Research Of Black Rock Series In Chang 7₃ Member Of Yanchang Formation In Tongchuan Area,Southeast Ordos Basin

In recent years,we found that pyroclastic deposition in black rock series in Chang 7 Member of Yanchang Formation are neglected and even predominant in partial areas.As the pyroclastic rocks usually mix with black organic matter and black asphalt,it has been mistakenly idenfied as the so-called normal terrigenous sedimentary such as oil shales,oil mudstones,siltstones and fine sandstones.Hydrothermal materials largely developed in pyroclastic rocks and oil shales.The new discovery of composition of black rock series in Chang 7 Member of Yanchang Formation indicates that volcanic movement and hydrothermal deposition participated in deposition of the black rock series.As drilling datas are “One-sided understanding”,the observation of outcrop profile can more directly reflect the actual scales and vertical distribution of pyroclastic deposition and hydrothermal deposition.Basing on the research achievements of the predecessors,we focused on identifying composition of outcrop profile and analyzed the characteristics of the vertical distribution and the depositional background.In addition,we researched the primary hydrocarbon-generating organism and pore types of black rock series in Tongchuan region and discussed the relationship between enrichment of hydrocarbon and the hydrothermal deposition?the pyroclastic deposition,establishing the model of hydrocarbon accumulation.The main conclusions and understandings can be listed as follows: Firstly,we found that the so-called“black mudstone”are mainly composed of dust tuffs and some of so-called“siltstones and fine sandstones” may be Silt level-fine sand level tuffs when identified the pyroclastic rocks.Secondly,deposition of half-deep lake facies,shallow lake facies,deposition of Gravity flow and massive pyroclastic deposition overlap vertically in black rock series of Tongchuan area,rather than thenormal terrigenous clastic sedimentary under the control of half-deep lake facies.Moreover,hydrothermal deposition largely developed in black rock series in Tongchuan area,which include veins filled by anhydrite and typical hydrothermal minerals assemblage such as buddingtonite + jarosite + pyrite ? amorphous Si O2 and albitization of plagioclase.Generally speaking,The pyroclastic deposition and hydrothermal deposition largely developed in the black rock series in Tongchuan,which are good for accumulating of hydrocarban.