DECAMETER-SCALE BIT DROPS AT 7 KM DEPTH IN THE YIJIANFANG FORMATION, HALAHATANG OILFIELD, TARIM BASIN, CHINA: FAULT-RELATED DEEP-SEATED DISSOLUTION

Dissolution-modified carbonate rocks have heterogeneous porosity, therefore, dissolution and fracture–modified reservoirs are challenging for exploration. Productive oil-and-gas reservoirs in Ordovician carbonates of Yingshan to Lianglitage Formations, Tabei Uplift, Tarim Basin contain dissolution features, opening-mode fractures and faults and are in a setting of rapid, large-scale shortening. Outcrops contain cemented paleo-karst features, but Yijianfang Formation cores from Halahatang lack evidence of ancient, still-porous karst. Evidence exists for large voids, including bit drops of as much as 29 m at more than 7 km depth and large volume mud losses.

To understand the large voids and shed light on permeability, we conducted petrographic, isotopic, microstructual, fluid inclusion, and image log analyses and assessed tectonism using high-resolution GPS data. Results establish the paragenetic sequence of diagenetic events that led to voids. We conclude that bit drops occur in large fissures that follow NNE and NNW faults, possibly augmented by porous fault rock attested from outcrop studies. Wide, stable opening-mode fractures at depth are favored by current active, extensional stress regime, high displacement rates, and strong carbonate rocks (E as much as 80 Gpa, averaging 40 GPa). High opening rates, and presence of oil in fractures impedes closure by calcite cement. In cores, textural evidence from vugs supports dissolution along and adjacent to faults and localized near the top unconformity of the Yijianfang Formation. Diagenetically altered rock and paleo-karst near the unconformity may be prone to dissolution. Fluids from which fracture and vug cements precipitated included sulphur-rich oil and water of shallow and deep burial and hydrothermal origin. Porous fault rock, opening-mode fractures, and voids are conduits for dissolution-enhancing fluids and account for seismic bright spots. Fault arrays extend to near the high Tien Shan range (elevation ca. 6k), potentially providing source and head for deeply circulating water. CO₂-rich hydrothermal fluids, possibly mixed with meteoric waters, and sulphur-rich oil together are possible sources of dissolving fluids.