Sedimentary facies analysis and sequence stratigraphy characteristics of Shahejie Formation in Lixian Slope, Raoyang Sag, Bohai Bay Basin, Northern China

The Lixian Slope is located in the west of Raoyang Sag, Bohai Bay Basin, Northern China. The Shahejie Formation which has produced more than 90% of oil and gas over the past years is the main explorative target in this sag.... [ view full abstract ]

The Lixian Slope is located in the west of Raoyang Sag, Bohai Bay Basin, Northern China. The Shahejie Formation which has produced more than 90% of oil and gas over the past years is the main explorative target in this sag. The study on sedimentary facies and sequence stratigraphy has important practical significance about reservoir-forming geological conditions and further hydrocarbon exploration.
Based on the comprehensive analysis of core, well logging and seismic data as well as granularity data, the sedimentary face types of Paleogene strata in Lixian Slope were studied in detail. The result shows that, in the Shahejie Formation of Lixian Slope, braid delta facies are dominant, followed by lacustrine facies. On the basis of identification of sequence interfaces which mainly include unconformity surface, lithoface mutation surface and transformation surface between parasequence superimposed styles, it shows that, the Shahejie Formation in slope is actually a complete second-order sequence and can be divided into four third-order sequences. All third-order sequences from SQ1 to SQ4 are just composed by lacustrine transgressive system tract (LTST) and highstand system tract (HST), with the absence of lowstand system tract (LST). By studying depositional system in isochronal frame of sequence stratigraphy, the evolution rules of sedimentary facies and distribution of sandstones in Shahejie Formation of the Lixian Slope were studied systematically. The LTST in SQ1 to SQ3 are mainly composed of lacustrine facies and braid delta facies, but the lacustrine face is dominant in LTST of SQ4. The HST in SQ1 to SQ4 are mainly composed of braid delta facies. The reaseach also shows that the vertical distribution of sandstone is generally controlled by depositional system tracts. The sandstones in HST of SQ1 to SQ4 have been developed better than those in LTST. On the other hand, the planar distribution is mainly controlled by the palaeotopography, tectonic and provenance.