Fluid activities Controlled by Intra-craton Strike-slip Faults: A Case Study of Ordovician in Fuman Area in Tarim Basin

Deep fluid activities driven by strike-slip fault movement play important roles in the modification of carbonate and hydrocarbon accumulation. The complexities of deep fluid sources and temperature-pressure variations during strike-slip fault movement complicate fluid-rock reactions, diagenetic modification processes and the formation and evolution of reservoirs in deep to ultradeep carbonate strata. To understand the temporal-spatial coupling between strike-slip movement and deep fluid migration, we investigate the migration periods and sources of deep fluids along strike-slip fault belts in the Fuman Area of the Tarim Basin, considering the geometry of the strike-slip faults and analysing laser ablation U-Pb dating, clumped isotopes, REE, and fluid inclusions in diagenetic products such as calcite, chert, and quartz.

U-Pb age results indicate that vug-filling calcites were emplaced between 460.8 ± 3.4 and 448.6 ± 5.3 Ma, and at 335 ± 19 Ma during the early Hercynian orogeny, while the fracture-filling and megacrystalline calcites formed between 364 ± 53 and 282.9 ± 5.4 Ma, and during periods from 324 ± 23 to 300.9 ± 4.8 Ma and from 244.13 ± 13 to 240.5 ± 4.1 Ma, respectively. The latest fracture-filling calcites show a slightly younger U-Pb age of ca. 158 ± 17 Ma. In addition, the U-Pb ages for the chert and quartz in fractures (459 ± 57 Ma, 252 ± 56 Ma, and 174 ± 35 Ma) fall within the middle Caledonian, late Hercynian, and Yanshanian periods.

The combination of geochemical analyses on calcites, including clumped isotopes, d13C, d18O, and 87Sr/86Sr isotopes, REE, and fluid d18O calculation, suggests that these calcites were precipitated from formation fluids mostly of meteoric water origin with some input from hydrothermal fluids. Hydrothermal fluid flow resulted from strike-slip fault activity and volcanism, whereas meteoric water intruded from uplifted areas along the faults during tectonic quiescence. This study shows that the formation of fracture-related cavern reservoirs in the Fuman oil field is related to the early Hercynian, late Hercynian, and Yanshanian tectonic events and their associated fluid activity. The methodologies and outcomes of the present study may guide future hydrocarbon exploration in the Tarim Basin and be applied to other oil fields with similar tectonic backgrounds.