Organic Facies Type and Hydrocarbon Potential of Carbonates in Majiagou Fm. Lower Ordovician in Ordos Basin
-
摘要: 对鄂尔多斯盆地下奥陶统马家沟组碳酸盐岩的各种沉积环境的有机相特征进行了研究,结果表明平均有机碳丰度最高的是陆缘斜坡相碳酸盐岩(0.26%),其次是膏盐湖(0.22%)、膏云坪(0.21%),而开阔海、云坪和滩等环境的碳酸盐岩有机碳平均含量较低(<0.20%);碳酸盐岩干酪根样品多数为富氧、贫氢,显示其有机质类型较差;岩石地球化学特征和生物化石特征反映出陆缘斜坡处于相对还原的环境,而其他沉积环境则为氧化环境。鄂尔多斯盆地下奥陶统的大部分碳酸盐岩处于生烃潜力较差的C~D相。从有机质数量、有机相类型来判断,鄂尔多斯盆地下奥陶统碳酸盐岩不是好烃源岩,难以形成具有一定规模的气田。Abstract: Lower Ordovician carbonate in the Ordos basin is characteristed by its low organic matter content and high maturity, and its low residual TOC (total organic carbon) value can be not only regarded as its own initial feature but also contributed to the high maturity and high hydrocarbon potential. So whether it can be regarded as main source rock of the weathering-crust gas reservoir beneath the Upper Carboniferous/ Lower Ordovician parallel unconformity is still debatable. In this paper, the authors studied the hydrocarbon potential of Ordovician carbonates from the view of organic facies. Geochemical feature of different depositional environments includes: (1) Over 600 sample shows that carbonate formed in continental slope has a maximum average TOC value of 0.26%wt, then is that formed in gypsum lagoon (0.22%wt) and gypsodolomitic flat (0.21%); those deposited in dolomitic flat and open sea have lower TOC content (0.15%wt and 0.16%,respectively). TOC in carbonates formed in beach is minimum (0.15%). Even if taking the decrease of organic matter during maturity into account, the initial TOC value in the carbonate in its low-mature stage is still as little as in organic C~D proposed by Jones. (2) H/C and O/C atomic ratios of 20 kerogen samples all drop on the right part of the Van Krevelen chart. High O/C value indicates that the low H/C of organic matter in the carbonate is the feature during its formation instead the result of hydrocarbon generation, low H/C ratio also indicate a poor organic facies. (3) Mean Fe3+ /Fe2+ value is the highest of 33 carbonate samples formed in beach (8.55), then 5 open sea carbonate samples (0.78), 3 gypsum lagoon and gypsodolomitic plat carbonate samples (0.36). They are all higher than those in hydrocarbon generation or reservoir strata of Carboniferous-Permian (mean 0.05 of 12 samples) and Ordovician weathering crust (mean 0.07 of 15 samples), showing the lack of hydrocarbon and high oxidation potential. (4) Lithology and fossil feature also show a bad condition for the preservation of organic matter (oxidation and consumption of fauna). So most depositional environments of Lower Ordovician carbonate in the Ordos Basin (open sea, beach, dolomitic and gypsodolomitic flat) belong to organic facies C~D, with low hydrocarbon potential. Organic facies B can only exist in part of continental slope and part of gypsum lagoon, which is rather narrow. Organic facies study disproved the lower Ordovician carbonates to be good source rock in the Ordos Basin.
-
Key words:
- carbonate /
- organic facies /
- hydrocarbon potential /
- O rdos Basin
-
[1] 1 O ehler J H. Carbonate sou rce rocks i n the Ju rassic Smackover Trendof M ississippi, A labama, and Florida[A]. In:Palacas J G, ed.Petroleum Geochemistry and Sou rce Rock Potential of CarbonateRocks[C], AA PG, Tulsa, 1984. 63~70
2 Jones R W. O rganic facies[A]. In:Brooks J, Welte D, eds. A d-vances in Petroleum Geochemistry, Vol. 2[C]. London:AcademicPress, 1987. 1~90
3 冯增昭, 鲍志东, 张永生等. 鄂尔多斯盆地奥陶纪碳酸盐岩地层岩石岩相古地理[M]. 北京:地质出版社, 1998. 8~75
4 戴金星, 宋岩, 张厚福等. 中国天然气的聚集区带[M]. 北京:地质出版社, 1997. 61~63
5 程克明, 王兆云. 高成熟和过成熟海相碳酸盐岩生烃 条件评价方法研究[J]. 中国科学(D 辑), 1996, 26(6):537~543
6 刘宝泉, 郭树芝. 恢复原始有机碳 和生烃潜 力的简便 方法[J]. 古潜山, 1997(1):52~58
7 夏新宇, 洪峰, 赵林. 烃源岩生烃潜力的恢复探讨 -以鄂尔多斯盆地下奥陶统碳酸盐 岩为例[J]. 石油与天 然气地质, 1998, 19(4):307~312
8 邱中建, 张一伟, 梁狄刚等. 田吉兹、尤 罗勃钦碳 酸盐岩油 气田石油地质考察及对塔里木盆地寻找 大油气田 的启 示和 建议[J]. 海相油气地质, 1998, 3(1):49~56
9 戴金星, 夏新宇. 长庆气田奥陶系 风化壳气 藏气源研 究回顾[J].地学前缘, 1999, 6(增刊):195~203
10 夏新宇, 赵林, 戴金星. 鄂尔多斯盆地 奥陶系风 化壳气藏 中油型气成分的来源[J]. 石油勘探与开发, 1999, 26(4):23~25
11 夏新宇, 赵林, 李剑锋等. 长庆气田天 然气地球 化学特征 及奥陶系气藏成因[J]. 科学通报, 1999, 44(10):1 116~1 119
计量
- 文章访问数: 619
- HTML全文浏览量: 5
- PDF下载量: 518
- 被引次数: 0