The Analysis Of The Sequence Stratigraphy And The Favorable Reservoir Belts Of Liushagang Formation In Maichen Depression Of Beibu-Gulf Basin

Beibu-Gulf basin is a Cenozoic non-marine faulted basin which was developed from the continental shelf of Nanhai Sea since Paleocene Period. Maichen depression, a depression of Beibu-Gulf basin, is a favorable exploration potential area with low exploration degree. The framework of sequence of the Liushagang formation is established by the use of sequence stratigraphy, sedimentology, petroleum geology, technical geology and seismic stratum. There are two three-order sequences in the liushagang formation including SQL1 and SQL2. The thickness is bigger in the southeast zone beside the high slope, and there is trend that the thickness is decreased gradually toward the northwest. The three-order sequence stratigraphy consists of three phases of lowstand system tract, transgressive system tract and highstand system tract divided by the initial flood surface and max flood surface.The temporal and spatial evolution is revealed through the established chronostratigraphic framework with the research of sedimentary facies and the distribution of sand body. In the depression, there are depositional systems like the fan delta and delta, shallow lake, half-deep lake and fore-delta turbid fan and so on. There are three sedimentary zones including high slope in the southeast, middle part of the depression and slow slope in the northwest.By the analysis of seismic facies and seismic property, we construct the deposition system distribution in the sequence frame, and analysis the mechanism of the distribution and evolution of the deposition system. The basin has three resources to supply the sediments from south-east, north-west and east. The deposition systems are controlled by the fault and palaeogeomorphology, and form dissymmetry deposition model. The sequence stratigraphy and the sediment facies are controlled by the growth fault.There are two petroleum systems in the Liushagang formation. In the transgressive system tract and highstand system tract of the three-order sequence of liushagang formation, there is thick mud which can form good seal and main oil source rock in the deposition systems of half-deep lake and deep lake. In the lowstand system tract and the transgressive system tract of the three-order sequence of liushagang formation, there are deposition systems of delta front sandbar and underwater channel sand as the good reservoir.The favourable reservoir belts in the depression locate in the transition belt of the high slope in the southeast and the depression in the middle part, in the transition belt of the slope in the west and the depression in the middle part, in the place of the west slope.