A chemostratigraphic study of a 300 m long core recovered from the southeastern central Ordos depocenter reveals thick ($<10–65\mathrm{m}$) intervals of fine-grained, organic-rich lacustrine strata, interpreted to represent deepwater deposition under meromictic conditions (incomplete watermass overturn) during lake highstand phases, interspersed with thick (10–30 m) intervals of arkosic sandstones, reflective of fluvio-deltaic deposition during lake lowstand phases. Along with elevated concentrations of %Al, traditionally a proxy for clay content, maximum total-organic-carbon (TOC) values in the deepwater lacustrine facies reach 8%, with average values of approximately 3%. The fine-grained, organic-rich facies is also characterized by elevated S (up to 6%) and As concentrations, both proxies for pyrite, an indicator of more stagnant, reducing conditions in the hypolimnion (the volume of the lake below the surface mixing zone) during lake highstand phases. Enrichment factors for redox-sensitive trace elements (RSTEs) are not significantly elevated throughout the TOC-rich intervals, but they are punctuated in thinner intervals. Punctuated enrichments in RSTEs reflect episodes of enhanced suboxia/anoxia in the hypolimnion during lake highstand phases resulting from sustained meromixis (protracted episodes of incomplete watermass turnover). Although dramatic shifts between fluvio-deltaic and deepwater lacustrine deposition are recorded in the Yanchang strata, no evidence indicates that the lacustrine system ever built up a significant salt concentration, suggesting that an outlet was maintained throughout much of the depositional history. A chemofacies framework for the Yanchang Formation is developed based on hierarchical cluster analysis and ranking of major element chemostratigraphic results. The outcome of chemofacies analysis is similar to the lithofacies analysis, subdividing the stratigraphic record of calcite-cemented sandstone/siltstone lithologies (fluvio-deltaic facies) and silty to finer grained mudstone lithologies (deep-water lacustrine facies) based on changes in elemental concentrations that equate to shifts in the relative abundance of mineral contents (e.g., clays, quartz, feldspars, pyrite, and carbonates).

Along with the successful exploitation of shale oil, organic-rich shale has attracted great attention to the geologists. However, quantitatively differentiating the origin of organic matter in shale is still challenging due to the nature of its complex depositional processes. Organic matter in lacustrine shales could be derived from both lake water (produced by lake water column) and land input (transported into the lake by rivers). The Triassic Chang-7 shale in the Yanchang Formation is a typical lacustrine shale deposited in the central part of the Ordos Basin. It is rich in organic matter (TOC of 0.52−9.37%) and consists primarily of type II₁ kerogen. The Chang-7 shale is thermally mature and currently in the stage of generating oil and gas, with more structured than organic matter. In this study, quantitative models of organic matter in the Chang-7 shale with sources from both lake water and land is established using geochemical and carbon isotopes data and the results show that lake water and land input have the almost equal contribution to the organic enrichment in the study area.

Recent exploration activities on Triassic Yan-chang Formation has indicated significant potential for shale-gas resources. Even though some areas have been put into pilot production, challenge exists in effectively determining shale-gas content, whichmake it difficult to estimate reserve and forecast production. This is primarily due to the low maturity of the shale and complicated relationships between oil, water and gas. However, studies on the gas generation and accumulation of low-maturity shales are lacking in the literature and previous desorption experiments did not consider the effects of oil and water. In this study, eight core samples are used to run the gas-desorption experiment. Shale-gas composition and isotopic fractionation data are analyzed to provide insights on gas adsorption, expulsion and accumulation processes in the Yanchang Formation. The experiment results indicate that the overall methane content decreases (from 68.8% to 58.9%), heavy hydrocarbon-gas content (from 20.30% to 36.12%) and δ13C1 increase (from -51.43% to - 34.63%) during the desorption process. Gas yield reached up to 3.89 m3/t shale, and the content of lost gas can account for more than 20% of total desorbed gas during the coring process. The recoverable shale gas content is estimated to be 80% of the total gas-desorption volume. Desorbing the rest 20% requires extra heating under much higher temperature.