INFLUENCES OF MUD DIAPIRIC FLUID ON RESERVOIR RECONSTRUCTION AND GAS ACCUMULATION SEQUENCE IN THE YINGGEHAI BASIN, SOUTH CHINA

The Yinggehai basin in south China has several important natural-gas blocks, which are closely related to mud diapirs. However, how the mud diapirism influenced the mixing of hydrocarbon and CO₂and how the gas accumulation was modified during the diapirism are still open to further discussion, which should benefit future exploration in this area and for similar petroleum systems.

In this study, we investigated the core samples from different blocks and stratigraphic levels of the Yinggehai basin, aiming at (1) describing morphological features of internal fractures, (2) identifying composition of the filling materials and characterizing the abnormal porosity. Moreover, based on reservoir experiments (MTS), physical properties of the reservoir samples were measured at different conditions of temperature, pressure and fluid constituents. The experimental data were compared with samples from various structural positions with respect to the mud diapirs, in order to understand how the diapiric fluids migrated into the traps and modified the gas reservoir. Followed important findings are achieved: (1) Fracture morphologies and mineralogy of filling materials indicate three stages of development, which are characterized by the crystallization of pyrite, hematite and cryptocrystalline pyrrhotite, respectively. (2) The reservoir rocks underwent the MTS experiments show obvious modifications in morphology and porosity, indicating mechanical reshaping during the fluid injection and porosity modification due to water-rock reaction, which commonly generated secondary solution porosities in the reservoir. (3) The increase in pressure and injection of deeper CO2 and organic acid-rich fluids related to the mud diapiric fluid could modify chemical properties of formation water and reconstruct migration conduit and reservoir into better status for accumulating hydrocarbon. (4) During the expulsion of hot fluid from source rocks, more hydrocarbons could be expelled out of the source rock due to decompression and dissolved into the fluid; therefore, the CO₂, organic acid and hydrocarbon rich fluids resulted from mud diapirism could have not only improved reservoir quality but also promoted petroleum accumulation.