2002 Vol. 13, No. 4
Petroleum migration process of Bamianhe oilfield of Dongying depression, East China is evaluated on the basis of our detailed work on geochemistry of the crude oil and the source. Molecular tracers, especially nitrogen compounds, are employed in assessing style of oil migration and accumulation. Additionally, absolute quantitative results of heteroaromatics including dibenzofuran, dibenzothiophene and fluorene with similar basic frames to pyrrolic nitrogenic compounds are also used to evaluate the quantity of petroleum migration. According to apparent migration effects of the pyrrolic nitrogen compounds, it is indicated that most of Bamianhe oil was derived from potential source kitchen~Niu zhuang sag, migrated and accumulated at Bamianhe fault belt through selective paths according to a main filling point displayed. Generally, upward migration trend from the main point along the south slope is noted. There are observable migration effects from Guangli subsag to the north Bamianhe and Yangjiaogou oilfields showing Guangli subsag was a secondary kitchen accounting for the oils in the north portion. Significant vertical migration effects shown in several cross sections demonstrate that faults well developed in the area play an important role in oil migration and accumulation. Unformality, sandstone reservoirs distributed locally also suggest important fairways in connecting sources with traps. The tectonic and a combination of tectonic and lithological mechanism are identified as the critical mechanisms in entrapping hydrocarbons. Migration trend suggested by nitrogenic compounds agrees well with analysis of lateral compositional variety and thermal maturity gradient. This study also confirms well with our previous studies. It is the location of source rocks, tectonic and stratigraphic characteristics that have a comprehensive control on the model of hydrocarbon migration and accumulation in Bamianhe oilfield. Results also show that the oils are mostly derived from the deep parts of the Niuzhuang and Guangli sags within normal oil window.
Extensional basins include mainly grabens and half grabens displaced along a lower detachment. Based on area balance theory, there is a linear relationship between a height of regional and the lower detachment h on the outside of the basin and “lost area S” from the regional in the basin. The pre~growth beds above lower detachment are of the same extensional displacement so that an “S~h diagram” can be used to determine the depth to lower detachment and to calculate the total extensional displacement of the beds above the lower detachment. The extensional displacement is dominated by the heave of various scale normal faults. The displacement of obvious faults can be immediately figured out from the measured bed~length. The requisite extension calculated by area balance is the layer~parallel strain, which could be accommodated by displacement on sub~resolution faults. Accordingly, the layer~parallel strain can help us predict the magnitude and distribution of sub~resolution faults on the basis of analysis of the structural style and rheological behavior.
Trace fossils were discovered in the Middle Ordovician Badou Member of Fengfeng Formation and the Member 1 of Jinsushan Formation in Mt. Jinsu of the Fuping region which lies at the southeastern margin of the Ordos basin. The rocks of the fore mentioned parts contain a considerable amount of bioturbation structures, in which ichnogenus and ichnospecies can not be identified, and abundant stromatolites. The distributions and characteristics of the bioturbation structures are analyzed and summarized in this paper. The paleoenvironments of related formations and members were interpreted in the method of semiquantitative analysis of bioturbation structures with researches on stromatolites and sedimentology. It is suggested that Fuping zone, located in the southeastern margin of the Ordos basin, was a carbonate platform where stromatolite developing tidal flat and low~energy subtidal open platform environments dominated. Also that this zone experienced three paleogeographic evolutions from tidal flat to low~energy subtidal open platform environments, that is, three larger~scale eustatisms occurred during the sedimentary period of Middle Ordovician Badou Member of Fengfeng Formation to the Member 1 of Jinsushan Formation.