Subsidence and maturation evolution of a new lower-slope minibasin, Xingning Sag, northern South China Sea

Abstract

Study of Subsidence and maturation evolution of a continental slope basin without a drilled well is an important problem in the early stage of deep-water exploration. Lack of some parameters, such as stratigraphic-age, porosity-depth relationship, exact paleo-water depth and heat flow value are common problems in the study of subsidence and maturity history. Using 46 long 2D seismic lines and paleo-water depth data from paleontological data of a well on the slope toe, the present study combines backstripping with thermal modeling to reconstruct the subsidence and maturation evolution of a lower-slope minibasin, Xingning Sag (XS), on the southeast Pearl River Mouth Basin (PRMB) of the South China Sea (SCS), now a new prospecting area. Stratigraphic-ages, porosity-depth relationship, lithology of XS were estimated using data from an adjacent basin. Paleo-water depths were obtained by clinoform geometry using seismic facies and paleontological method. With these parameters, the subsidence curves were created by backstripping. Crustal stretching factors (β) were obtained by best fit of tectonic subsidence curve and thermal modeling. With these stretching factors, the theoretical subsidence and heat flow values were obtained at the same time. Based on the heat flow value and the backstripping parameters, the maturation history of this basin was simulated. The result shows XS evolved through three cycles of subsidence during 46–23.03 Ma, 23.03–13.82 Ma and 13.82–0 Ma, respectively. Each cycle shows a rifting phase of relatively rapid tectonic subsidence followed a cooling phase of slow subsidence. Potential Eocene lacustrine source rocks are distributed primarily in the central depression area (Xingning main sub-sag), and would have entered the main oil-generation window and main gas generation during early- and mid-Miocene respectively.