane concentration and depleted [Formula: see text] values of OM in the middle of the Chang 7 may indicate the presence of freshwater cyanobacteria blooms that corresponds to a period of maximum lake expansion. The OM deposited in deeper parts of the lake is dominated by oil-prone type I or type II kerogen or a mixture of both. The OM deposited in shallower settings is characterized by increased terrestrial input with a mixture of types II and III kerogen. These source rocks are in the oil window, with maturity increasing with burial depth. The measured solid-bitumen reflectance and calculated vitrinite reflectance from the temperature at maximum release of hydrocarbons occurs during Rock-Eval pyrolysis ([Formula: see text]) and the methylphenanthrene index (MPI-1) chemical maturity parameters range from 0.8 to [Formula: see text]. Because the thermal labilities of OM are associated with the kerogen type, the required thermal stress for oil generation from types I and II mixed kerogen has a higher and narrower range of temperature for hydrocarbon generation than that of OM dominated by type II kerogen or types II and III mixed kerogen deposited in the prodelta and delta front.

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The Structural Characteristics and Distribution of Sand Body of the Layer 7 of YanChang Formation in Ordos Basin

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.628.360 ◽

2014 ◽

Vol 628 ◽

pp. 360-365

Author(s):

Qi Qi Lv ◽

Shun She Luo ◽

Lin Jing Li ◽

Rong Dai ◽

Yu Dong Li

Keyword(s):

Depositional Environment ◽

Structural Characteristics ◽

Ordos Basin ◽

Sedimentary Facies ◽

Research Area ◽

Uniform Thickness ◽

Yanchang Formation ◽

Genetic Types ◽

Sand Body ◽

Pelvic Shape

The tight sand in the layer 7 of YanChang Formation, Ordos Basin major develops braided river delta and gravity flow deposits. In this paper, based on previous studies on lake pelvic shape, provenance and hydrodynamic, the sand body in the research area has been systematically studied through core observation, logging data, and sedimentary theory. Various genetic types of sedimentary sand body are developed in this area, mainly as delta deposition, sandy debris flow deposits, turbidites (classic turbidites). We can identify 6 kinds of sand vertical combination type, they are the superimposed sand body (A-type), thick and uniform thickness sand body (B type), thin and uniform thickness sand body (C-type), up thinning sand body (D-type), thickening up sand body (E-type) , thick and thin interbed sand body (F-type). The 6 types mainly controlled by sedimentary facies. The A-type sand body mainly developed in the delta depositional environment, the B type sand body is visible both in the delta and slope belt, while the D-type, E-type, F-type sand body are mainly developed in the deep lake. The distribution of sand body in the plane is zonal pattern.

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