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For assessing the density and viscosity of tight oil and their geochemical controls, 47 tight oil samples across the Ordos Basin were analyzed. The oil density and viscosity lay in the ranges of 0.81-0.88 g/ml and 1.9-13.36 mPa·s, which are low in overall. The average saturated hydrocarbon content is higher than 80%. The molecular compositions suggest a source dominated organic matter by lacustrine type I/II with minor input of terrestrial type III kerogen. The oils were mainly generated at relatively high maturities. Fractionation during the oil expulsion may also result in the depletion of heavy-ends compounds. These are the main controlling factors for the overall good quality of the Ordos tight oil. [ABSTRACT FROM AUTHOR]

Organic matter (OM)-rich shale in the Yanchang Formation is recognized as a promising hybrid shale oil/shale gas system in lacustrine strata in China. Being mainly in the oil window, both the type and distribution of the pores in the shale have been affected by several factors, including petroleum expulsion and retention, mineral and organic composition, and compaction. To obtain a better understanding of the factors controlling OM pore development, 10 core samples were selected with various OM content from the Chang 7 and Chang 9 members of the Yanchang Formation for pore characterization. The investigation combined microscopic observation, both of thin sections and ion milled surfaces of samples with low-pressure carbon dioxide and nitrogen adsorption and bulk porosity measurements, with the main emphasis being on OM-associated pores. The selected samples generally have relatively low total porosity, with the pores being poorly connected. Most pore types found in marine shales were present. The size of porous OM is usually small, mostly measuring several hundreds of nanometers to a few micrometers, which suggest a general compaction effect on pore development. The pore development is related to different petroleum expulsion processes. Shale with very porous OM was a very important feature. The OM coexisting with fluorescent lipitinites was present in a cross-linked nanofiber structure, possibly related to altered extracellular polymeric substances (EPS) by compaction and maturation. Various OM types that varied in both size and shape did not contain any visible pores (e.g. woody relics, migrated solid bitumen and OM laminae in close association with clay minerals). Unusually low meso- and macropore volume in the sample with the highest total organic carbon (TOC) content was confirmed by the predominance of OM laminae without visible pores; however, abundant micropores were indicated by CO 2 adsorption analysis. Meso- and macropores in this sample had probably not developed due to a significant compaction effect. Due to the small numbers of studied samples, these OM pore characteristics need to be viewed with caution. [ABSTRACT FROM AUTHOR]

Following the successful exploration of shale oil in North America, China recently started to explore unconventional oil resources. Industrial shale oil flows were obtained from some strata in lacustrine basins, especially in the Chang7 Member of the Ordos Basin that may reach 150 barrels a day (e.g., in well Geng 295, 2655 m). So one of the most significant questions now being asked focuses on the shale oil storage within the reservoir interval. In the present study, five organic rich shale samples were collected from the Chang 7 Member of the Yanchang Formation in the Ordos Basin and were analyzed by field emission scanning electron microscopy to characterize pores and fractures in the shales. The results show that microfractures are predominant storage space for shale oil produced from the Chang 7 Member. Three types of fractures and pores were recognized, including (i) fractures and pores associated with interstitial organic matter and organic lamina, (ii) fractures and pores occurring in minerals such as pyrite framboid, mica, and feldspar, and (iii) fractures and pores that are present along the contact interface of organic matter and minerals. These fractures and pores were mainly formed by oil generation, tectonic stress, or dissolution. The total organic carbon and brittle minerals are main factors controlling the abundance of pores and fractures. High total organic carbon and brittle minerals are favorable for the formation and preservation of pores and fractures in shale. These results may provide an insight that helps to locate sweet spots for shale oil exploration in Ordos Basin and affect oil exploration in shales in other lacustrine basins of China. [ABSTRACT FROM AUTHOR]