Shale Porosity Compaction and Gas Leakage During Exhuming: A Case From the Fuling Shale Gas Field, Eastern Sichuan Basin, China

Abstract

The Fuling shale gas field, located on the Eastern fold-thrust belt of the Sichuan Basin, has cumulatively produced >10¹⁰ m³ of shale gas. Compared with the gas shales of North America, the Wufeng-Longmaxi shale in the Fuling gas field is characterized by an older age (O₃-S₁), higher maturity (R_o >2.0 %), and stronger exhumation (>3,000 m). The retention of gas in the pore space of shale largely depends on the deformation process, which varies with location in the fold-thrust belt.

Integrating the inorganic/organic components, helium porosity, and nuclear magnetic water saturation (S_w) of cores in various areas of the Fuling gas field, the following is observed:

1) The porosity and S_w of organic-rich shale are generally <6 % and >20 %, respectively, and they are rarely as favorable for gas production as the shales of North America;

2) Shale porosity correlates positively with TOC and clay content in the boreholes of the northeast (NE) and southwest (SW) areas, respectively, indicating that the organic matter-hosted and clay mineral-hosted pores are considered as the dominant contributor to total porosity in the NE and SW areas, respectively;

3) At equivalent TOC content and burial depth, shale porosity is relatively lower in the SW area. Despite the maximum burial of both areas once up to 6,000 m, shale porosity correlates inversely with depth in the SW area (the change in porosity is ~2 % per 100 m thickness). Due to tectonic deformation, the shale porosity in the SW area was lost during folding;

4) The stronger fluid pressure in the boreholes of SW area relative to NE area at equivalent TOC content and burial depth reveals that the initial maturation induced overpressure of SW area seems to be supplemented by tectonic compaction;

5) The S_w correlates inversely with TOC content and positively with clay content in the boreholes of the NE area, but it can be abnormally high (>40 %) at high TOC content (>3~4 %) or at low clay content (<40 %) in the boreholes of the SW area. Furthermore, in the SW area, the S_w of organic-rich interval is abnormally high near the top/bottom boundary (even higher than the S_w of organic-poor roof/floor), indicating that gas has leaked from the organic-rich interval;

6) The S_w of both the retention and leakage areas has the same relation with porosity, implying that the water scarcely drained from shale during the gas leakage.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018