Upper Triassic Potential Source Rocks in the Qiangtang Basin, Tibet: Organic Geochemical Characteristics

Abstract

Upper Triassic coal‐bearing strata in the Qiangtang Basin (Tibet) are known to have source rock potential. For this study, the organic geochemical characteristics of mudstones and calcareous shales in the Upper Triassic Tumengela and Zangxiahe Formations were investigated to reconstruct depositional settings and to assess hydrocarbon potential. Outcrop samples of the Tumengela and Zangxiahe Formations from four locations in the Qiangtang Basin were analysed. The locations were Xiaochaka in the southern Qiangtang depression, and Woruo Mountain, Quemo Co and Zangxiahe in the northern Qiangtang depression. At Quemo Co in the NE of the basin, calcareous shale samples from the Tumengela Formation have total organic carbon (TOC) contents of up to 1.66 wt.%, chloroform bitumen A contents of up to 734 ppm, and a hydrocarbon generation capacity (Rock‐Eval S1+ S2) of up to 1.94 mg/g. The shales have moderate to good source rock potential. Vitrinite reflectance (Rr) values of 1.30% to 1.46%, and Rock‐Eval Tmax values of 464 to 475 °C indicate that the organic matter is at a highly mature stage corresponding to condensate / wet gas generation. The shales contain Type II kerogen, and have low carbon number molecular compositions with relatively high C21−/C21+ (2.15–2.93), Pr/Ph ratios of 1.40–1.72, high S/C ratios (>0.04) in some samples, abundant gammacerane (GI of 0.50–2.04) and a predominance of C27 steranes, indicating shallow‐marine sub‐anoxic and hypersaline depositional conditions with some input of terrestrial organic matter. Tumengela and Zangxiahe Formation mudstone samples from Xiaochaka in the southern Qiangtang depression, and from Woruo Mountain and Zangxiahe in the northern depression, have low contents of marine organic matter (Type II kerogen), indicating relatively poor hydrocarbon generation potential. Rr values and Tmax data indicate that the organic matter is overmature corresponding to dry gas generation.

Elemental geochemistry of the early Jurassic black shales in the Qiangtang Basin, eastern Tethys: constraints for palaeoenvironment conditions

“…The Qiangtang Basin is the biggest marine basin in the Tibetan Plateau of the eastern Tethys, covering an area of about 220 × 10 3 km 2 (Wang et al ., ). Over the past two decades, the basin has become a frontier for oil exploration (Zhao and Li, ; C. S. Wang et al ., 2004; Chen et al ., , ; Guo et al ., ; Wang et al ., , ; Ding et al ., , ; Zeng et al ., ). The early Jurassic black shales are the most important source rocks in the basin (Wang et al ., ).…”

New insights about petroleum geology and exploration of Qiangtang Basin, northern Tibet, China: A model for low-degree exploration

“…The mudstone, marl interbedded coals occur mainly along the margin of the basin, and represent deposition in coastal swamp environments. The shallow-marine shale and sandstone were widely developed in the northern part of the basin (Zeng et al, 2013). Our new well data reveal that the shelf facies shale is widespread in the central part of the eastern Qiangtang Basin.…”

“…Finally, the extracted kerogens were rinsed using de-ionized water, then frozen and freeze-dried for separation. Vitrinite reflectance (Ro) measurements were performed using a Leica MPV Compact II reflected light microscope fitted with an oil-immersion photomicrometer (Zeng et al, 2013).…”

“…In 2001, new oil and gas reconnaissance and exploration actives were undertaken by the Chengdu Institute of Geology and Mineral Resources. According to the regional survey for oil and gas, more than 200 surface oil and gas shows have been found in Mesozoic formations (Wang et al, 2004b;Fu et al, 2008;Zeng et al, 2013). However, the exploration level of oil and gas in the Qinghai-Tibet Plateau is the lowest in China due to complicated tectonic evolution (Kapp et al, 2003;Yin and Harrison, 2000) and poor natural environmental conditions (e.g., extreme altitude, lower levels of oxygen, and severe climatic conditions).…”

“…A schematic diagram showing major stratigraphic units, and petroleum and sedimentary characteristics of the Qiangtang Basin (modified fromZeng et al, 2013). NQD-North Qiangtang depression; SQD-South Qiangtang depression.…”