Abstract:

The No.2 fault zone of the Weixi′nan Sag in the Beibu Gulf Basin is composed of a group of en echelon normal faults， with a large transition zone formed at the contact parts of faults in overlapping. However， there has been a lack of systematic study on the effect of the transition zone on hydrocarbon accumulation. Based on 3D seismic and drilling data， this study systematically analyzes the distribution pattern and evolution of No. 2 fault transition zone， and its role in controlling hydrocarbon enrichment. As affected by two stages of extensional activities since the Cenozoic， the development of No.2 fault transition zone featured resurrection of basement faults in the early Eocene as controlled by NW-SE transtensional stress， resulting in a continuous single fault of large scale； while in the mid-to-late Oligocene， the transtensional stress turned to strike SN， accompanied by weakened basement fault activities， resulting in segmented en echelon faults developed in the upper caprock， and the contact parts of faults growing to be transition slope serving to control the development of gravity flow deposits in the first member of Liushagang Formation （Liu 1 Member） of B low. At the sedimentary period of Liu 1 Member， the large-scale delta sourced from the NW entered B low through the transition slopes of the segmented faults and formed sand-rich sub-lacustrine fan under the secondary transportation with gravity. In addition， drilling data confirm that the gravity flow deposits in B low is mainly of sandy debris flow， trending NE in parallel with the No.2 fault zone in en echelon arrangement. The understanding provides theoretical guidance in the exploration of lithologic traps in the study area with good effects achieved in exploration.

Key words: sub-lacustrine fan, gravity flow deposits, fault transition zone, lithologic trap, south Weixi′nan Sag, Beibu Gulf Basin