Fluid production of L field, Bohai Bay Basin, offshore eastern China is mainly from the Palaeogene Dongying D interval. The stratigraphy framework and depositional model of the braided-river delta system within the D interval are investigated using seismic, well log, core data and production response. The D interval is interpreted as having been deposited in a lowstand system tract. And two progradational successions are recognized, including in ascending order the D2 and D1. The younger sandstone in the D1 interval is of greater thickness and larger distribution area than the elder sandstone in the D2 interval. Six core facies and five log patterns are recognized and interpreted to be underwater distributary channels, levees, overbank splays, mouth bars, sheet sand and shalier interchannel deposits. Channel deposits occur along the trend of thickest D2 and D1 sandstones. The sandstones that flank each side of the channel deposits are interpreted to be levee and overbank splay deposits. The sandstones that develop at terminal distributary channel mouth are interpreted to be mouth bar and sheet sand deposits. Channel-flank deposits can form good-quality reservoir sandstones, but they contain interbedded siltstones and thus have lower porosity and permeability than do channel deposits. The facies distributions predicted for the D interval match trends of the daily total fluid production. Knowledge gained from study of the L field has application to the development of other fields with similar depositional and diagenetic histories.

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Five wells of L oilfield in Bohai bay basin have drilled 10-15 meters thick oil layer in the Paleogene delta. Due to the deep-buried reservoir and the poor seismic performance, it is difficult to identify the reservoir genesis, and predict reservoir distribution. By analyzing core, well logging and seismic data, a stable mudstone section is selected as the correlation marker to establish a stratigraphic framework. The paleogeomorphology is reconstructed after decompaction correction and paleobathymetric analysis. Based on the differences of paleotopography and sedimentary facies, the study area mainly develops two delta systems: low gradient coarse-grain delta system and steep gradient delta-turbidite system. The favorable reservoir of low gradient coarse-grain delta, which is thick and has good lateral connectivity, mainly locates in the delta front. The favorable reservoir of steep gradient delta-turbidite system locates in the delta front and turbidite facies, and the delta front deposits are thin and have poor lateral connectivity. The boundary of delta front is first depicted on the basis of paleotopography. In combination with reservoir architecture and forward modeling analysis, the seismic attribute is then optimized to predict the distribution of favorable reservoir. Using this method, several sets of oil-bearing sandbodies have been drilled in L oilfield, and the prediction accuracy of reservoir distribution is proved to be high. This study demonstrates that the paleogeomorphology plays an important role in controlling the genesis and distribution of the delta reservoir and provides reference for the reservoir prediction in similar oilfields.

Through the comprehensive analysis of the data related to cores, logging and seismic, the author holded that there were two kinds of sedimentary systems in the study area, which were braided river deposits and meandering river deposits. According to the sequence stratigraphy method of fluvial facies proposed by Catuneanu et al, we established sequence stratigraphic framework in the Guantao formation of south gentle slope belt, Huanghekou Sag. It was divided into 2 third-order sequences, named as SQGU and SQGL, each of the third-order sequences contains a high accommodation systems tract (HAST) and a low accommodation systems tract (LAST). On this basis, the author summarized the sedimentary sequence evolution characteristics in the Guantao formation of south gentle slope belt, Huanghekou Sag. In the period of LAST in the SQGL, there were mainly composed of braided river facies in the study area, main developed multistage compound sand body. In the period of HAST in the SQGL, there were mainly composed of meandering river facies in the study area, main developed isolated point bar sand body and a high proportion of flood plain mud. The evolution characteristic of the SQGU was similar to the SQGL. Because of the accommodation space of the SQGU was at a high level, the grain size of sandstone and the percentage of sandstone are smaller than SQGL. It is generally accepted that it was mainly composed of braided river facies in Guantao formation of Huanghekou Sag, difficult to form oil and gas reservoir due to lack of mudstone cap rock. The exploration of Guantao formation has not attracted enough attention. Through this analysis, the author thinked that two sets of meandering river deposition can provide partial sealing and created conditions for oil-gas accumulation in the Guantao formation of south gentle slope belt, Huanghekou Sag. Under the guidance of this research, we have obtaineda certain oil and gas discovery in the study area.