Oil retention and intrasource migration in the organic-rich lacustrine Chang 7 shale of the Upper Triassic Yanchang Formation, Ordos Basin, central China

Abstract

The Zhang 22 well was drilled in the Ordos Basin, penetrating the Chang 7 Member of the Yanchang Formation, which has more than 80 m cumulative black organic-rich shale of oil window maturity. Utilizing seventy-six samples collected every 1 meter from the well the effects of stratigraphic fractionation and petroleum expulsion within five intervals of the Chang 7 shale were qualitatively and quantitatively documented. The organic-rich intervals-1,-2 and-5, having an average TOC content of 6.79 wt% and pyrolyzable hydrocarbon potential S2 of 9.40 mg/g rock, are defined as "generative units" in the Chang 7 shale system, compared to the "in-source reservoirs" or "sweet spots"-the third and fourth intervals-which contain lower average TOC content of 4.19 wt% and an average S2 value of 7.17 mg/g rock, but the highest amount of free oil (av. total oil of 7.35 mg/g rock). Geochemical and molecular compositions display distinctive differences between samples from these source and reservoir groupings. For example, bitumens from the generative units proportionally possess lower saturated hydrocarbons (56% to 66%) than those from the in-source reservoirs (up to 81%). The proportions of aromatic and polar compounds in the generative units are accordingly higher than in their counterpart. The individual molecular weight distribution of sample extracts displays more light-end moieties being enriched in the generative units. By applying the compositional mass balance calculation, the overall and compound-specific expulsion efficiencies in the in-source reservoirs are abnormally negative compared to the positive values in the generative 2 intervals. This finding in conjunction with the effects of the preferential retention of aliphatic hydrocarbons and the differential expulsion of light molecular weight compounds in the in-source reservoirs together indicate a short-distance intrasource migration of generated petroleum into the sweet spot intervals (intervals-3 and-4) from the overlying units (intervals-1 and-2) and the underlying interval-5. Furthermore, when quantifying the total amount of retained petroleum in the shale system, an amended assessment has been introduced to overcome the systematic misestimations if only unextracted S1 values were considered. Thus, the oil crossover effect, Tmax shift phenomenon and the HI being shifted to higher values after extraction all account for identifying the intervals-3 and-4 as the in-source reservoirs. In this study, we have not only identified a set of promising in-source target for shale oil exploration and production, but we also presented the chemical and molecular composition for these shale oils. We have additionally speculated for the intrasource migration model, and further discussed the different expulsion efficiencies in the shale system upon the compositional mass balance calculation, and the stratigraphic fractionation on differentiating the chemical compositions during migration. The improved oil quality by fractionation, the extra storage potential derived from microfos...

Geochemical Properties and Pore Structure Control on Oil Extraction of Shale

“…The Yanchang Formation could be subdivided into 10 members, and the sediment infilling of which is of continental lacustrine origin, among them, the Chang 7 Member, which is overlain by Chang 8 Member, preserve more than 50% of shale oil in the basin (Figure 1) [39,41]. The Chang 7 Member was studied in detail using various experimental methods and porous media models, which was also the target in this research [42][43][44].…”

Quantitative Characterization of Pore Connectivity and Movable Fluid Distribution of Tight Sandstones: A Case Study of the Upper Triassic Chang 7 Member, Yanchang Formation in Ordos Basin, China

“…The tight sandstone reservoirs are mainly distributed in the Chang 7 1 and Chang 7 2 layers of the local area, where subaqueous distributary channels and estuarine dam microfacies of the delta front have developed. The Chang 7 3 layer is a major hydrocarbon source in Mesozoic oil-bearing systems [32,33]. The tight sandstone of the Chang 7 Member, which formed at the most expansive stage of the lacustrine basin, is lithologically complex and dominated by fine sandstone, siltstone, and argillaceous siltstone ( Figure 1(b)).…”

Evaluation method for resource potential of shale oil in the Triassic Yanchang Formation of the Ordos Basin, China

“…Shale oil exploration has made major progress in the Permian Basin, Williston Basin, Western Gulf, Appalachian Basin, and Western Canada Sedimentary Basin in North America (Abouelresh and Slatt, 2012;Attar and Pranter, 2016;Bernard and Horsfield, 2014;Browning et al, 2013;Chen and Jiang, 2016;EIA, 2015EIA, , 2016EIA, , 2017Hackley and Cardott, 2016;Han et al, 2019;Kuske et al, 2019;Lewan and Pawlewicz, 2017;Marra, 2018). Breakthroughs in shale oil have also been made in the Ordos Basin, Bohai Bay Basin, Songliao Basin, and Junggar Basin in China (Liang et al, 2017;Liu et al, 2019;Zhang et al, 2018;Zou et al, 2019a). Shale oil, with great resource potential, will be the main target for oil and gas exploration and development in the future.…”

“…Shale oil, with great resource potential, will be the main target for oil and gas exploration and development in the future. With wide distribution and large thickness, the Chang 7 Shale in the Upper Triassic in the Ordos Basin has the most abundant shale oil in China (Guo et al., 2019; Zou et al., 2019b).…”