Characteristics of Continuing Water-Transgressive Sequences of the Lower Assemblage in BN Sub-sag,Bongor Basin

摘要

Bongor basin of Chad is a Mesozoic rift basin of Central African Shear Zone(CASZ).The straitigraphies include the pre-Cambrian basement,the lower Cretaceous(P/M/K/R/B Fm.)and a few Cenozoic straitigraphies.The lower assemblage composed by P Fm.and M Fm.is one of the main exploratory straitigraphy in the basin.During the lower assemblage deposition Bongor basin was in initial stage and experienced intensive faulting.Accordingly,in P Fm.Bongor basin was composed of many individual sub-sags.It was not until M Fm.these sub-sags had a monolithic lake level.Among these sub-sags,BN sub-sag has the biggest resource potential.Intensive faulting in the lower assemblage results in long-term subsidence and continuing water-transgressive in the sub-sags.BN sub-sag developed continuing Water-transgressive Sequences which composed of interactive formations between thick sandstones and thick dark mudstones with gravity current characteristics in the lower assemblage.The similarity of lithology cycles in different depth and different wells make it difficult in stratigraphic sequence division.In this article based on the relationship between the structure activities and strati-graphic sequence,considerate of episodic activities of gravity current sedimentation,stratigraphic sequence of BN sub-sag was divided into 4 sequences.SQ4 deposited a set of rapid transgressive sequence in initial intensive faulting stage of rift basin.SQ3 and SQ2 deposited in a relative stable environment with continuing water-transgressing and formed near-source fans in deep water.SQ1 was deposited during the provenance degenerate and formed a monolithic lake level.SQ2 is the most important sequence in oil exploration.Five main near-bank submerged alluvial fans and fan deltas were found for further oil exploration.