**5. Sequence stratigraphy of carbonate-dominated shales**

The sequence of carbonate-dominated shale has been studied using the Niobrara shale in Wyoming and Colorado (USA), for example. For the carbonate-dominated sequence, its sealevel response is different from that one of siliciclastic-dominated shales. For the siliciclastic shales, the sea-level rise has responses of increase in organic matter content and gamma ray (GR) of uranium. For the carbonate-dominated sequence, the sea-level rise means less input in siliciclastic clay and deposition of limestone in clean and quiet water. For the Niobrara equivalent formation in the Cretaceous Seaway, the sequence boundary is indicated by the maximum input of siliciclastic clay indicated by the highest GR in proximal Utah and clayrich marl in Colorado (**Figure 5**). The TST is indicated by upward decrease in GR and deposit

**Figure 5.** Sequence stratigraphy of carbonated dominated Niobrara shale in U.S.

of chalk. The HST is indicated by increase in GR and deposit of marl. The MFS is indicated by the lowest GR, the highest Rt, and clean chalk. The case study of Well Libsack 43,027 in Colorado in **Figure 5** clearly shows the sequence stratigraphic framework of Niobrara shale formation consisting of chalk and marl.

the deposition of the late highstand systems tract (LHST), a further fall of sea level resulted in an increased input of siltstone and sandstone within the shales (see silty layer in core images of the LHST shale in **Figure 4**). During this time interval, sedimentary environment changes to oxygenated shallow marine setting and the coarsening-upward sequence made up of carbonatic shale and silty shales progrades basinward. The shale in LHST is organic-lean and clay-rich shale due to the dilution of clastic input. Generally, the Paleozoic marine shale in TST and EHST has high TOC, high quartz content, and high gas content and is the main target for shale gas play in China. The intervals with high TOC and quartz content in TST and EHST have higher porosity. The porosity in LHST is the highest due to the contribution of silty shale. The sandstone or carbonate in the LHST may trap gas migrated from the TST and EHST shale

The sequence of carbonate-dominated shale has been studied using the Niobrara shale in Wyoming and Colorado (USA), for example. For the carbonate-dominated sequence, its sealevel response is different from that one of siliciclastic-dominated shales. For the siliciclastic shales, the sea-level rise has responses of increase in organic matter content and gamma ray (GR) of uranium. For the carbonate-dominated sequence, the sea-level rise means less input in siliciclastic clay and deposition of limestone in clean and quiet water. For the Niobrara equivalent formation in the Cretaceous Seaway, the sequence boundary is indicated by the maximum input of siliciclastic clay indicated by the highest GR in proximal Utah and clayrich marl in Colorado (**Figure 5**). The TST is indicated by upward decrease in GR and deposit

source rock, and they can form tight sand/carbonate gas play (**Figure 4**).

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

**5. Sequence stratigraphy of carbonate-dominated shales**

**Figure 5.** Sequence stratigraphy of carbonated dominated Niobrara shale in U.S.
