Formation Mechanism And Occurrences Of Lithilogical Petroleum Reservoirs Of Es3 In Linnan Subsag

With the process of petroleum exploration, the concealments of residual petroleum resources were more prominent and lithological oil reservoirs were attracted more attention. As the most important subsag in Huimin sag, Linnan subsag will lay strong emphasis on lithological oil reservoirs exploration after about forty years exploration of structural oil reservoirs. But some deficiencies were fronted in the relative researches in this subsag. Such as lack of establishment of sequence framework for the whole area, ambiguity of depositional systems constrained the formation of lithological traps, especially the distributions and evolutions of depositional systems in sequence framework and their implications, unawareness of the key controlling factors and mechanism of lithological oil reservoirs, vagueness of occurrences of lithological oil reservoirs in lateral and vertical, all of which restraint the predictions of potential targets.Focused on the formation and searching of the lithological oil reservoirs, high resolution sequence stratigraphy and sedimentology were used. Incorporated with regional tectonic evolutions, Es₃ in Linnan subsag was believed to be a long base level cycle. According the eight isochronous sequence interfaces, four middle base level cycles were classified and sequence framework of the whole area was established in detail.The analysis of lithologic characteristics, lithologic texture, sedimentary structures, palaeontology, log curves, seismic reflections and vertical sedimentary series of deposits in Es3 indicated that delta, fan delta, fluxoturbidite fan and lake facies were developed in this subsag and formed five depositional systems in space. During the course of base-level rose, Linyi delta, Jishan delta, Shuangfeng fan delta and Jiangjiadian-Wawu fan delta prograded to the subsag center, while during the course of base-level falling sandbodies of delta fronts would yield fluxoturbidite fans or deepwater turbidite fan in deepwater areas, influencing the formation of kinds of lithologic traps. Researches on formation mechanism of petroleum accumulations suggested that complicated carrier networks were made up of connective sanbodies, faults of various levels activated in different stages, multiple hiatus and regional unconformities, igneous rocks and fissures in mudstones. Abnormal high pore fluid pressure was the chief force driving hydrocarbons secondary migration. Lithologic oil reservoirs in Linnan subsag was under the control of tectonic settings, sedimentary facies, carrier systems and matching of formation conditions.Lens-shaped sandbodies were easy to be yielded in the deepwater areas and fluxoturbidites of the delta fronts, while upclined pinch-out traps formed in the flanks of paleogeosyncline or slope areas of subsag. Associated with faults or paleogeomorphology, lithologic or structural-lithologic oil reservoirs can be formed. Occurrences of Lithologic oil reservoirs in Linnan subsag were around the Jishan sandbody, sandbodies in distributary channel of delta plain and turbidite of prodelta in Linnan delta, sandbodies in delta front of Jiangjiadian fan delta, fluxoturbidite in delta front of Yingzijie delta and sand-gravel bodies in south slope of Huimin sag. In terms of tectonic performances, depositional characteristics, distribution rules of sandbodies, formation conditions and present exploration achievements, over twenty potential targets for lithologic oil reservoir were predicted in Linnan subsag.