陕甘宁盆地中部马五段上部准层序划分及其特征

Identification and Description of Parasequences in Upper Ma5 Member, Central Ordos Basin

根据多种资料在陕甘宁盆地中部马五段上部划分出8个准层序，对每个准层序作了详细的描述，它们大多与自然伽玛曲线对应良好，且均为米级规模，顶部多为含膏云坪微相，反映持续暴露的证据充分，认为它们是由他放回作用形成的。受海平面升降的控制，准层序内δ13C值先升后降，并且由于其形成的早期和晚期石膏和石盐含量各不相同准层序内孔隙度和残余含盐量逐渐升高。准层序内泥质和有机质的含量变化也导致酸不溶物含量逐渐降低。岩石的高孔隙带多分布于暴露型准层序的上部或顶部。

Based on various information, total 8 allocyclic parasequences are identified in Upper Ma5 Member,Central Ordos Basin and most of them are in good coincidence with the Gamma Ray logging curves. In this paper, each parasequence is described in detail The upper parts of these meter-scale parasequences are mostly deposited in dolomite flat enriched with gypsum, which abound in various evidences of durable exposure. Controled by sea-level fluctuation, δ13C values in each parascquence ascend and follow by reduction. As shown in parasequence No. 6, δ13C va1ue varies upward from -3. 0‰ to -0. 7%,, and finally reaches to1.7‰, But because each parasequence is located at the different part of the third-order sequence, the averages of δ13C values may change. These changes in higher-order sequence show a similar rule as that in parasequences. Different levels of sea-level fluctuation shaped sequences of different levels or even parasequences,thus indicated the similar ru1es in sequences and parasequences. On the basis of numerous test data, the content of gypsum and salt precipitated in different period of the formation of parasequences seem to change regularly, leading to the porosity and content of remnant gypsum steadily increase. While content of material which can not be dissolved in acid gradually descend, as it was mainly constituted by mud and organic matters. High porosity zone mostly distributes at the top or upper part of exposed parasequences.