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 Triassic Yanchang Formation, which, cumulatively, has more than 80 m (>260 ft) of black organic-rich shale of oil window maturity. Using 76 samples collected every 1 m (3 ft) 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 total organic carbon (TOC) of 6.79 wt. % and pyrolyzable hydrocarbon potential of 9.40 mg/g rock (i.e., the amount of hydrocarbons generated by pyrolysis between 300°C and 650°C [S2]), are defined as “generative units” in the Chang 7 shale system, compared to the “in-source reservoirs” or “sweet spots” (i.e., the third and fourth intervals), which contain a lower average TOC of 4.19 wt. % and an average S2 value of 7.17 mg/g rock, but the highest amount of free oil (average 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%–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 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, indicates 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 values for the amount of thermally extractable hydrocarbons volatilized at 300°C (mg HC/g rock) were considered. Thus, the oil crossover effect, temperature at which the rate of S2 generation is at maximum shift phenomenon, and the hydrogen index being shifted to higher values after extraction all account for identifying intervals 3 and 4 as the in-source reservoirs. In this study, we have not only identified a set of promising in-source targets 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 as well as the stratigraphic fractionation on differentiating the chemical compositions during migration. The improved oil quality by fractionation, the extra storage potential derived from microfossil quartz, the weak adsorptive affinity of oil to organic matter, and the good shale susceptibility to hydraulic fracturing all give a promising prospective for exploring and producing shale oil from the Chang 7 shale system in the Ordos Basin.
Author Listing： Caineng Zou;Songqi Pan;Brian Horsfield;Zhi Yang;Siying Hao;Entao Liu;Leifu Zhang
Volume： 103
Pages： 2627-2663
DOI： 10.1306/01301917052
Language： English
Journal： AAPG Bulletin

AAPG BULLETIN

AAPG BULL

影响因子：2.7 是否综述期刊：否 是否OA：否 是否预警：不在预警名单内 发行时间：1974 ISSN：0149-1423 发刊频率：Monthly 收录数据库：SCIE/Scopus收录 出版国家/地区：UNITED STATES 出版社：American Association of Petroleum Geologists

期刊介绍

While the 21st-century AAPG Bulletin has undergone some changes since 1917, enlarging to 8 ½ x 11” size to incorporate more material and being published digitally as well as in print, it continues to adhere to the primary purpose of the organization, which is to advance the science of geology especially as it relates to petroleum, natural gas, other subsurface fluids, and mineral resources.Delivered digitally or in print monthly to each AAPG Member as a part of membership dues, the AAPG Bulletin is one of the most respected, peer-reviewed technical journals in existence, with recent issues containing papers focused on such topics as the Middle East, channel detection, China, permeability, subseismic fault prediction, the U.S., and Africa.

虽然21世纪的AAPG公报自1917年以来经历了一些变化，扩大到8 ½ x 11”的大小，以纳入更多的材料，并出版数字和印刷，它继续坚持该组织的主要宗旨，即促进地质科学，特别是与石油，天然气，其他地下流体，和矿物资源。作为会费的一部分，每月向每个AAPG成员提供数字或印刷版，AAPG公报是现有的最受尊敬的、同行评审的技术期刊之一，最近的几期包含专注于诸如中东、通道检测、中国、渗透性亚地震断层预测，美国，和非洲。

年发文量 79
国人发稿量 30
国人发文占比 37.97%
自引率 7.4%
平均录取率 较易
平均审稿周期 平均1.0个月
版面费 -
偏重研究方向 工程技术-地球科学综合
期刊官网 http://aapgbull.geoscienceworld.org/
投稿链接 https://www.editorialmanager.com/BLTN

质量指标占比

研究类文章占比 OA被引用占比 撤稿占比 出版后修正文章占比
97.47% 0.00% 0.00% 0.00%

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