The Recognition Of The Fourth Member Of Shahejie Formation In Central And Northern Parts Of The Huanghua Depression And Its Corresponding Significance Discussion

The Fourth Member of Shahejie Formation was only discovered in the constrained areas around Wangguantun in south Huanghua Depression in the past. Though it shares a complete Sha. IV Member sequense, the upper submember is not a favorable oil sourcing bed and it seizes little attention. In the early 1990s it was the paleontologists that doubted the Fourth Member of Shahejie Fm. may also distribute in the Beitang Sag in north Huanghua Depression according to some spore pollen fossil data from several wells. With limited fossil evidence and lower geological exploration level this say was not convincible and had to remain a prediction. This paper mainly demonstrates that the underlying neighbor layer of The Third Member of Shahejie Formation that used to be put in the Mesozoic shows quite typical characteristics of Shehejie Formation of Eocene in fossil assemblage lithology electricity, as well as seismic reflectance profiles. The sequence correlation shows that this layer should be put in The Forth Member of Shahejie Formation.The Fourth Member of Shahejie Fm. in both central and northern parts of Huanghua Depression is mainly purple or grey mudstone, silts with interbedded dark grey marl, limestone, oil shale, tuff breccia and totally 465m in thickness (set Well Banshen? as the example ). The lithology in Beitang Sag is comparably finer than that in Banqiao Sag. The electrify of this layer shows comb shaped resistance rate curve with frequent ups and downs crowded bayonets subresistance rate curve. The natural electric potential curve is comparative straight and the natural gamma curve also shows frequent ups and downs like crowded fingers, the sonic wave difference curve shows comb shaped ups and downs. So both the electrify and the lithology characteristics can be compared with The Fourth Member of Shahejie Fm. Of south Huanghua Depression and its neighbor depressions.It contains abundant fossils such as ostracodes gastropodes algae,etc., and the obtained algae fossils are so much in amount and typical that they can be enough to make up an assemblage named Campenia-Pediastrum Assemblage. This assemblage is characterized by occurring a large number of chlorophyta elements like Campenia and Pediastrum, besides, other kind of algae also can be seen in this assemblage, they are Bohaidina, Parabohaidina, Conicoidium, Palaeostomocystis minor, Deflandrea. Among the above algae, Campenia is a special member that is considered with constrained strata distribution of the second and the fourth members of Shahhejie Fm.. Deflandrea is the representing element of The Fourth Member of Shahejie Fm. in Bohai Bay Basin. However others of this algae assemblage often occur in the Third Member of Shahejie Fm., and also with a little amount in The Fourth Member of Shahejie Fm.. Another feature of this assemblage is that it contains both elements reflecting fresh water environment and elements representing brackish water or half-salty water sedimentary environment.Based on the spore and pollen assemblage Taxodiaceae High Content Assemblage steadily distributing in Eocene in Bohai Bay Basin, we further established a subassemblage named P'maceae-Taxodiaceaepollenites-Ulmoideipites Subassemblage. It mainly contains angiosperm and gymnosperm pollens and rarepteridophyte spores, shows a succession of the spore and pollen assemblage of The Second Member of Kongdian Fm.. It is also quite comparable with the spore pollen assemblages distributing in The Fourth Member of Shahejie Fm. of the neighbor depressions to Huanghua Depression in Bohai Bay Basin.In addition, some ostracodes gastropodes and charophytes are found in The Fourth Member of Shahejie Fm. in Banqiao and Beitang sags. They are Eucypris albata , Cypris bella, C. sp., Cyprois magna, Cyprinotus sp.; Hydrobia liuqiaoensis, Stonothyra, Valvata; Shandongochara decorosa, Obtusochara jianglingensis, Grambastichara sp. etc.. All these members are the common elements from The Second Member to The Fourth Member of Shahejie Fm. in Bohai Bay Basin.The seismic profiles sho...