**6. Sequence stratigraphy of fine-grained shale interval in lacustrine basin exampled by Ordos Basin in China**

The late Cretaceous evolution of lakes and the corresponding depositional systems in the Ordos Basin of Northern China have been controlled by the changes in the lake level, the sediment supply, and the tectonic setting. The lakes expanded and shrank similarly to littoral to shallow marine environments [36]. Therefore, the sequence stratigraphy and system tracts developed in littoral to shallow marine environment can be applied in the lacustrine Ordos Basin [37] due to similarities of geologic processes in marine and lacustrine settings. To this aim, we have adopted methodology and terminologies of classical sequence stratigraphy also in lacustrine settings, such as the lowstand systems tract (LST), the transgressive systems tract (TST), the highstand systems tract (HST) [1, 2, 8].

For the shale play located in the Chang7 source rock of the Ordos Basin, we have used the first lacustrine shale gas well, LP177, for the shale play located in the Chang7 source rock of the Ordos Basin in order to reconstruct the lacustrine sequence stratigraphy. The Chang7 interval in well LP177 is characterized by organic-rich black shales overlain by gray organiclean shales with sandstone interbeds (**Figure 6**). The SB is characterized by a change in depositional environments from semideep lake to shallow lake and by a corresponding change in lithofacies from thin siltstone within shale to thicker siltstones within shales. The LST is characterized by siltstone within shale. The transgressive surface bounding the LST and the TST has been identified through the sharp difference in color from gray shale to black shale and through the variations of the TOC content. The MFS is characterized by the highest gamma ray, the highest acoustic transit time (AC), high resistivity, and the highest TOC. HST is identified by coarsening-upward interval. The lower part of HST is characterized by organic-rich shale representing early highstand systems tract (EHST). The upper part of HST is characterized by silty interval representing later highstand systems tract (LHST). Within this sequence stratigraphic framework, the organic-rich shale having TOC of 2–6% was deposited during TST and EHST (**Figure 6**). Tests on five samples from the organic-rich Chang7 shale have indicated a porosity of 3–3.5% and a permeability of 60–100 nD. The 1.5–8 mg/g S1 in the shale reservoir interval indicates the accumulation of free oil in the shale or the presence of shale oil. The siltstone and sandstone intervals in LST and LHST are interpreted as turbidite deposits due to their position in semi to deep lake settings and their association with thick shale. GR curves and lithologies show sharp contact with the underlying and overlying shales confirming the nature of the turbidite deposits. Tests on the lower sandbody indicate a porosity of 11% and permeability of 0.1 mD (millidarcy), and it is classified as a tight oil play. The upper sandbody was tested to have a porosity of 14% and permeability of 1.3 D (darcy) and is classified as a conventional turbidite sandstone reservoir in large part due to the darcy-range permeability (**Figure 6**). The maturity of 0.77–0.95% of this organic-rich shale puts the Chang7 shale in the oil to wet gas window. The reported gas production of 2350 m<sup>3</sup> /day (82,990 ft<sup>3</sup> /


**Figure 6.** Sequence stratigraphy and its application of Triassic lacustrine Yanchang Formation, LP177 well in Ordos Basin, China.

day) from this organic-rich shale interval is probably wet shale gas indicated by the maturity. These results indicate that the LST and LHST within the sequence stratigraphic framework of lacustrine sediments tend to develop conventional and unconventional tight (sandstone) reservoirs, while TST and EHST are prone to develop shale gas and oil reservoirs.
