**8. Conclusions**

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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)

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

As shown above, an understanding of sequence stratigraphy can be a very powerful tool for building a sequence stratigraphic framework for the lithofacies prediction. It can also help to interpret the depositional history and to predict fine-grained unconventional resource plays for a single well to a basin (**Figure 3**–**6**). **Figure 4** shows the successful production of 200,000 m<sup>3</sup>

day shale gas is from the predicted high-quality shale reservoir in TST and EHST interval with high TOC, high quartz content, and high gas content. At a regional scale, sequence stratigraphy can be used for the correlation of the stratigraphy, the lithology, the mineralogy, the geochemistry, and even rock fabrics. **Figure 7** shows that TST has the highest TOC content, total gas content and quartz content from Jiaoye1 well to Pengye1 well. At the same time, the TOC content has the similar changing patterns to gas content within Sequence A (lower member of the Longmaxi Fm): both show decreasing trends from TST to LHST. Also, gas content decreases with decreasing quartz content from TST to LHST, which indicates the shale in TST is highquality fracturing susceptible shale gas reservoir with the highest TOC, gas content, and brittle minerals content (**Figure 7**). The positive relationship between TOC, quartz content, and shale gas content in the Silurian Longmaxi shale is similar to that one of the Bartnett shale in the USA, which is attributed to the formation of biogenic quartz in an anoxic environment that is favorable for organic matter preservation [24]. Laterally, shale gas content and quartz content in TST decreases from Jiaoye1 well to Pengye1 well even though they have similar TOC. These

reservoirs, while TST and EHST are prone to develop shale gas and oil reservoirs.

28 Seismic and Sequence Stratigraphy and Integrated Stratigraphy - New Insights and Contributions

**7. Application**

Basin, China.

The theory, approach, and terminology of clastic sequence stratigraphy developed for relatively coarse-grained deposits can be applied to fine-grained "shale" since their depositions are both controlled by sea- or lake-level changes. The sequence stratigraphic studies of marine and lacustrine shales in the USA and China have indicated that the sequence boundary of shales coincides with an unconformity formed during a relative sea-level or lake-level fall. The different and eventual occurrence of LST, TST, and HST has delineated the occurrence of both complete and/or incomplete depositional sequences, depending on their location. Many sequences of marine shales in the USA and China are incomplete, since they have developed only TST and HST. On the other hand, the lacustrine shale represented by the Chang7 shale in the Ordos Basin has developed complete depositional sequences, including LST, TST, and HST. HST can be further divided into EHST and LHST.

The investigation of siliciclastic-dominated fine-grained "shale" and carbonate-dominated fine-grained "shale" has revealed that lithofacies and well log responses to sea- and lake-level changes are strongly different. Previous proposals stated that high gamma ray and organic-rich shale are indicative of TST or the highest gamma ray and TOC are criteria for identifying MFS of siliciclastic-dominated fine-grained shales exampled by Barnett, Woodford, Marcellus, Mowry, and Longmaxi shales do not work for carbonate-dominated fine-grained shale. The sequence stratigraphy of the Niobrara carbonate-dominated fine-grained shale in this chapter shows that the rise in the sea level resulted in deposit of fine-grained low gamma-ray organic-lean chalky carbonate in clean water and the fall in the sea level resulted in clay-rich, high gamma ray, and organic-rich marl due to the increase in clay input during the sea-level drop.

Both siliciclastic-dominated "shale" and carbonate-dominated "shale" are usually located in anoxic depositional settings, for example, intra-shelf low and slope. The brittle minerals in rich siliciclastic marine and lacustrine shales with high TOC and high hydrocarbon (oil and gas) content occur in the transgressive systems tract (TST) up to the early highstand systems tract (EHST). The brittle condensed section of MFS has the best reservoir quality and completion quality.

Similarly to clastic sequence stratigraphic models developed for sandstones and carbonates, the sequence stratigraphy of fine-grained shale provides a powerful tool for predicting and mapping stratigraphy, variations of lithology, geochemistry, mineralogy, and shale gas content in time and space, and ultimately the most productive facies of unconventional gas shales.
