Neogene-Quaternary Tectonic Reactivity And Its Control On Petroleum Accumulation In The Bohai Bay, East China

The Bohai Bay is the offshore portion of the Bohai Bay Basin which belongs to the rifting basin. The evolution of this basin can be clearly divided into two stages:the Paleogene rifting stage and Neogene-Quaternary postrift stage. This region is the important area for hydrocarbon production in East China, and with a complex geologic feature resulting into the variety of the hydrocarbon reservoirs. The Bohai Bay has a significant and special regional location and some special aspects in its evolutionary process, providing excellent material for study of regional geology, tectonic and structural geology, and petroleum geology. Thus the Bohai Bay became a research focus for petroleum and geologic scientist in recent years. Previous studies about this area mainly focus on the Paleogene, whereas the systematic investigation on the Neogene-Quaternary of the basin is poor. That is because the Paleogene is the main target for hydrocarbon exploration in this basin for many past years. And thus, this study mainly focuses on Neogene-Quaternary structural characteristics and tectonic evolutionary process of this area with abundant exploration data.The final investigation via abundant seismic and well data shows that the Bohai Bay is atypical in terms of its Neogene-Quaternary postrift subsidence history in that it experienced intensive tectonic reactivation, rather than the relative tectonic quiescence experienced during this stage by most rift basins. This Neogene-Quaternary tectonic reactivation arose principally in response to two tectonic events:(1) activity on a dense array of shallow faults and (2) accelerated tectonic subsidence that occurred during the postrift stage; these two events were neither strictly temporally nor spatially equivalent. The dense array of shallow faults form a NW-SE trending belt in the central part of the basin, corresponding to the NW-trending Zhangjiakou-Penglai fault zone, with displacement on them having been induced by the reactivation of older northeast-and northwest-trending and E-W-trending basement faults and an associated substantial component of strike-slip displacement occurring after5.3Ma. The intense reactivation of these faults contributed to the atypical accelerated rate of postrift tectonic subsidence of the basin, which commenced～12Ma. However, this was not the sole cause of this accelerated tectonic subsidence:a combination of geological activity at a deep level within the crust led to the build-up of intraplate stresses and this, combined with on-going thermal subsidence, acted as additional contributory factors that drove unusually high rates of subsidence for this basin. The postrift tectonic reactivation in Bohai Bay indicates that structural deformation associated with postrift tectonic reactivation has a different expression to that which occurred during the synrift stage:it is less intense overall, yet arose in response to a complex and dynamic formative mechanism; the amount of the fault was increased dramatically but with small throw; the subsidence rate was accelerated, but the differential subsidence caused by faulting was weaker on plan. This postrift structural deformation may comprise two parts:one requires the reactivation of the synrift structural elements (i.e., fault development), whereas the other develops in a manner unrelated to synrift structural deformation (i.e., anomalous tectonic subsidence).Several significant issues relating to the tectonic reactivity has not previously been thoroughly examined in detail, including the documentation and analysis of the specific characteristics relating to the distribution and development of postrift renewed faulting, the timing and location of zones of accelerated subsidence across the basin, and the relationship of one to the other, as a consequence, these issues are discussed in detail in this study. Based on these investigations, the dynamic of Neogene-Quaternary tectonic evolution of the Bohai Bay, and implications for geodynamics of East China are also be discussed. Finally, using all the new understands about the Neogene-Quaternary evolution, the rule of hydrocarbon accumulation in the Bohai Bay is analyzed. The result demonstrates that postrift tectonic reactivation results in conditions that are favorable to hydrocarbon accumulation in a rift-basin setting.