| Located in the southeast portion of Linqing sub-basin, Dongpu depression is one of the most petroliferous depressions in Bohaiwan basin. The Luxi high and NeiHuang high are to the east and west and apart from the depression by lanliao fault reaching to the basement and Chang Yuan fault respectively. Similarly, the KaiFeng sub-basin and ShenXian depression are to the South and north and apart from the depression by LanKao High and MaLing fault.The strata deposited in this depression are more than 10000 meters. From bottom to top, these strata are made up of the lower Paleozoic (Ordovician) carbonate, the upper Paleozoic (carboniferous-Permian) core-bearing marine-terregenous clastics (the first oil source rock), the Mesozoic (Triassic-Jurassic) red clastics, the Paleogene salt-bearing clastics (the second oil source rock) and the Neogen fluvial clastics. Paleogene strata can be divided into Kongdian formation, Shahejie formation and Dongyeng formation. From lower to upper, the Shaheje formation can be divided into four members, namely, member1, member 2,member3 and member4, in which the microfossils, such as ostracode, gastropod, charophyte, sporopollen, dinoflagellate and other algae are abundant. Different types of fossil make up different fossil assemblies. In this depression, there are 8 assemblies and 3 sub-assemblies of orstracoder, 4 gastropodes assemblies, 5 charophyto assemblies,3 assemblies and 9 sub-assemblies of sporopllen, 3 assemblies and 5 sub-assemblies of dinoflagellate and other algae.Kongdian formation was deposited in the early Eocene, member 4 and member 3 of Shahejie formation late Eocene, member 2 early Oligocene, member 1 middle Oligocene, Dongying formation late Oligocene. The Paleogene fossils in Dongpu depression can be divided into 7 biofacies. The climate of Paleogene of this depression can be determined as the sub-tropical frequently alternative semi-drought and semi-humid.This paper concludes that the evolution history of Dongpu depression was a process of a change from widespreading basin in member 4 period to multiple depocenters in member 3 period. The Shijiaji fault and Wenxi-Weixi fault were active during the period of member 3. Four low-relief trends striking NEE such as Qiaokou-Baimiao, Hu7-Wen205, Hu19-Wen10 and Mazai-pucheng exist in the depression. Their long-term activity had the profound influence to the formation of depositional framework. The model of salt crystallization in shallow water of deep basin is summarized in this paper, and the evidence of the formation of salt by terrestrial evaporation has been found. For instance, the content of Br is less than 200 PPM,δS34 is rich, the value of V/Ni is less than 1,the sequence of salt is composed of salt with thickness of 10-30cm and interbeded layers of gypsum (has perfect cleavage planes can be called alabaster), claystone (usually the claystone is in grey or buff or red color). The range of the lake waters changed dramatically and the lake standline had a vast scope during salt crystallization period,it is the main reason of sandstones in member 3 and member 4 widely spreading with thin thickness and of the conditions for the formation of subtle reservoirs. More data have proved that the source of salt in Dongpu depression not only is terregenous but also came from marine. The stratigraphic sequences are studied and the sequence stratigraphic framework of third and fourth unit in member 3 is established in this paper. It is concluded that the slop break belts are the place on which the Sandbodies are well developed. Meanwhile, three slop break models, such as break slop, gentle slope and paleo-trend are set up. These models will be the basis of the prediction for the subtle reservoirs.Based on the theory of sequence stratigraphy, this paper summarizes the patterns of distribution of sandbodies and depositional facies, and pointed out initially that the sandstones in central trend was deposited in the environment of lower stand point bar, and in the condition of drag flow dynamic. Six types of sand body ex... |
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