山东东营凹陷沙河街组纹层年龄估算方法探讨

A PROBING AGE-CALCULATION METHOD FOR THE LAMINAE OF THE SHAHEJIE FORMATION IN THE DONGYING DEPRESSION,SHANDONG

渤海湾盆地位于北纬35°—42°之间,中始新世沙河街组四段上部和三段沉积期间,在气候上属于中亚热带,湖泊水体较为稳定,并出现季节性分层,因此沉积常显示出暗灰和浅灰色的年纹层(或称偶层)。根据不少国内外湖泊纹泥统计和14C测定和频谱分析的对比,认为纹层可以作为地层定年工具。纹层年龄计算方法研究对含油盆地成烃成藏定量研究具有重要意义。东营凹陷年纹层计算方法有两种,或选定含纹层岩芯样品进行岩石薄片和电子探针观察和测定,取得年纹层间隔与元素峰值统计;或选取一段含纹层的测井数据作天文400ka、40ka或20ka周期地层年龄计算。通过两种方法求出东营凹陷通29井等沙三段—沙四上亚段堆积速率的比较,前者薄片和探针方法得到的平均速率为0.1078mm/a,后者天文的方法得到的是0.1280mm/a,两者计算差只有0.0172mm/a。因此应用两种方法来计算地层年龄和对比是可行的。前者求纹层方法更为直观,但岩芯和电子探针的测定费用较大,而相比而言,用油田测井数据作天文地层堆积速率和年龄的计算,在经济上、时间上可能胜于前者。

The Bohai Gulf is located between 35°and 42° N, where the upper part of Member 4 and the Member 3 of the Shahejie Formation belonged climatically to the middle subtropical zone and the lake water was stable, with seasonally deposited beds occurring. Thus, dark and light grey laminae often appeared. Based on the comparison between the statistics for the lacustrine laminae in China and abroad, the 14 C dating, and the frequency spectrum analysis, it is considered that laminae may be used as a tool for stratigraphic dating. Therefore, the study of the method for calculating lamina age is of important significance in the quantitative study of hydrocarbon accumulation in the petroliferous basin. There are two kinds of methods for calculating lamina age of the Dongying Depression: to select lamina-bearing cores for conducting the rock thin section and electron probe observation and determination and obtaining lamine intervals and element peak value statistics; to select a sector of lamina-bearing logging data to carry out cyclostratigraphic age calculation of astronomic periods of 400ka, 40ka or 20ka. A comparison of the accumulation rates between the Member 3 and the Upper Submember of Member 4 of the Shahejie Formation in the Tong-29 Well of the Dongying Depression, by using these tow kinds of methods, shows that an average accumulation rate of 0.7078mm/a was obtained by means of the thin section and probe method and the rate of 0.1280 mm/a was acquired by using astronomic method. There is a difference of only 0.0172mm/a.Using these two kinds of methods to calculate and compare stratigraphic ages is viable. By comparison using the former method is of more direct observation, but the cost is high; using the latter method is better economically and temporally.