Stratigraphic framework of theWufeng-Longmaxi shale in and around the Sichuan Basin,China:Implications for targeting shale gas

Stratigraphic division and correlation are crucial for the identification of sweet spots and drilling design of shale gas.In this study,a stratigraphic division and correlation was carried out for the Wufeng-Longmaxi Formations in southern China from the prospective of lithostratigraphy,sea level changes,and biostratigraphy using data from seismic investigation,wells,and outcrops.The Wufeng and Longmaxi Formations were respectively divided into four members,Wu 1 and Wu2 for the former and Long 1 and Long 2 for the latter.Of the members,Long 1 was subdivided and its first subdivision(Long 11)was further divided into 4 layers(Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),and Long 1^(4)_(1)).Three eustatic cycles were recognized in the Wufeng-Longmaxi Formations.Cycle I corresponds to the Wufeng Formation with the maximum flooding surface at the top ofWu 1.Cycle II corresponds to Long 1,with the maximum flooding surface at the top of Long 1^(3)_(1).CycleⅢⅢcorresponds to Long 2.Furthermore,4 graptolite biozones(WF1 to WF4)were identified in the Wufeng Formation and 9 graptolite biozones(LM1 to LM9)in the Longmaxi Formation.WF1-2 and WF3-4 correspond to Wu 1 and Wu 2,respectively;and LM1,LM2-4,LM5,LM6,and LM7-9 correspond to Long 1^(1)_(1),Long 1^(2)_(1),Long 1^(3)_(1),Long 1^(4)_(1),and Long 12 and Long 2,respectively.Highquality shales mainly occur in the Wufeng Formation and Long 11.The major intervals that should be investigated with regards to shale gas production include LM1eLM5(10m thick)in the Weiyuan Block and WF1eLM5(20e35m thick)in the Changning Block.Long 1^(1)_(1)is believed to be an optimal target for drilling due to its high TOC content,siliceous content,porosity,microfracture density,and horizontal/vertical permeability ratio.

CO_(2)geological sequestration potential of the low-rank coals in the southern margin of the Junggar Basin

Carbon capture,utilization,and storage(CCUS)is considered one of the most effective measures to achieve net-zero carbon emissions by 2050,and low-rank coal reservoirs are commonly recognized as potential CO_(2)storage sites for carbon sequestration.To evaluate the geological CO_(2)sequestration potential of the low-rank coal reservoirs in the southern margin of the Junggar Basin,multiple experiments were performed on coal samples from that area,including high-pressure mercury porosimetry,low-temperature N2 adsorption,overburden porosity and permeability measurements,and high-pressure CH4 and CO_(2)isothermal adsorption measurements.Combined with the geological properties of the potential reservoir,including coal seam development and hydrodynamic characteristics,the areas between Santun River and Sigong River in the Junggar Basin were found to be suitable for CO_(2)sequestration.Consequently,the coal-bearing strata from Santun River to Sigong River can be defined as“potentially favorable areas for CO_(2)eequetfraiion”To better guide the future field test of CO_(2)storage in these areas,three CO_(2)sequestration modes were defined:1)the broad syncline and faulted anticline mode;2)the monoclinic mode;3)the syncline and strike-slip fault mode.