Enrichment mechanism of shallow lithologic reservoirs and newly discovered billion-ton oilfield in depression zone of Laizhou Bay Sag:A case study of KL10-2 Oilfield
In view of the key problem of unclear oil and gas accumulation and enrichment mechanism in the shallow lower member of Minghuazhen Formation in the deep depression zone of Laizhou Bay Sag,the sand body distribution rule and reservoir migration and accumulation pattern were studied by conducting a comprehensive analysis of drilling,logging,and 3D seismic data,as well as analysis of paleoenvironment,modern sedimentary analogy,and geological and chemical indexes.The sedimentary paleoenvironment and the development mechanism of large-scale sand bodies in the lower member of the Minghuazhen Formation of the depression zone in Laizhou Bay Sag were revealed,and the oil and gas migration model of lithologic reservoirs and the enrichment model of large-scale lithologic reservoirs were established.The results show that:①There is a high proportion of aquatic herbs in the Ⅳ oil group and the V oil group in the lower member of the Minghuazhen Formation,and the fossil abundance and diversity are between lacustrine facies and fluvial facies,which is a typical sedimentary environment featuring river-lake interaction.The well logging curves reflect the positive cycle of fluvial sand bodies,the reverse cycle of lacustrine action,and the positive and reverse cycles.Due to the frequent interaction and rapid transformation of the river-lake sedimentary environment,the"dendritic"contiguous sand bodies with large-area distribution and wide internal connectivity are formed.② Oil and gas accumulate through the deep sand bodies as a transfer station in the steep slope zone,then migrate to the shallow layer through the boundary oil source fault,and finally move southward along the shallow contiguous sand bodies for shallower layers.The geological and chemical indexes all have long-distance migration effects,showing the characteristics of"circuitous"reservoir formation.③ The enrichment mechanism of lithologic reservoirs in the lower member of the Minghuazhen Formation is jointly controlled by the distribution range of contiguous sand bodies,sand body configuration,and mudstone sealing ability.Oil and gas are first charged along the river line and then adjusted to the surface of the lake-flat sand bodies,forming a large-area contiguous oil surface in the shallow sand body,with"first dendritic and then contiguous features".The difference in sand body configuration at different periods controls the degree of oil and gas enrichment.Among them,the storage capacity of"dendritic"continuous sand bodies is better than that of isolated channel sand bodies,and the maximum oil-bearing range can reach 117 m.The new understanding successfully guided the discovery of KL10-2 Oilfield,the first large-scale high-yield lithologic oilfield in the shallow deep depression zone in China,which broke through the traditional"forbidden area"for shallow exploration in Bohai Oilfield and first discovered a large-scale oilfield in shallow layers of deep depression zone in Bohai Bay Basin.
Laizhou Bay SagKL10-2lower member of Minghuazhen Formationdendriticcircuitousdepression zoneexploration and discovery
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