Distribution and geological significance of 17α(H)-diahopanes from different hydrocarbon source rocks of Yanchang Formation in Ordos Basin

Abstract

Based on GC-MS testing data of many saturated hydrocarbon samples, 17α(H)-C 30 diahopanes (C 30 * ) are extensively distributed in the lacustrine hydrocarbon source rocks of the Yanchang Formation in OrdosBasin, but show remarkable differences in relative abundance among various source rocks. Generally, Chang 7 high-quality source rock (oil shale) developed in deep lake anoxic environment shows lower C 30 * content, whereas Chang 6-9 dark mudstone developed in shallow to semi-deep lake, sub-oxidizing environment shows relatively high to high C 30 * value. Particularly, Chang 7 and Chang 9 black mudstones in Zhidan region in the northeast of the lake basin show extremely high C 30 * value. A comparative analysis was made based on lithology, organic types and various geochemical parameters indicative of redox environment, and the results indicate that environmental factors such as redox settings and lithology are key factors that control the C 30 * relative abundance, while organic types and maturity may be minor factors. High to extremely high C 30 * values are indicative of sub-oxidizing environment of fresh-brackish water and shallow to semi-deep lake. Therefore, research on C 30 * relative content and distribution in lacustrine hydrocarbon source rocks in the Yanchang Formation, especially on the difference in C 30 * between Chang 7 high-quality source rocks (oil shale) and Chang 6-9 1 source rocks (dark mudstone), will provide an important approach for classification of Mesozoic lacustrine crudes and detailed oil-source correlation in the basin.diahopane, neohopane, sedimentary environment, Yanchang Formation, Ordos Basin

The composition and its impact on the methane sorption of lacustrine shales from the Upper Triassic Yanchang Formation, Ordos Basin, China

“…The lake basin subsequently evolved into its final stage, and Chang 1, 2 and 3 sections were mainly deposited in the resulting deltaic environment. Oil shale, black shale and carbonaceous shale are very common in semi-deep and deep-lake conditions in the Chang 7 section, and petroleum resources have been well correlated with the shales in this section, which contains abundant organic matters (Duan et al, 2008;Zhang et al, 2009). The Chang 9 shales also have a very high potential for oil generation (Zhang et al, 2007).…”

Prediction of diagenetic facies using well logs – A case study from the upper Triassic Yanchang Formation, Ordos Basin, China

“…The sequence of the Yanchang Formation is subdivided into 10 informal oil reservoir units named Chang 1 to Chang 10 from top to bottom, based on marker beds, sedimentary cycles and lithological associations (Fig 2; (Guo et al, 2014;Ji et al, 2008;Qiu et al, 2015). Oil shale, black shale and carbonaceous shale in bottom of Chang 7 constitute the best source rocks of Yanchang formation (Duan, 2012;Duan et al, 2008;Zhang et al, 2009). The source rocks are mainly humic-sapropel type and minor sapropel type and the R o values of the source rocks range from 0.66% to 1.07% (Duan et al, 2008;Zhao et al, 1996).…”

“…In the last decades the source rocks from Chang 7 unit, which could be used for oil-source rock correlation and thus facilitate hydrocarbon exploration, have attracted much attention (e.g. (Ma et al, 2005;Xi et al, 2008;Zhang et al, 2009). However, in recent years, debris flows and turbidity sandstones at the top of Chang 7 have been found as oil and gas reservoirs, which are known as tight sandstone reservoirs (Yang et al, 2010).…”

Identification of the four rearranged hopane series in geological bodies and their geochemical significances

“…Although the precursors of rearranged hopanes are not directly derived from terrestrial higher plants, a genetic connection between them seems apparent. Zhang et al (2009) suggested that high to extremely high C 30 diahopane values are indicative of sub-oxidizing environment of fresh-brackish water and shallow to semideep lake. Previous studies indicate that the Songliao Basin develops a brackish-saline lacustrine environment with a mixed input, whereas coal-measure source rocks develop well in sedimentary environments of the Tarim and Ordos Basins (Zhang and Zhu 1996;Zhang et al 2013;Cheng et al 2014).…”

“…In recent years, rearranged hopanes have received increasing attention as biological markers with applications for geochemical studies of petroleum source rocks and crude oils (Huang et al 2003;Li et al 2009;Zhang et al 2009;Cheng et al 2014). However, systematic identification of the four rearranged hopane series is not reported in the domestic and overseas literature.…”