Evidence for episodic expulsion of hot fluids along faults near diapiric structures of the Yinggehai Basin, South China Sea

Diapiric structures are well developed and occur in most of the central par t of the Yinggehai Basin, on the western side of the South China margin. A strong thermal anomaly due to hot fluid flows occurs in the diapiric zone, as evidenced from vitrinite reflectance (R-0), clay mineral transformation, and fluid inclusion homogenization temperatures. This anomaly results from hydrothermal fluid flow along vertical faults from overpressured compartme nts into the overlying Late Miocene and Quaternary sand-rich layers. The ma gnitude of thermal anomaly is related not only to the distance to which the vertical fault is hydraulically open, but the permeability of rocks interc onnected with the faults. Intense heat transfer for convection of fluids oc curs in the sand-rich intervals adjacent to vertical faults. Abnormal organ ic-matter maturation, together with rapid transformation of clay minerals, which occurs at certain intervals within the present-day normally pressured system and normal conductive temperatures in a diapir, can be used to iden tify palaeo high pressure zones. Abnormal high temperatures measured from a dr-ill-stem test in a diapir can be inferred to be the results of recent e xpulsion of hydrothermal fluid flow. The results of this study suggest that thermal fluid expulsion along faults plays an important role in the modifi cation of thermal regimes, the enhancement of organic-matter maturation, an d rapid transformation of clay minerals, as well as the accumulation of hyd rocarbons in diapiric structures of the Yinggehai Basin, South China Sea. ( C) 2001 Elsevier Science Ltd. All rights reserved.