Controlling Mechanism On Sandbody And Favorable Prospecting Directions In Fault Lacustrine Basin

Since Paleogene,the basins were small and the water was deep in Shenxian sag Jizhong depression, thus,attitude of stratum was steep. Furthermore, double contemporaneous faults controlled the development of the lacustrine basins, rift system was complicated, the reservoir of intended interval was small, of which facies belt was narrow and lateral variation of sands was complicated, whose exploration degree was also lower. It has great theoretic and practical significance for structural stratigraphic hydrocarbon reservoir exploration of Shenxian sag and similar areas to study the sequence stratigraphic characteristics and sedimentary characteristics of the lacustrine basins and its mechanism about how to control the sands.On the basis of core observation, according to the data of geo-logging, well logging and seismic, applying the principle and methodology of high resolution sequence stratigraphy and sedimentology, adopting many techniques such as strata correlation by combinating well with seismic, seismic attribute analysis, sand ratio calculation by multiple regression with well and seismic data, waveform classification and seismic facies analysis, palaeogeomorphology reversion, flattening seismic strata, balanced section, study of coupling relations of structure and sedimentation, etc, following the research path of point(well)-line(profile)-map(plane map)-tract(temporal evolution and spacial distribution)-point(trap), the depositional system's planar distribution and vertical evolution of intended intervals of each period in the region of interest were studied in the HR isochronous stratigraphic framework and then the sequence packing model and mechanism about how to control the sands of the downfaulted lacustrine basins were also researched. Finally, structural stratigraphic hydrocarbon reservoirs were predicted and beneficial explortion targets were ascertained. Through the above studies, the following achievements were acquired.1. According to the stratigraphic cyclicity and superimposed patterns, stratigraphic division and correlation were made using the base level as sequence boundary.The intended intervals were divided into three second-order sequences, eleven third-order sequences and twenty-two four-order sequences. It was the first time to complete the high resolution sequence divisions and establish framework in the whole sag;2. The conclusion that grain size of southern area was large and that of northern area was small, fan delta was developed in southern area and braided delta was in northern area was generalized and analyse its origins.The regularity about how to control sands in Shenxian sag was summarized, which was that structure controls provenance, physiognomy controlled sedimentation, regulator belt was in gentle slope and down cutting valley was in abrupt slope;3. Five kinds of depositional facies were distinguished,which were alluvial fan,fan delta, braided delta, lacustrine facies and fluvial facies. On the whole, they were lacustrine sedimentations,which present the typical characteristics of sediment fill in downfaulted basins. Ek-SQ～Es4-SQ were the depositions of initial chasmic stage of lake basin.At that time, relief was high; water was separate;alluvial fan, fan delta and large braided delta were relatively developed well; a big set of coarse glutinites and sands were deposited and hydrocarbon show was less. Es3-SQ1～Ed-SQ3 were the depositions that suffered the complete process of downfaulted lake basin expansion, tectonic uplift, tectonic reversion and lake basin consumption.At early stage, kinds of proximal fan delta and braided delta were developed in the lake basins. Meanwhile, dark mudstones at the dislocation of rising half cycle and falling half cycle in Es3-SQ2 sequence and Es1-SQ1 sequence offered good mother rocks for hydrocarbon generation.The late sands of fluvial channel, channel bar and point bar in fluvial facies were the dominate reservoir rocks;4.The characteristics and control factors of play in each divided area were evaluated in the sequence stratigraphic frame.Two beneficial targets were analysed(slope of Shenxi sag and downthrow block of Hengshui fault). Through the above analysises, the combination of structural hydrocarbon reservoir and lithologic hydrocarbon reservoir should be taken as the chief strategy.