Chemostratigraphy and Sequence Stratigraphic Investigation of the Lower Silurian - Upper Ordovician Hot Shale, Southeastern of the Sichuan Basin

摘要

The southeastern Sichuan Basin is potentially one of the most prolific Shale Gas play in China. There is an obvious and pervasive stratigraphic correlation (high Gamma reading) at the lower Longmaxi Formation, which has been defined as the Hot Shale interval. However, it is difficult to make a meaningful detailed geological correlation within the Hot Shale interval due to relative poor data coverage of seismic and petrophysical log signatures. Chemostratigraphy under the right geological setting is a proven tool that uses changes in bulk elemental composition to understand controls on reservoir quality, such as sediment provenance (proximal or distal) and paleo-environment in order to establish a chemostsratigraphic correlation for a given interval. For this study, a total of 377 cuttings and 20 core samples from the Upper Longmaxi, Lower Longmaxi to Wufeng (Hot Shale) and Linxiang/Baota formations of 4 exploration wells have been analyzed using ICP-OES and ICP-MS. Final integrated analyses have been made by combining with petrophysical logs, TOC, X-Ray Diffraction (XRD), petrography and Scanning Electron Microscope (SEM) data. Prior to any interpretation, cutting sample quality was checked by comparison between chemical gamma (using U, K and Th data from the sample) and downhole gamma which suggested the cutting samples were largely representative of the formation drilled. Elements and element ratios can be directly linked to the XRD data which reflects the influence of sediment lithology/ mineralogy variation, it also demonstrates the elemental data is qualified to be used. The enrichment factor of vanadium (EFV), which is sensitive to changing paleo-environments, suggests the lower Hot Shale was deposited in an overall more anoxic environment compared to the overlying and underlying formations. Within the Hot Shale interval, there is a positive relationship between TOC and EFV, indicating that TOC preservation is predominantly controlled by changes in paleo-environmental settings in the southeastern Sichuan basin. It further implies that EFV can also be used as a proxy for modeling TOC to assist in predicting lateral changes of paleo-environment between widely spaced data points. Changes in the relative amount of terrigenous input (TiO2 + Al2O3 + K2O + Na2O) and biogenetic silica (SiO2/Zr) are also modeled, the former being related to changes in sediment provenance and the latter to depositional conditions. Terrigenous input increases upward with the lower Hot Shale having lower terrigenous content indicating a period of sediment starvation in relatively deep water. The high biogenic silica contents in the lower Hot Shale interval are consistent with the pervasive presence of sponge spicules in the microscope view of thin sections. The high biogenetic silica contents results in the sediments being more brittle, thus favorable for Hydraulic Frac Stimulation. In summary, a clear sequence stratigraphy framework within hot shale interval has been built up. The sequences of good reservoir quality (with high TOC and brittleness) are clearly identified in the Squence-2 using bulk elemental data. The interpretation of elemental data presented in this study is consistent with petrophysical logs, petrographical data and biostratigraphy analysis of the 4 wells. Future studies will focus on the lateral variation of the inorganic geochemical characterizations in the Southeastern Sichuan Basin.